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Future Scaling Potential of Particulate Media in Magnetic Tape Recording
C H A P T E R
T H R E E
Future Scaling Potential of Particulate Media in Magnetic Tape Recording☆ Mark A. Lantz* and Evangelos Elefteriou Contents 1. Introduction 2. Media Considerations 2.1. Metal particle media 2.2. BaFe particulate media 2.3. Future scaling potential of BaFe media 3. 29.5-Gbit/in2 Areal Recording Demonstration 3.1. Introduction 3.2. Media 3.3. Low-friction head technology 3.4. Tape path and hardware platform 3.5. Servo pattern design 3.6. Synchronous servo channel 3.7. Design of the track-following controller 3.8. Track-following performance 3.9. Read channel and recording performance 3.10. Summary of demo results 4. Continued Future Scaling Potential of Magnetic Tape Technology 4.1. Introduction 4.2. Write and read-head technology 4.3. Head–tape interaction 4.4. Data detection
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*Corresponding author: [email protected] IBM Research - Zurich, Sa¨umerstrasse, Ru¨schlikon, Switzerland ☆
This chapter is an updated and expanded version of the information published in the following two papers: “Scaling tape recording areal densities to 100 Gb/in2” by A. J. Argumedo, D. Berman, R. G. Biskeborn, G. Cherubini, R. D. Cideciyan, E. Eleftheriou, et al., IBM J. Res. Develop., Vol. 52, No. 4/5, pp. 513–527, July/September 2008, and “29.5-Gb/in2 recording areal density on barium ferrite tape” by G. Cherubini, R.D. Cideciyan, L. Dellmann, E. Eleftheriou, W. Haeberle, J. Jelitto et al., IEEE Trans. Magnet., Vol. 47, No. 1, pp. 137–147, January 2011.
Handbook of Magnetic Materials, Volume 22 ISSN 1567-2719, http://dx.doi.org/10.1016/B978-0-444-63291-3.00003-9
Copyright # 2014 Elsevier B.V. All rights reserved.
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4.5. Format efficiency 4.6. Track density limits 5. Summary and Conclusions Acknowledgments References
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1. Introduction The volume of digital data produced each year is growing at an overwhelming pace. According to a recent International Data Corporation (IDC) study (IDC, 2011), 1.2 zettabytes (1.2 1021) of data were created and replicated in 2010. In the future, this staggering volume of data is projected to grow at a compound annual growth rate (CAGR) of 50% or more, significantly faster than the expected growth rate of storage capacity. New regulatory requirements imply that an ever-larger fraction of these data will have to be preserved for much longer periods of time. All of this translates into a growing need for cost-effective digital archives, in which tape plays an integral part in current tiered storage infrastructure. Tape systems are particularly well suited for low-cost, long-term storage of data and for backup and disaster recovery purposes. Tape technology offers several important advantages, including energy savings, security, lifetime, reliability, and cost. Moreover, in such applications, the main drawback of tape, namely, its slow access time, does not have a major impact on the system performance. Once data have been recorded in tape systems, the medium is passive: it simply sits in a rack and no power is consumed. Compared with similar HDD (hard disk drive)-based systems, a tape-based archive consumes roughly 290 less power (Reine and Kahn, 2008). Tape systems also offer additional security in that once data have been recorded and the cartridge has been removed from the tape drive, the data are inaccessible until the cartridge is reinstalled in a drive. Security is further enhanced by drive-level encryption, which was introduced in Linear Tape-Open1 generation-4 (LTO-4) drives, and is also standard in enterprise-level tape drives. The tape medium has a lifetime of 30þ years (Fujifilm, 2011; Watson et al., 2008); however, this is rarely used because the rapid advances in tape hardware lead to significant cost savings associated with migration to higher-capacity newer-format cartridges. In terms of reliability, the latest LTO tape drives have a bit error rate (BER) that is several orders of magnitude better than that of a SAS (serial attached scsi) 1
Linear Tape-Open and LTO are trademarks of HP, IBM, and Quantum in the United States and other countries. IBM is a trademark of International Business Machines Corporation, registered in many jurisdictions worldwide. Other product and service names might be trademarks of IBM or other companies.
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HDD. Moreover, the fact that tape media are removable and interchangeable means that, in contrast to HDDs, mechanical failure of a tape drive does not lead to data loss because a cartridge can simply be mounted in another drive. All of the earlier-mentioned advantages contribute to the two net advantages of tape: cost and reliability. Estimates of the savings of the total cost of ownership of a tape backup system, relative to HDD backup, range from a factor of three to more than 20, even if technologies such as data deduplication are taken into account. In archival applications, where data deduplication is ineffective, the cost savings are even higher. Historically, the cost and the size of the media recording area of both tape cartridges and disk drives have remained relatively constant. Reductions in the cost per Gbyte, which are fundamental to the continued viability of these technologies, have primarily resulted from increases in the areal recording density. Figure 3.1 compares the evolution over time of the areal densities of commercially available tape drives and HDDs and also shows recent tape areal density demonstrations. The figure illustrates that historically, tape drives have operated at an areal density roughly two orders of magnitude lower than that of HDDs. However, tape drives maintain a lower cost/Gbyte relative to disk because tape media can be produced at a very low cost per unit area and because the media are removable, allowing the cost of the drive to be amortized over many cartridges. The cost/Gbyte difference is the key reason that tape technology has remained successful, even though the investment into research and development in tape drives is far lower than that for HDDs. From another perspective, the areal density
Figure 3.1 Recording areal density of hard disk drive and tape products and tape demonstrations. Adapted from Berman et al. (2007).
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difference between tape and disk systems presents a great opportunity for achieving a much lower cost/Gbyte for tape if the areal density of tape can be brought closer to that of HDDs by skillful engineering. Moreover, Fig. 3.1 also indicates that even though the gap between the areal densities of HDD and tape products has remained essentially constant in recent years, tape areal density demonstrations exhibit a slope of about 60% CAGR, indicating the potential to reduce the areal density gap between tape and HDD. We can gain insight into how this rapid improvement in tape areal density demonstrations was achieved by comparing the bit aspect ratios (BARs) of tape drives and HDDs. The typical BAR (bit width to bit length) in recent HDD technology demonstrations is on the order of 6:1 (Bandic and Victoria, 2008). In contrast, LTO generation-5 (LTO-5) tape drives operate at an areal density of 1.2 Gbit/in2 with a BAR of about 123:1 (IBM, 2010). This comparison indicates that from a magnetic recording physics perspective, there is considerable room to reduce the track width in tape technologies. In the past, one of the obstacles to a more widespread use of tape technology has been the difficulty of using tape in a general or standalone context. Hard disks provide random access to data and generally contain a file index managed by a file system. These files can be accessed using standard sets of application programming interfaces (APIs) via various operating systems and applications. Tape, in contrast, is written in a linear, sequential fashion using a technique called “shingling,” which not only provides backward-write functionality but also imposes the restriction that new data can only be appended and that previously written areas can only be reclaimed if the entire cartridge is reclaimed. In traditional tape systems, an index of the files recorded on a given cartridge is typically kept only in an external database managed by an application such as a proprietary backup application. The requirement to access an external database to retrieve data renders data on tape much less portable and accessible than data on alternative storage methods, such as a HDD or a USB flash drive. To address these deficiencies, a new long-term file system (LTFS) has recently been introduced into the LTO-5 tape-drive systems and recent enterprise tape systems to enable efficient access to tape using standard system tools and interfaces (Pease et al., 2010). LTFS is implemented using the dual-partition capabilities supported in the LTO-5 format initially and now also in recent enterprise products. A so-called index partition is used for writing the index, and the second, much larger partition for the data itself. With this new file system, files and directories show up on the desktop with a directory listing. Users can “drag and drop” files to and from tape and can run applications developed for disk systems. All of these features help to reduce the costs associated with using tape and eliminate the dependency on a middleware layer. In addition, tape becomes
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cross-platform portable (Linux, Mac, and Windows2), enabling and facilitating the sharing of data between platforms. These features render significant new use cases for tape such as video archives and storage of medical images possible. Considering the cost advantages of tape over other storage solutions and the increasing usability of tape provided by advances such as LTFS, tape is set to play an important role in the rapidly expanding market for archival data storage solutions. How large a role tape will play—and for how long— will depend on the continued scaling of tape to higher areal densities at a constant cost so that the cost per Gbyte advantage of tape over other technologies either remains constant or increases. In this chapter, we will explore the future scaling potential of magnetic tape technology based on low-cost particulate media. In Section 2, we discuss the state of the art of particulate media. In Section 3, we review the achievements and the technologies behind the recent 29.5 Gbit/in2 areal density demonstration using low-cost particulate BaFe media. In Section 4, we examine the potential for further scaling, posing the questions: “Can magnetic tape be scaled to 100 Gbit/in2?” and “What technologies are required to do so?” (Argumedo et al., 2008). Finally, in Section 5, a summary and conclusions are presented.
2. Media Considerations 2.1. Metal particle media Most of the present and legacy commercial magnetic tape media use iron– cobalt-based metal particles (MPs). In this section, we will discuss various particulate media considerations to achieve areal densities beyond 50 Gbit/in2, which according to the INSIC 2012 roadmap (INSIC, 2012) will bring us to cartridge capacities of 128 TB by 2022. In addition, we will examine the potential to scale particulate tape beyond this point towards 100 Gbit/in2. The following parameters limit the areal recording density that can be achieved on tape: the magnetic stability of the medium, the maximum achievable write-head field, the minimum bit length that can be recorded in the medium, and the signal-to-noise ratio (SNR). The broadband SNR varies with the number of particles per bit volume, and model calculations have shown that to achieve reasonable SNR, about 100 particles per bit are required (Mallinson, 1974). Thus, particles must become smaller as the areal density increases. However, to preclude 2
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thermally induced switching of the particles, the anisotropy energy KUV of each particle must be much larger than the thermal energy in the environment, kBT. Here, KU is the uniaxial anisotropy energy density, V the volume of the particle, kB the Boltzmann constant, and T the absolute temperature. A widely used rule for data retention is that KUV > 60 kBT. Particles with a higher anisotropy also require larger magnetic fields for magnetization reversal. Unfortunately, head fields are limited by the saturation magnetization (Bs) of the head-pole materials, and thus, there is a limit to how much the particle volume can be reduced. Current tape drives use write heads with a saturation magnetization on the order of 16–18 kG, and more than 20 kG is considered large. The limit of existing materials is on the order of about 24 kG (e.g., Co35Fe65 has a Bs of 24 kG). Following Weller and Moser (1999) and Zhou and Bertram (1999), the minimum bit length Bmin in a thin longitudinal media is approximately sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 D B min p 0:35a2WC þ 2
where D is the particle diameter and aWC the Williams–Comstock transition width parameter. In SI units, aWC can be expressed as (Bertram, 1994; Williams and Comstock, 1971) ð1 S Þðd þ t=2Þ þ aWC pQ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 S Þðd þ t=2Þ 2 Mr t ðd þ t=2Þ : þ pQ pQHC
Here, d is the head/medium spacing, t the medium thickness, Mr the remanent magnetization, S the hysteresis squareness, and Q the head field gradient. HC denotes the medium coercivity, which is proportional to the medium anisotropy field Ha. For imperfectly oriented medium at typical recording timescales, it can be approximated by 0.5Ha or KU/Ms, where Ms is the saturation magnetization (Sharrock, 1999). Using these equations, the minimum bit length for longitudinal recording can be estimated as a function of medium thickness (i.e., thickness of the magnetic layer on the tape), head/medium spacing, coercive field, and medium saturation magnetization. Figure 3.2 presents the calculated minimum bit length achievable, assuming a maximum coercive field of 5300 Oe (hence, a Bs of 20 kG, 80% usable deep gap field, and HC ¼ 1/3 of the deep gap field to account for head/medium spacing) and a saturation magnetization of 550 emu/cc. These values correspond to a required particle anisotropy constant of 2.9 106 erg/cc and a particle diameter of at least 12 nm. These calculations also show that at a head/medium spacing of, for example, 20 nm, longitudinal media can sustain a linear density of 800 kbpi if the medium is no thicker than 15 nm.
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Figure 3.2 Minimum bit length versus medium thickness calculated for three different head/medium spacings. Hc ¼ 5300 Oe, Ms ¼ 550 emu/cc, Q ¼ 0.8, S ¼ 0.8. With these values, the maximum particle anisotropy equals of 2.9 106 erg/cc, and the minimum particle diameter for magnetic stability is 12 nm. Reprinted with permission from Argumedo et al. (2008).
For such a linear density and a medium thickness of 15 nm, the minimum bit area is 32 nm 380 nm if we assume 100 particles of 12 nm diameter per bit with a 50% volume packing fraction. To account for servoing error and dimensional stability, a 50% margin should be added to the track pitch, which yields a track density of 45 ktpi and a maximum areal density of 36 Gbit/in2, in agreement with earlier projections (Charap et al., 1997). To achieve an areal density of 100 Gbit/in2 with a linear density of 800 kbpi, the track density needs to be 125 ktpi, which corresponds to a minimum bit area of 32 nm 135 nm (note that 50% of the track width has been removed to account for track-following error) or 36 particles of 12 nm diameter with a 50% volume packing fraction. This corresponds to a loss in SNR of 4.5 dB compared with the SNR in the 100-particle case, which has been defined as 20 log(N1/2). To compensate for this loss in SNR, the signal processing would have to be improved. Other routes to achieving 100 Gbit/in2 with longitudinal media have been proposed, but these put even more constraints on the track density or the head/medium spacing (Bertram and Williams, 2006; Dee, 2006; Mallinson, 1974). In summary, achieving a linear density of 800 kbpi, although theoretically possible, appears extremely challenging with longitudinal recording. A more attractive alternative for achieving such high linear densities is the migration to perpendicular recording. In perpendicular recording,
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demagnetizing fields actually favor high linear densities. Moreover, thicker films could be used for better thermal stability. For example, linear densities of 1500 kbpi have been demonstrated for a HDD using pole heads and perpendicular media with a soft underlayer (Seagate, 2006). On tape, achieving 800 kbpi linear density will require very thin coatings, probably containing only a monolayer of particles. To achieve low noise for low bit sizes, the particles must be magnetically isolated but packed closely and uniformly with their neighbors. Packing would be easiest for uniformly sized spherical particles, and recently, nanoparticles have been successfully synthesized and arranged uniformly on surfaces (Lin and Samia, 2006; Pyun, 2007). To render the use of these particles attractive for magnetic tape will require inexpensive synthesis and a rapid method for coating them uniformly on the tape, especially as typical tape lengths are approaching 1 km (some products have even exceeded this length). An alternative to particles would be the formation of thin magnetic films using vacuum deposition. Evaporated metal films have already been introduced in tape products (Motohashi et al., 2007); they have the advantages that they can be deposited at high rates and that inexpensive metals can be used. Sputtered metal films, which have been successfully developed for HDD, have the potential for much larger areal density; however, the cost of the raw materials required to produce sputtered magnetic tapes would likely result in a significant increase in cartridge cost. While it may be possible to transfer that technology in an economically viable solution to tape media, this remains challenging (Lee et al., 2005; Moriwaki et al., 2005). In summary, sputtered magnetic films are promising for tape, but optimized particulate and evaporated metal films may still have the potential to achieve 100 Gbit/in2. In the next section, we will focus on low-cost particulate media, specifically BaFe media, which hold the promise to bring us to areal densities beyond 50 Gbit/in2 and may even be extendible to 100 Gbit/in2.
2.2. BaFe particulate media As discussed in the preceding text, one approach to increase the recording capacity of magnetic tape is to reduce the volume of the magnetic particles used in the recording layer. In this section, we investigate the use of BaFe particles as a potential replacement of MPs in future tape media. As the coercivity of MP media originates in the shape anisotropy, it is difficult to maintain a high coercivity when the particle volume is reduced. In contrast, the coercivity of BaFe particles arises from the crystalline magnetic anisotropy, rendering the scaling of BaFe particles more favorable. Another issue with traditional MP media results from the oxidation of the iron–cobalt that is typically used. To prevent this, the FeCo particles are covered with a
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protective, nonmagnetic “shell,” which is a further issue in scaling to finer particle sizes. BaFe, in contrast, is already an oxide and therefore does not need a protective shell. Another advantage of BaFe particles is their unique platelet shape, which makes it easier to orient the easy axis in the perpendicular direction of the medium surface. Note that perpendicular orientation results not only in an increase in signal amplitude but also in a reduction of the demagnetizing field at high linear densities (Thompson, 1997). Together, these properties make the scaling of BaFe to finer particle sizes much more favorable than MP technologies. Therefore, the use of BaFe particles (Berman et al., 2007; Harasawa et al., 2010; Kubo et al., 1982; ¨ lc¸er Matsumoto et al., 2010; McClelland et al., 2009; Nagata et al., 2006; O et al., 2009; Shimizu et al., 2010; Watson et al., 2008) in future generations of magnetic recording media has been investigated extensively. To improve the performance of BaFe media even further relative to that of previous media generations (Berman et al., 2007; Harasawa et al., 2010; Kubo et al., 1982; Matsumoto et al., 2010; McClelland et al., 2009; Nagata ¨ lc¸er et al., 2009; Shimizu et al., 2010; Watson et al., 2008), et al., 2006; O three new technologies were applied in developing the medium. First, the average particle volume was reduced to 1600 nm3. Second, the easy axis of the particles was oriented in the perpendicular direction during the coating process, resulting in a squareness ratio of 0.86 in the perpendicular direction. Finally, the surface waviness was reduced, achieving a surface roughness (Ra) of 0.7 nm, as measured with optical interferometry, which neglects the small asperities, and an Ra of 2.1 nm measured using atomic force microscopy (AFM). The properties of this new perpendicular BaFe tape (tape A) are summarized in Table 3.1. For comparison, we have also included the properties of the BaFe medium used in an earlier areal density demonstration of 6.7 Gbit/in2 (tape B) (Berman et al., 2007) and the properties of commercial MP media (LTO-5). Note that LTO-5 operates at an areal density of 1.2 Gbit/in2 with a cartridge capacity of 1.5 TB (IBM, 2010). The magnetic layer of all three media types is approx. 60 nm thick. 2.2.1. Fine-particle BaFe In general, as the size of a magnetic particle is reduced, also the thermal stability factor KUV/kBT will be reduced. For BaFe particles, the anisotropy can be adjusted by substitution of Fe with other elements (Kubo et al., 1982). To compensate for the reduced particle volume of tape A, the type and amount of substitution elements used in the BaFe particles were adjusted to increase KU. By optimizing the formulation of the 1600 nm3 particles, a thermal stability factor of 75, measured as described in Matsumoto et al. (2010), was achieved. This provides sufficient stability for long-term archival applications.
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Table 3.1 Properties of magnetic particles and tapes using barium ferrite (BaFe) and metal particle (MP) media (Cherubini et al., 2011) Medium type
Tape A
Tape B
LTO
Magnetic particle material Volume (nm3) Coercivity (Oe) Saturation magnetization (emu/g) Orientation direction
BaFe 1600 2250 48 Perpendicular
BaFe 2100 2300 52 Nonoriented
MP 2800 2400 103 Longitudinal
1970 0.40 0.31
2730 0.86 1.39
2660 0.61
– –
0.40
–
2.0
2.0
2.0
2.5
Magnetic properties measured in longitudinal direction Hc (Oe) 1480 SQ 0.22 Mrt (memu/cm2) 0.18 Magnetic properties measured in perpendicular direction Hc (Oe) 2940 SQ (with demagnetization 0.86 compensation) Mrt (memu/cm2) 0.56 Surface roughness Ra (nm) Optical interferometry (measured 0.7 with a WYKO HD2000 instrument) AFM 2.1
2.2.2. Perpendicular orientation Figure 3.3 presents transmission electron microscopy (TEM) images of tapes A and B. Note that in tape A, the platelet-shaped BaFe particles are well aligned, whereas in tape B, they appear randomly oriented. The more regular orientation of the particles in tape A results in an increased squareness ratio and improved performance (Shimizu et al., 2010). This improved squareness ratio was achieved through the use of intensive dispersion prior to coating and the application of a magnetic field in the drying zone of the coating process. The M—H loops of these tapes are shown in Fig. 3.4. The M—H loop in the perpendicular direction of tape A is very similar to that of the LTO-5 tape in the longitudinal direction, indicating a similar degree of orientation. 2.2.3. Low-waviness surface To reduce the head/medium spacing, the surface of the medium should be as smooth as possible. However, the fundamental dilemma in contact magnetic recording is that such an increase in medium smoothness may result in increased friction and hence may reduce the durability and
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Figure 3.3 TEM images of cross sections of (a) tape A (perpendicular orientation) and (b) tape B (non-oriented). Scale bars are 20 nm. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Figure 3.4 M—H loop in the perpendicular direction of tapes A and B, and M—H loop in the longitudinal direction of LTO-5 tape. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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runnability of the medium. To resolve this dilemma, the long-range surface roughness, referred to as “waviness,” was reduced. Simultaneously, a moderate roughness when measured over a shorter length scale was maintained. Figure 3.5 presents 180 mm 240 mm surface profiles of tapes A and B, measured with optical interferometry. The data show a waviness of Ra ¼ 0.7 nm for tape A and an Ra ¼ 0.2 nm for tape B. Figure 3.6 presents 40 mm 40 mm surface profiles of both tapes measured with AFM. Note that compared with tape B, tape A has many small asperities. This combination of low surface waviness and moderate short-range roughness in tape A results in an increased signal from the medium while maintaining excellent durability and runnability.
2.3. Future scaling potential of BaFe media The perpendicularly oriented BaFe tape described previously (tape A) was used to perform a 29.5-Gbit/in2 areal density recording demonstration (Cherubini et al., 2011). The details of this demonstration are described in
Figure 3.5 Optical interferometry measurements of surface roughness (waviness): (a) tape A and (b) tape B. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Figure 3.6 AFM images of tape surface profiles: (a) tape A and (b) tape B. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Section 3. This areal density achievement does not, however, represent the limit of the scaling potential of BaFe media, but rather is a snapshot of the state of the art of the technology at the time the demonstration was performed. There remains considerable potential to continue scaling BaFe technology beyond this point. For example, the particle volume of 1600 nm3 used in tape A could be reduced to significantly smaller volumes, leading to even better SNR values. However, the coercivity of the particles would then have to be increased to ensure the archival stability of the medium. This in turn necessitates the development of write heads with a higher saturation magnetization than that of the heads used for the demonstration. Second, the magnetic spacing could be further reduced in future generations of the media if the tape–head friction can be controlled through technologies such as those discussed in Sections 3 and 4. Third, further improvements in the orientation of the particles will result in additional SNR gains. Finally, the media noise can be further reduced through decreasing the variations in particle size and magnetic properties and by improving the dispersion of the particles. We believe that with such improvements, it should be possible to scale BaFe-based tape to areal densities of 50 Gbit/in2 and beyond, which are required for a 128-TB cartridge. To achieve an areal density of 100 Gbit/in2 will require major advances in all of the areas discussed in the preceding text. However, even when BaFe reaches its scaling limits, there is potential to continue scaling magnetic tape technology through the development of new particle technologies or by transitioning to sputtered media.
3. 29.5-Gbit/in2 Areal Recording Demonstration 3.1. Introduction We can gain insight into the state of the art of magnetic tape-recording technology and the experimentally established potential for future areal density scaling by reviewing the results of a recent single-channel areal recording demonstration of 29.5 Gbit/in2 (Cherubini et al., 2011). This was achieved by increasing the track density by more than 18-fold relative to that of LTO-5 and by dramatically improving the performance of the track-following servo system. The servo improvements were realized through the use of an optimized servo pattern in combination with a new servo channel, an H1-based track-following controller, and a new low-friction head technology. This aggressive scaling of the track density necessitates the use of much narrower read elements, leading to a reduced readback amplitude. To compensate for this reduction, a new magnetic tape based on ultrafine, perpendicularly oriented BaFe magnetic particles was employed that enables high density without the need for expensive
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vacuum-coating methods and provides a much better SNR than standard MP tape media. Finally, an advanced noise-predictive maximum-likelihood (NPML) detection method was implemented that supported linear densities of up to 518 kbpi with an ultranarrow, 0.2 mm wide data reader, leading to an additional gain in areal density. In this section, each of these technologies will be discussed, followed by a description of how they can be combined to achieve an areal density of 29.5 Gbit/in2. This areal density corresponds to approx. 25 the areal density of LTO-5. However, it is important to note that this was a single-channel demonstration, in which effects such as tape dimensional stability and head fabrication tolerances, which are important for multichannel parallel tape systems, were not taken into account. The impact of such effects and the potential to scale tape beyond 29.5 Gbit/in2 towards 100 Gbit/in2 are discussed in Section 4.
3.2. Media A detailed discussion of particulate tape media was presented in Section 2 of this chapter. The properties of the two types of tape investigated in the 29.5 Gbit/in2 demonstration were summarized in Table 3.1, in which also the properties of LTO-5 tape were included for comparison.
3.3. Low-friction head technology The tape magnetic spacing, which is defined as the distance between the magnetic coating on the tape and the read/write transducers in the head, is a critical parameter for scaling the linear density in magnetic recording systems. The 2012 INSIC roadmap (INSIC, 2012) predicts that, in order to scale the linear density of a tape system from the current value of 400–500 kbpi to around 1000 kbpi by the 2022 time frame, the magnetic spacing will have to be reduced from the current (2012) value of approx. 36 nm to about 19 nm by 2022. Unlike in HDDs, there is no air bearing between the head and the storage media in tape systems. Instead, tape heads are designed with a sharp leading “skiving” edge that prevents air from entering the tape–head interface. This results in a reduced air pressure below the tape, such that the higher air pressure above the tape pushes the tape into contact with the head (Biskeborn and Eaton, 2002). In conventional tape head designs, the head surface is smoother than the tape, such that the head–tape contact area is determined primarily by the tape roughness. In addition, the magnetic spacing is determined by the roughness of the tape and any recession of the pole tips in the head, which can occur over the lifetime of the head because of wear. Several factors contribute to the tape surface roughness, including the base film roughness, coating uniformity, and the effect of asperities engineered to control the head–tape contact area and for head cleaning.
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To reduce the magnetic spacing, the tape roughness must be reduced. In a conventional flat-head geometry, this results in an increase in contact area, which in turn causes an increase in friction and stiction forces (Biskeborn and Eaton, 2002). Variations in the tape velocity caused by varying friction forces can degrade the performance of the timing recovery loop in the read channel, leading to a degradation in detector performance and an increase in the raw error rate at the output of the detector. In extreme cases, stiction can cause the tape transport to stall and even result in tape damage. The dynamic effect of head–tape friction is primarily to excite a compression wave in the tape, which reflects back and forth between the two rollers on either side of the head at a frequency determined by the mechanical properties of the tape and the distance between the rollers. Typical frequencies are on the order of several tens of kilohertz. The effect of the resulting timing variation is largest at low tape speeds, at which the timescale is fastest compared with the single-channel data rate. Although a low magnetic spacing is required at the read/write and servo elements, these elements span slightly less than one-quarter of the tape width, that is, one out of four data bands. In the LTO 1–5 tape formats, these elements’ span is only 2.9 mm out of the 12.65-mm tape width. Although the entire width of the tape must be supported by the head, the regions distant from the read/write elements can be just as effectively supported by an air bearing as by the hard contact required in the central region of the head. A simple and elegant method to produce an air bearing at the periphery of the head while maintaining intimate contact in the central region occupied by the read/write transducers is to bevel or round the skiving edge of a conventional head in the regions distant from the read/ write transducers. This approach was used in the 29.5-Gbit/in2 areal density demonstration to enable the use of smoother media with reduced magnetic spacing while maintaining tolerable levels of friction and stiction. In addition, the beveled head technology has been adopted in the latest IBM tape drives. The basic head design is illustrated in Fig. 3.7: The skiving edge appears only near the write/read elements, whereas elsewhere, the contour is approximately that of a circle with a radius of 6 mm into which a straight section has been inserted. The edge slope is 2.2 . For a cylindrical bearing of radius R, the flying height h at velocity v is given by h ¼ 0.643 (6 mv/ (T/w))2/3R (Eshel and Elrod, 1965), where m is the air viscosity and T and w are the tension and width of the tape, respectively. Thus, for velocities between 1.4 and 7 m/s, the fly height varies between 0.75 and 2.2 mm. The portions of the head that are rounded during the beveling process are intentionally roughened to exhibit a roughness with an rms value of about 12 nm. This roughness is well below the air-bearing thickness such that there is no contact with the tape flying over the air bearing during tape motion. However, the roughness is much larger than the tape roughness and therefore reduces the contact area between the tape and head at zero velocity and
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Figure 3.7 Schematics (not to scale) and photograph of the beveled head. (a) Illustration of a head attached to mounting beam (brown). (b) Photograph of the central section of a beveled head, showing the location of the cross sections in (c) and (d). (c) Illustration of a cross section of the flat area of the head, with the tape in contact. (d) Illustration of a cross section of the beveled region of the head, illustrating the air bearing between the tape and the head. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
hence the static friction at start-up and when the tape velocity goes through zero when the tape direction is reversed. Because the tape contacts the rough area only while starting and stopping, the tape is not damaged by the rough surface, and the rough areas of the head will not be polished smooth. Compared with a conventional flat tape head, such a beveled head reduces running friction on smooth prototype BaFe media from 0.3 to 0.1 N and start-up stiction from >2 to <0.8 N. The reduction in running friction results in a reduction in high-frequency variations in tape velocity, which in turn leads to an improvement in error-rate performance, particularly at low tape speed. Figure 3.8 compares the recording quality and readback performance of a commercial tape drive fitted with a beveled and a nonbeveled head on a smooth formulation of BaFe tape (Berman et al., 2007) written at 2 m/s at a linear density of 480 kbpi and read with a 3.8-mm wide giant magnetoresistance (GMR) reader. At high velocities, the error rate with the beveled head is reduced by a factor of 2–4, whereas at very low tape velocities, the error rate is reduced by up to a factor of 20 relative to the conventional head (McClelland et al., 2009).
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Figure 3.8 Recording quality versus velocity for a conventional flat and a beveled head. The recording quality (MbTF) is the mean number of bits between error events, that is, the inverse of the error rate. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
3.4. Tape path and hardware platform As discussed, aggressive track-density scaling is the key to the continued scaling of tape areal densities. An important tool to investigate the potential for track-density scaling, which was used in the 29.5-Gbit/in2 demonstration, was an experimental platform developed for closed-loop track-following experiments. The setup consists of a commercially available experimental tape transport (manufactured by Mountain Engineering II Inc., Longmont, CO, United States), the main electronics card and track-following actuator from an IBM TS1130 tape drive, a prototype beveled GMR tape head equipped with 2.5-mm wide servo readers, an FPGA (field-programmable gate array) board, a FPGA/DSP (digital signal processor) board, and a host computer. A photograph of the tape path, electronics card, and head-actuator assembly and a block diagram of the experimental platform are shown in Fig. 3.9. The tape path is composed of 10 porous ceramic externally pressurized air-bearing tape guides that use hard-edge guiding and two precision aluminum reels. The low-friction air-bearing guides in combination with a long tape path result in a low magnitude of lateral tape motion (LTM) during tape transport. With this tape-guiding approach, the typically observed LTM is strongly influenced by the edge roughness of the tape. For the prototype BaFe samples used in the demonstration, an LTM with a standard deviation of 1 mm was typically observed for tape transport velocities of 2 m/s. The tape transport is also equipped with two tension sensors and performs velocity and
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Figure 3.9 (a) Photograph of the tape path, TS1130 electronics card, and the headactuator assembly. (b) Block diagram of the prototype experimental setup for trackfollowing experiments. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
tension control without the need to read and decode servo information written on the tape. The TS1130 electronics card functions as the analog front-end for the read/write head and provides bias currents to the head and amplification and analog-to-digital conversion of the signals from two servo and 16 data readers in each of the two head modules. Digital samples of the readback signal from the two servo readers are fed from the TS1130 card to the FPGA board in which the servo channel, described in Section 3.6, has been implemented. The servo channel decodes the servo readback waveform and provides estimates of the lateral position of the head to the DSP/FPGA board, which in turn runs the track-following controller described in Section 3.7. The track-following controller operates in a synchronous mode in which the FPGA board provides an interrupt to the DSP board whenever a new estimate of the head lateral position is available. The DSP/FPGA board also provides a digital-to-analog converter and a voltage-to-current amplifier that are used to produce a control signal that is fed back to the head actuator. Data and control parameters are transferred between a host computer and the DSP/FPGA board using a FireWire interface, and user parameters are entered using a graphical user interface implemented in MatLabTM.
3.5. Servo pattern design In modern tape drives, encoded position information is prewritten onto the tape during manufacturing in dedicated areas, referred to as servo bands. During tape-drive operation, the encoded position information is read back continuously by dedicated servo read elements. The servo readback signal is decoded by a servo channel that generates estimates of the tape velocity and
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Figure 3.10 Illustration of the servo pattern used in the 29.5-Gbit/in2 demonstration. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
the lateral position of the head relative to the tape. These estimates are in turn used for closed-loop feedback control of the position of the head to compensate for and track lateral tape motion. The geometry of the servo pattern has a major influence on the overall servo system performance and hence on the achievable track density. In the 29.5-Gbit/in2 demo, the basic servo principle described in the LTO standard (ECMA, 2001) was adopted; however, the pattern geometry was modified to enhance the performance of the servo system as illustrated in Fig. 3.10. The influence of the individual servo-pattern parameters on the overall servo performance was analyzed using a model of the servo readback signal that includes the servo reader width W, the azimuth angle a, and the pulse width at 50% amplitude P as parameters (Cherubini et al., 2008, 2009). The primary design goal was to enhance the track-following performance by improving the quality of the position and velocity estimates and by increasing the update rate of the estimates, which enables higher bandwidth control and/or averaging of the estimates. As illustrated in Fig. 3.10, the geometry of the servo pattern can be described by six parameters: the azimuth angle a, the servo stripe distance s, the servo stripe width t, the servo subframe distance d, the number of servo bursts (four in the LTO standard, designated A, B, C, and D), and the number of stripes per servo burst. The geometry of the pattern influences the shape of the readback waveform and the amount of energy and the frequency content contained in the signal. In addition, the shape of the servo readback waveform is influenced by the tape medium properties such as orientation and by the servo reader width W. The lateral position of the head is determined by measuring the distance x between adjacent servo bursts along the longitudinal tape direction and then estimating the lateral position y using the relationship (Barrett et al., 1998): yest ¼ cxest
1 ; 2 tan ðaÞ
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where xest is the estimated distance between two adjacent servo bursts at a lateral position y and c is a pattern-dependent constant. The term 1/tan(a) describes the amplification of the measurement error associated with the measurement of x that results from the relatively shallow azimuth angle a. The quality of the parameter estimates can therefore be improved by increasing a. For example, by increasing a from 6 to 12 , the error amplification is reduced by a factor of about 2. Increasing a, however, has additional implications on the system performance. First, if the other parameters are kept fixed, a larger azimuth angle results in a longer servo frame length and hence a lower update rate and a longer position estimation delay. To partially compensate this pattern extension, the pattern can be compressed by reducing the servo stripe width t and spacing s. Second, increasing the azimuth angle also results in an increase in the dispersion of the readback waveform. This can be compensated for by reducing the servo reader width, as depicted in Fig. 3.11. A narrow servo reader width results in a smaller servo signal amplitude and hence in a lower SNR. Increasing the azimuth angle, reducing the servo stripe width, and reducing the servo reader width result in an energy loss in the servo readback waveform. This energy loss limits the performance gains that can be achieved from the reduced error amplification associated with an increased azimuth angle. To find a good performance trade-off between azimuth angle, servo reader width, and servo stripe width, extensive VHDL level simulations
Figure 3.11 Simulated dibit readback waveform shape plotted as a function of the azimuth angle, reader width, and servo stripe width. The dibit shapes have been calculated using the dibit response model described in Cherubini et al. (2008) and Cherubini et al. (2009). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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of the servo-channel and track-following controller were performed in combination with extensive experiments. This work indicated that despite the energy loss, a significant improvement in track-following performance can be obtained by using an azimuth angle of 18 and a servo pattern that was compressed by a factor of 5/3 relative to the standard LTO pattern. Compression of the servo pattern by a factor of 5/3 does not suffice to compensate for the pattern-length increase due to increasing the azimuth angle to 18 . To compensate for the larger angle and to further increase the servo update rate relative to the standard LTO pattern, the pattern height was reduced to 23.35 mm, a reduction by a factor of 8 relative to the LTO specification. This servo-pattern height enables a reduction of the servo frame length from 200 mm in LTO to 100 mm for the demo pattern, hence doubling the update rate of the estimates. The final step in the servo-pattern optimization process was to adapt the servo-formatting process for the new servo-pattern geometry and the new tape medium. One aspect of this was selecting a suitable pre-erase method to control the magnetization of the media before the servo pattern is written to the perpendicularly oriented BaFe medium, which also determines the orientation of the medium in the regions of the pattern between the stripes. Additional aspects included optimizing the write gap width of the servo format head to achieve the desired stripe width and optimizing the applied servo write current in combination with the pre-erase optimization to obtain a symmetric readback waveform with high SNR. An optical micrograph of the servo pattern written on perpendicular BaFe tape and decorated with ferrofluid is presented in Fig. 3.12. The details of the overall geometry were summarized in Fig. 3.10.
3.6. Synchronous servo channel A typical servo waveform resulting from the readback of the demo servo pattern written on perpendicular BaFe tape and captured with 2.5-mm wide servo reader is shown in Fig. 3.12b. The current tape velocity and lateral position of the head reading the servo patterns are calculated from the relative timing of the servo bursts in the readback waveform. The position and velocity are computed by a digital servo channel that processes samples of the servo signal obtained by an analog-to-digital converter (ADC) operating at a fixed clock frequency. The position measurements are subsequently fed into the track-following servo controller. In this work, a synchronous architecture was adopted for the servochannel implementation (Cherubini et al., 2007). Optimum filtering of the servo signal is achieved using a digital matched-filter interpolator/correlator to minimize measurement noise in the readback signal. As illustrated in the block diagram of Fig. 3.13, the interpolator/correlator is implemented in the synchronous servo channel prior to calculation of the servo parameters,
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Figure 3.12 (a) Optical micrograph of demo servo pattern written on perpendicular BaFe tape and decorated with ferrofluid. (b) Servo readback waveform of demo servo pattern written on perpendicular BaFe medium and readback with a 2.5-mm wide servo reader. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
that is, tape velocity, head lateral position, and detection of the longitudinal position (LPOS) information that is encoded in the servo pattern. Interpolated servo samples are generated at a predetermined rate that is independent of the tape velocity and defined in terms of samples per unit of tape length. This rate is different from the ADC sampling rate, which is specified in terms of samples per unit of time. A timing-base reference unit provides the interpolation instants for a desired interpolation distance. Hence, the matched-filter interpolator/correlator achieves a close approximation of the optimum filter for the detection of the servo signal. The time instants at which the output signal of the correlator exhibits maximum values are used to compute the tape velocity and head lateral position. This correlation method results in a significant improvement in the measurement quality compared with measurements based on the peak arrival times of the servo bursts (aka peak detection). The improved performance can be intuitively understood by noting that the matchedfiltering technique exploits all of the available signal energy or information encoded in the servo waveform rather than only the positive or negative peaks of the signal.
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Figure 3.13 Block diagram of the digital synchronous servo channel. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
The resolution of the synchronous servo channel is on the order of 1 nm for an ideal (noise-free) servo signal and for the finite arithmetic precision of the detection algorithms implemented. By further considering the effects of the finite SNR of the servo readback signal, the noise floor of the position measurements can be estimated to be less 10 nm by simulating closed-loop track-following control, assuming negligible LTM, a proportional-integralderivative (PID) controller, a tape formatted with the 18 demo pattern described in Section 3.5, and an SNR of 28 dB measured at the servochannel input (as measured on the perpendicular BaFe used for the demo).
3.7. Design of the track-following controller The primary function of the tape drive’s track-following controller is to ensure that the write/read elements in the head are positioned on or near the centerline of the data tracks during write and read operations. The performance of the system is typically measured in terms of the position error signal (PES) or the standard deviation of the PES, where the PES is specified as the difference between the desired and the measured head position. The primary disturbance that the control system has to compensate for originates from LTM during tape transport, which causes a misalignment of the read/write transducers relative to the track locations (Cherubini et al., 2007; Pantazi et al., 2010a). The track-following feedback
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controller utilizes the position measurements generated by the servo channel to control the lateral position of the head via a voice coil actuator to follow the lateral tape motion and to keep the read/write transducers centered on a specified track location. A one-degree-of-freedom track-following controller, described by the block diagram shown in Fig. 3.14, was implemented for the demo. In the block diagram, G is a transfer function that describes the dynamics of the head actuator in the lateral direction and is determined by measuring the frequency response of the actuator, which is obtained by applying a chirp excitation to the actuator coil and then determining the position of the head using a laser Doppler vibrometer (Polytec GmbH, Waldbronn, Germany). The controller K was designed using numerical optimization and weighting functions that specify the desired requirements or attributes of the closedloop transfer functions. These results were confirmed by simultaneously measuring the longitudinal head position via the decoded servo pattern at a tape velocity of 2 m/s. Both experimentally obtained frequency responses are presented in Fig. 3.15. The dynamics of the actuator are dominated by the fundamental resonance mode and can be accurately described by a second-order model. The additional delay in the phase response data measured using the servo pattern, relative to the vibrometer data, primarily arises from the position measurement delay, which depends on the servopattern format and the tape speed. The bandwidth requirements for the track-following control system depend on the frequency and amplitude characteristics of the LTM. An approximate measurement of the LTM can be obtained by capturing the decoded position information while maintaining a fixed lateral position of the actuator. Such a measurement also contains effects arising from measurement noise, which is denoted as n in Fig. 3.14. Several factors contribute to the measurement noise in the decoded position signal, including the format of the servo pattern, the method used for detecting and decoding the position information, the characteristics of the servo reader, the analog
Figure 3.14 Schematic diagram of the track-following control system. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Figure 3.15 Head-actuator frequency response measured using a laser Doppler vibrometer (black) and using the decoded servo pattern (blue). A second-order model fit to the data is shown in red. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
front-end of the servo reader, the magnetic properties of the medium including medium noise, defects in the written servo pattern, and imperfections in the servo-formatting process such as written-in velocity noise. Figure 3.16 presents a plot of the power spectral density of open-loop head-position data captured using an LTO-4 tape formatted with the standard 6 azimuth angle LTO servo pattern and using the new perpendicular BaFe medium formatted with the 18 demo pattern described in Section 3.5. Both data sets were captured using a tape speed of 2 m/s. The low-frequency regions of both spectra are dominated by LTM, which is primarily determined by the mechanical component of the tape transport system, the roughness of the tape edge, and the tape speed. Comparing the two spectra in the high-frequency regime, it can be seen that the data captured from the demo pattern exhibit a higher resolution or a lower noise floor because of the larger azimuth angle of the pattern combined with the higher SNR of the new BaFe medium. The H1 control framework (Skogestad and Postletwaithe, 1996) was used to design the track-following control used for the demo. The weighting functions used in the design of controller K were specified to balance the system requirements in terms of disturbance-rejection capabilities and sensitivity to measurement noise and to minimize the track-following error
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Figure 3.16 Power spectral density of open-loop head-position data captured using an LTO-4 tape formatted with the standard 6 azimuth angle LTO servo pattern (blue) and using the new perpendicular BaFe medium formatted with the 18 demo pattern (red). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
(PES) under the experimental conditions of the demo. For the one-degreeof-freedom feedback loop of Fig. 3.14, the transfer function that serves as a measure of the system’s capability to follow the LTM disturbance, that is, that relates the disturbance d to the error e, is given by S¼
1 : ð1 þ GK Þ
The performance weight wp was specified such that the magnitude of S is small in the low-frequency range, where the LTM is largest. The transfer function T¼
GK ð1 þ GK Þ
relates the noise signal n to the output y and hence is a measure of the effect of measurement noise on the output of the controller. The weight wT was specified to shape the transfer function T such that the controller rolls off sufficiently fast at high frequencies and hence to reduce performance degradation arising from measurement noise. An additional weight wu was specified to impose a constraint on the maximum magnitude of the control
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signal u. The objective of the H1 control design methodology is to find a stabilizing controller K by solving the problem: min K kN ðK Þk1
in which N(K) is defined as 2
3 wp S N ðK Þ ¼ 4 w T T 5 : wu KS
In practice, the specified requirements will be met if a controller is found such that kN ðK Þk1 < 1:
For the areal density demonstration, a 7th-order controller was designed using H1 synthesis. The magnitude responses of the controller’s closedloop transfer functions S and T are plotted in Fig. 3.17. The closed-loop transfer function that relates the LTM to the PES exhibits a bandwidth measured at the 3 dB point of approx. 650 Hz.
Figure 3.17 Magnitude responses of the closed-loop transfer functions S and T. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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3.8. Track-following performance Track-following experiments were performed at a tape transport velocity of 2 m/s using the same perpendicular BaFe tape medium as used in the recording experiments described below. The track-following controller described in Section 3.7 was implemented for synchronous operation using the DSP of the experimental platform. The 40-kHz sampling frequency of the controller was determined by the 2-m/s tape speed and the 50-mm distance between position estimates derived from the demo servo pattern. A 7.3-s long capture of the closed-loop PES that is representative of the performance achieved is plotted in Fig. 3.18a. Two metrics were used to assess the fidelity of the track-following performance: (1) the standard deviation of the PES (sPES) and (2) the 99.9% cumulative distribution function (CDF99.9) of the PES. The CDF99.9 is a measure of the distance
Figure 3.18 (a) Plot of a 7.3-s capture of the closed-loop PES demonstrating a standard deviation of 23.4 nm. (b) CDF99.9 of the PES data plotted in (a). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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within which 99.9% of the magnitude of the position error estimates fall. For a Gaussian distribution, CDF99.9 ¼ 3.3s. Figure 3.18b presents a plot of the CDF99.9 of the data plotted in (a). The data exhibit a CDF99.9 of 87 nm and a standard deviation of 23.4 nm. For single-channel recording, in which head tolerance and tape dimensional stability effects can be neglected, the minimum track width (TW) that can be supported with a reader width RW and the achieved track-following fidelity can be estimated using the model described by the Information Storage Industry Consortium (INSIC, 2008): pffiffiffi TW ¼ 2 2 CDF99:9 þ RW
or pffiffiffi TW ¼ 2 2 3:3sPES þ RW
if the PES data exhibit a Gaussian distribution. If we now assume a reader width of 0.2 mm, as used in the recording experiments described in Section 3.9, and track-following performance exhibiting a CDF99.9 of 87 nm, as measured from the data of Fig. 3.18, we can estimate the minimum reliable track width to be 0.446 mm and an achievable track density of 57 ktpi.
3.9. Read channel and recording performance Magnetic-recording channels that use BaFe particulate tape exhibit distinct characteristics in their system responses and noise properties relative to those based on MP tape. For example, the dibit response of a tape channel that uses longitudinally oriented MP tape is plotted in Fig. 3.19a and can be compared with the dibit responses of two recording channels using a non-oriented BaFe tape (tape B) and a perpendicularly oriented BaFe tape (tape A) that are plotted in Fig. 3.19b. Compared with the MP dibit response, the non-oriented BaFe medium exhibits a relatively asymmetric dibit shape and the perpendicular BaFe medium exhibits a strong initial undershoot, both of which are features that result from the specific orientation of the BaFe particles. It can also be observed in Fig. 3.19 that the dibit responses from the BaFe tape samples decay more rapidly than that from the MP media and thus have more high-frequency content. The pulse can be further characterized by the PW50, defined as the pulse width at 50% peak amplitude. Fitting a Lorentzian approximation to the BaFe dibit response results in a best match of PW50/T 2.1 for both BaFe particle orientations at a linear density of 343 kbpi, where T is the bit duration. Note that only uncoded data were used in the experiments presented in this section and that the linear densities have therefore been normalized using the LTO-4 channel linear density
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Figure 3.19 Dibit responses of (a) an LTO-4 recording channel with MP tape captured using a GMR read head with 3.8 mm reader width and 0.18 mm shield-to-shield gap length and (b) the dibit responses of recording channels using non-oriented and perpendicularly oriented BaFe tape, both captured using a GMR read head with 0.2 mm reader width and 0.08 mm shield-to-shield gap length. For all three dibit responses, the linear density was 343 kbpi and no write equalization was employed. On the scale of the y-axis, T denotes the bit duration. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
of 343 kfci (after modulation coding) rather than the user linear density of 323 kbpi. Increasing the linear density to 600 kbpi results in a best fit of PW50/T 3.6 for both BaFe particle orientations. Also, the write equalization scheme typically used with MP tape (a spectral shaping technique that employs preemphasis to boost the high-frequency content in the MP recording channel) is not well matched to the channel response of BaFe tape and hence is not appropriate for this medium. In fact, the use of write equalization with BaFe tape leads to a loss of signal energy through the recording channel and hence to a performance degradation. As mentioned, MP and BaFe particulate media also exhibit differences in the noise components in their readback signals. Specifically, the ratios of stationary electronics and medium noise to nonstationary data-dependent noise vary significantly between the MP tape and the two tapes based on BaFe particles. The differences can be quantified using a noise decomposition technique described in Pozidis (2004). The results of such a study are presented in Fig. 3.20, in which the linear density has been normalized relative to the LTO-4 linear density of 343 kbpi. At the relative linear density of 1, nonstationary noise for the LTO-4 MP medium contributes about 70% of the total noise power, whereas for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe), nonstationary noise contributes only about 25% and 30%, respectively, to the total noise power.
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Figure 3.20 Contribution of data-dependent noise to the total noise power for the MP (LTO-4) and the non-oriented and perpendicular BaFe tapes. The recording conditions were the same as for Fig. 3.19. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Although the fraction of nonstationary noise exhibited by the two BaFe tapes is relatively smaller than that for the MP tape, the nonnegligible contribution to the total noise power still warrants the use of datadependent noise-predictive detection techniques, as discussed later. Note that the increase in data-dependent noise power at higher frequencies observed for the BaFe may result partly from increased jitter in the timing phase of the signals used for noise decomposition. The power spectral densities of the total noise for tape B and tape A at two linear recording densities are plotted in Fig. 3.21. Compared with tape B, tape A exhibits a significantly lower total noise power owing to the combination of the perpendicular orientation of the particles, the reduced particle volume, the improved particle dispersion, and the reduced surface roughness of the tape. To determine the maximum useful linear recording density that can be achieved with the most advanced formulation of the BaFe medium (tape A), a single track of data was recorded in a loop tester using a write transducer having a saturation magnetic flux density of 18 kG and write gap length of 0.17 mm. Readback of the data was performed using an ultranarrow 0.2-mm GMR reader having a shield-to-shield gap length of 0.08 mm. No tracking control of the read head was applied during the readback process; however, the written track was wide enough relative to the reader to ensure that there
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Figure 3.21 Power spectra of the total noise power of the readback signals for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe) at (a) 343 kbpi and (b) 600 kbpi. T denotes the bit duration and f is the frequency. The dip observed at 0.8 and the peak between 0.8 and 1 are artifacts due to aliasing that results from the 5/4 sampling rate used to capture the readback signals, which were then resampled at a fivefold higher rate. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
was no signal degradation due to track misregistration during readback. The written data consisted of a repeating pseudorandom binary sequence of length 255, which was recorded at linear densities ranging from 343 to 600 kbpi. A software read channel that implements all functions of an actual indrive read channel was used to process the captured readback waveforms. Note, however, that unlike an in-drive hardware read channel, the software channel uses full precision for the signal and coefficient representations. The data-detection schemes were optimized to minimize the BER at the output of the detector, despite the relatively low channel SNR values that resulted from the ultranarrow 0.2-mm reader and the high linear recording densities. To maximize the linear recording density and still achieve our target BER of 104 at the output of the detector, it was necessary to move away from the traditional approach of extended partial-response class 4 (EPR4) signal shaping and detection and implement a read-channel and detector design based on “generalized” partial-response signaling. In this approach, the target response of the overall channel can be chosen such that a good match to the actual response of the recording channel is ensured while effectively whitening the noise process and reducing its power at the input of the detector. This objective can be achieved using NPML detection
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(Chevillat, Eleftheriou and Maiwald, 1992; Coker et al., 1998; Eleftheriou, 2003; Eleftheriou and Hirt, 1996). The detection performance can be further improved by extending the noise-prediction concept implemented in NPML sequence detection to take into account the data-dependent nature of some of the distortion components. The notion of handling time-varying nonlinear and linear distortion, which encompasses data-dependent distortion, was introduced by Eleftheriou and Hirt (1996), Coker et al. (1998), and Eleftheriou (2003). In this approach, a whitening filter is implemented in the forward path and as input to the sequence detector, whereas the data-dependent distortion is handled in a RAM-based NPML detector by updating the contents of the RAM lookup table with known adaptation techniques. Past decisions taken from the RAM-based NMPL detector form a binary address for the RAM, with the addressed value being used to compute the branch metric. In this way, together with predictor coefficients, the contents of the RAM can be updated to provide near-optimal performance in the presence of linear, nonlinear, and/or data-dependent distortion. Caroselli et al. (1997) proposed an extension of NMPL that uses a plurality of noise-prediction filters that are obtained conditioned on data patterns taken from the path memory of the NMPL detector. In this approach, also referred to as data-dependent NPML (DD-NPML), the branch-metric computation of the NPML sequence detector involves data-dependent noise prediction such that the predictor coefficients and prediction error both depend on the local data pattern. This approach was developed further and analyzed by Kavcic and Moura (2000) and Moon and Park (2001). An overview of the generic NPML read-channel architecture and its various instantiations can be found in Eleftheriou (2003). For the NPML and DD-NPML detectors used in this work, a target polynomial of the form F ð D Þ ¼ 1 D 2 1 p1 D p 2 D 2
was used. For the DD-NPML case, the coefficients p1 and p2 of the noiseprediction filter are data-dependent. Equalization of the readback signal towards the (1 D2) class-4 PR target has the advantage of placing a spectral null and hence mitigating noise components at both dc and the Nyquist frequency. The 4th-order target polynomial F(D) leads to an NPML detector with 16 states. For the DD-NPML detector, the length of the data pattern on which noise prediction is conditioned determines the number of prediction filters needed. Here, it was assumed that data patterns consist of 5 consecutive channel bits, and therefore, a separate noise-prediction filter was implemented for each of the 32 branches in the detector trellis. Although it is possible to further improve the performance of DD-NPML detection by
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using even higher-order target polynomials, this approach was not pursued in order to limit the evaluation to detectors that are realistic in their complexity of hardware implementation for multichannel parallel recording. In Fig. 3.22, the SNR at the detector input for an EPR4 read-channel is plotted as a function of the relative linear density for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe). Comparing the two data sets indicates that tape A provides a substantial improvement of up to 2.9 dB in available SNR relative to tape B. In Fig. 3.23, the BER performance data versus the normalized linear density for the two BaFe tapes are plotted for 8-state EPR4, 16-state NPML, and 16-state DD-NPML detection. Using EPR4 detection and a relative linear density of 1.4, the BER of tape A (perpendicularly oriented BaFe) is more than 40 lower than that of tape B (non-oriented medium). The detection performance is significantly improved by using 16-state NPML and even more so when using 16-state DD-NPML. For the postdetection BER target of 10–4 used in this demonstration, 16-state DD-NPML allows operation at a relative linear density of up to 1.51, corresponding to a linear density of 518 kbpi. Combining this result with the potential for operation at a track density of 57 ktpi demonstrated in Section 3.8 leads to the conclusion at an areal recording density of up to 29.5 Gbit/in2 can be supported by the perpendicular BaFe media.
Figure 3.22 SNR versus relative linear density of tapes A and B at the output of an EPR4 read channel. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Figure 3.23 BER performance versus relative linear density for tapes B and A obtained using 8-state EPR4 (circles), 16-state NPML (diamonds), and 16-state DD-NPML detection (squares). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
3.10. Summary of demo results Using flexible tape media based on ultrafine, perpendicularly oriented BaFe particles, an areal recording density of 29.5 Gb/in2 was demonstrated on a single recorded channel. Specifically, using a 0.2-mm wide GMR reader, a postdetection BER target of 104 was achieved at a linear density of 518 kbpi using a 16-state DD-NPML read channel. In addition, a state space-based track-following controller, a novel synchronous servo channel, and a new servo pattern were described, which in combination with the improved SNR of the BaFe media allowed a track-following performance with a 23.4-nm standard deviation of the track-following error to be achieved. The minimum track width supported by this degree of trackfollowing fidelity and a 0.2-mm wide reader is 0.446 mm, corresponding to a track density of 57 ktpi. The combination of these two results leads to an areal density of up to 29.5 Gb/in2. Note that this was a single-channel recording demonstration that focused on the performance of the head/ medium/read-channel combination and on the performance of the trackfollowing servo system. Effects, such as erase bands between tracks and the side-reading behavior of the read sensor that will likely become important at submicron track widths, were not considered. Moreover, head tolerances and tape dimensional stability, which are important in parallel-channel tape recording, were not considered.
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4. Continued Future Scaling Potential of Magnetic Tape Technology 4.1. Introduction In this section, we examine the feasibility and technologies required to continue scaling parallel-channel linear magnetic tape-recording systems towards a target operating point of 100 Gbit/in2. Extrapolating from the latest 2012 INSIC Magnetic Tape Storage Roadmap (INSIC, 2012), such an operating point would enable the continued scaling of tape cartridge capacities at historical rates of doubling the capacity every 2 years, up to the 2024–2025 time frame. The current state of HDD technology and demonstrations of tape areal densities including both helical scan and linear tape drives (Berman et al., 2007; Childers et al., 2003; Dee, 2006; Matsumoto et al., 2006; Matsunuma et al., 2011; Motohashi et al., 2007; Nagata et al., 2006; Ozue et al., 2002; Xiao et al., 2006) provide important insight into critical technology choices and highlight key parameters that must be considered for further advances in areal density. In contrast to many linear tape areal density demonstrations reported in the literature that used only a single channel, here, we will also consider the effects on the achievable areal density that arise from the parallel recording nature of linear tape drives, such as from tape dimensional stability. To establish potential operating points to achieve an areal density of 100 Gbit/in2, we take as a starting point the operating point of the 29.5-Gbit/in2 demo, which used a linear density of 518 kbpi and track density of 57 ktpi corresponding to a track width of 446 nm. To achieve 100 Gbit/in2, we need to scale the linear and the track density by a combined factor of 3.4. The simplest approach would be to divide the required scaling equally between the track and the linear density. However, in this areal density range, such an approach is not optimal because the penalty in terms of SNR loss of linear density scaling is much larger than that of track-density scaling. For example, doubling the track density can be achieved by reducing the reader width by a factor of two and also reducing the tracking margin by a factor of two. For a given medium, reducing the reader width by a factor of two results in a loss in SNR of 3 dB, whereas doubling the linear density results in a much larger loss in SNR (see Fig. 3.23). This loss in SNR must be compensated by improvements in the read head and the tape medium and by improvements in the data channel that facilitate reliable operation at lower levels of SNR. It is therefore more favorable to scale the track density more aggressively, despite the additional need for scaling the tracking margin this entails. Here, we consider two linear density options: (1) a modest linear density increase to 650 kbpi and (2) a more aggressive increase to 800 kbpi. If we
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assume a reader width of 100 nm for both cases, we can use the data of Fig. 3.23 to estimate the required uplift in SNR over the head/medium combination used for the 29.5-Gbit/in2 demo to be 6 dB and 11 dB for option 1 and 2, respectively. At these linear densities, an areal density of 100 Gbit/in2 then requires a track pitch of 165 nm for option 1 and 200 nm for option 2. The first option requires more aggressive scaling of the tracking margin; however, the required uplift in SNR is more likely to be achievable using low-cost BaFe-based media than with the second option. The second option, which somewhat relaxes the requirements on tracking margin, requires a much larger performance improvement in the SNR of the medium and hence would likely require a transition to another media type such as sputtered media.
4.2. Write and read-head technology Unlike HDDs, which operate a single write/read head at a time, the recording heads in modern linear tape drives use arrays of write and read transducers operating simultaneously to enable not only high-speed writing and reading but also read-while-write verification of the written data. During write operations, the data are immediately read and verified by a set of read transducers placed downstream of and aligned with the write elements. Operating multiple parallel write/read channels in tape systems is necessary to achieve data rates that are competitive with HDDs because, compared with disk drives, tape moves past the head at a lower velocity (4–6 m/s for tape vs. more than 14 m/s for disk) and data are recorded to tape at 3–4 lower linear density. The use of multiple parallel transducers also limits the time required to write or read an entire half-inch tape cartridge to a few hours. In addition, it also reduces the number of passes required to write or read back a full cartridge, which in turn reduces media wear. The time and number of passes required to fill a cartridge has not changed much since tape cartridges were introduced more than 25 years ago, because of the gradual increase in the number of parallel channels used. Hence, a future paradigm in which the data rate does not scale with capacity will result in longer fill times. In addition, keeping the number of transducers fixed combined with aggressive track-density scaling, which will likely be needed to achieve future capacity increases, will result in a large increase in the number of passes to fill a tape and hence an increase in tape durability requirements and wear issues. The large investment in cartridges and tape library automations and the need for maintaining a cost advantage over disk drive-based libraries provide strong incentives to preserve the existing halfinch tape and cartridge formats. Thus, a challenge in the design of future tape systems and operating points is to increase the areal density and tape cartridge capacity while maintaining the current cartridge form factors and
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preserving or adapting the expectations of tape user in terms of backup and restore times. A cartridge containing a 1200-m long tape recorded at 100 Gbit/in2 will hold approx. 180 TB of data, that is, more than 80 the capacity of an LTO-5 cartridge, assuming the same system overheads and coding efficiencies as in LTO-5. Having 32 active write heads—the maximum today in a commercially available drive—and 60,000 data tracks, a simple calculation shows that 1875 one-way passes would be required to fill this cartridge. At 6 m/s tape speed, this translates into more than 100 h! This could force a change in backup management strategy. Beyond data-rate considerations, there may also be challenges associated with managing a library composed of such large individual objects. Doubling both tape speed and the number of channels reduces the fill time to 25 h, a value still much in excess of what is currently likely to be acceptable. Increasing the tape speed is challenging because of media-handling issues, such as the need to improve tension control as tapes become thinner, and air entrainment effects, which render reel-to-reel control more challenging at high speeds. In addition, LTM, which limits track-following fidelity and hence the achievable track density, also tends to increase with tape speed. Moreover, there remains an ongoing need to increase the number of active channels regardless of the tape speed to limit increases in the number of passes required to fill a cartridge. However, simply increasing the number of active transducers will be challenging and is not sufficient for scaling to 100 Gbit/in2. As will be discussed below, transducer dimensions will have to be shrunk to enable writing smaller tracks at higher linear density. In addition, the span occupied by the transducers must be reduced to reduce the effects of tape dimensional stability (TDS), which may improve only to about 100–200 ppm from about 800 ppm in current LTO tape cartridges. This final requirement for reducing the head span may potentially be relaxed if a scheme to measure and actively compensate TDS effects can be implemented. In any case, in the future, more transducers will have to fit into the current or a smaller span, leading to the need for smaller transducer dimensions. To illustrate the future scaling requirements of write and read transducers, we first consider the span of transducers in an LTO-5 head, which consists of two symmetrical halves, called “modules.” As mentioned, two such modules are required to read-verify data in real time as they are written. Each module consists of 16 write, 16 data read, and 2 or more dedicated servo read transducers. The write and data read transducers are piggybacked, as in disk heads, and the pitch (distance between the centerlines of adjacent write/read pairs) is 166.5 mm. The distance or span between the outermost pairs is (16 1) 166.5 mm or about 2.5 mm. A schematic cross section of a pair of write transducer is shown in Fig. 3.24. The spacing between the elements in Fig. 3.24 has been reduced
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Figure 3.24 Schematic of a cross-sectional (half-symmetry) view of a pair of magnetic write transducers. The yokes (green, purple, and blue) are sectioned to show front and back gaps and the yoke length. The coils (red) are shown nonsectioned for clarity. The spacing in this example schematic has been reduced relative to the LTO-5 spacing of 166.5 mm so that two transducers can be shown at a reasonable scale on the same page. Reprinted with permission from Argumedo et al. (2008).
relative to that of an LTO-5 head so that two writers can be displayed at a reasonable scale and to highlight a key difference between tape and HDD heads, which have only a single write transducer. An environmentally induced 200 ppm change in tape width between writing and subsequent readback could produce up to 0.25 mm of misregistration between an ideal head with a span of 2.5 mm and the tape, assuming tracking is done in reference to the center of the head span. This is more than the entire projected track width for the 100-Gbit/in2 operating point. To reduce this to a reasonable fraction of the tracking budget, an at least threefold reduction in the head span will be needed. Considering further that at least 64 active channels will be required to support continued data-rate scaling, the transducer centerline spacing must shrink to 14 mm, as shown schematically in Fig. 3.25b and (c). At this spacing, writers may interfere, or “cross talk,” a phenomenon in which the magnetic field produced by one write transducer alters the magnetic fields produced by its nearest neighbors (Biskeborn et al., 2008). To minimize such effects, the writer must be designed such that the yokes do not saturate, particularly in the portion wound by the coils, referred to as the “back-gap” region, as this can divert stray flux into neighboring write
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Figure 3.25 Schematic illustrations comparing the layouts and geometries of current and potential future write heads. All dimensions, including the transducer pitch, write pole width, yoke length, and back-gap width, are shown in microns. (a) Schematic of an exemplary LTO-5 write transducer consisting of two coil layers (only 1 shown), with seven turns in each layer. The pitch between write transducers is 166.5 mm; thus, only one transducer fits into this figure at the scale used. (b) Schematic of a future write transducer at the same scale having three coil layers (only 1 shown), with two turns in each layer. The pitch between transducers is 14 mm, which enables 64 writers (4 LTO-5) to fit in 1/3 of the LTO-5 span. (c) Cross section of the layout shown in (b). Reprinted with permission from Argumedo et al. (2008).
gaps. This can be achieved by using high-Bs pole materials and wide back gaps (wider than top pole). A target residual track width of 0.2 mm can be achieved by shingle writing with a front pole width in the range of 0.5–1.0 mm. Such a writer requires a back gap of at least 2 mm to avoid the saturation effects mentioned earlier. This leaves a distance of approximately 12 mm between the edges of neighboring write yokes, which is sufficient to accommodate two coil turns on a 1.75-mm pitch, assuming a conventional pancake coil design. These coil dimensions are within the capabilities of current thin-film microfabrication technology (see Fig. 3.25b and c). Crosstalk effects due to coil proximity are expected to be small compared with effects arising from magnetic stray field coupling between yokes. Therefore, traditional design methodology would suggest that the coils in tape write transducers at this pitch will be limited to having between 2 (1 layer) and 6 (3 layers) turns.
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Scaling future tape writers to dimensions that limit the coil design to a maximum of only 2–6 turns creates additional challenges. Although modern HDD write transducers are typically designed with 3–5 coil turns, current tape writers typically use 8–14 turns. This discrepancy arises from a combination of factors. In disk drives, current-mode write drivers are mounted on the actuator arm in close proximity to the head, and therefore, cable impedance is not critical. In addition, HDD write drivers, which supply only a single channel at a time, can supply more than 100 mA to the writer coil, which in turn enables a sufficient write field to be produced with fewer turns (write field is proportional to the number of turns times the current). In tape drives, the write driver chips are located on a printed circuit board (PCB) with the other drive electronics at the fixed end of the flex cables, because of heat dissipation issues, space requirements of multiple parallel channels, row-access-strobe latency, and other restrictions. The flex cables connecting the head to the PCB must be long enough to enable a sufficient range of motion such that the heads can access the entire half-inch width of the tape. Typical flex cable lengths are greater than 10 cm, and hence, cable impedance becomes critical. As a result, voltage-mode write drivers, in which the steady-state write current is limited by a series resistance, are typically used for tape. This in turn reduces the maximum available write current and increases the minimum required number of coil turns. Reducing the operating voltage range of the write process can potentially enable the use of series write resistors in combination with a lower cable impedance, which ultimately affects the minimum acceptable turn count. Impedance matching the driver, flex cable, and head can also be beneficial to minimize the 10–90 rise time. However, up to 50% overshoot has been used in both tape and disk systems, although for different reasons. In tape, the relationship between overshoot and error rates, especially for thinner magnetic coatings and non-oriented particulate tapes, is a subject of active research and debate (INSIC, 2005). Another important consideration is that lowering the write voltage results in less power dissipation in the write driver and series resistors. This is of particular importance for 32 or potentially even 64 simultaneously active writers. Even without this benefit, lower-voltage operation may become a necessity as higher-voltage mixed analog–digital CMOS technologies approach the end of life and are replaced by lower-voltage and higher-integration technologies. One potential solution to all of these design challenges would be to develop a write driver that can be mounted on the flex cable near the head, as is done in HDD. Here, the main challenge is in designing a sufficiently low-power write driver such that the waste heat from 32 or more parallel channels can be dissipated by means of innovative cooling solutions. Another factor that can potentially limit write-head coil turns is electromigration, which imposes a limit on the maximum acceptable current owing to reliability concerns arising from the small cross-sectional area of the coil, which leads
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to high current densities. Continued scaling of the areal density imposes the need for media with higher coercivity, which in turn require larger head fields, leading to the need for more amp turns for a given head design. However, as the linear density is scaled, media coatings are expected to become thinner, enabling the use of smaller write gaps. This in turn results in an improved head efficiency, which partially alleviates the increased current requirements. In summary, six coil turns appear to be an aggressive but potentially workable lower bound. Hence, a 14-mm scaled transducer pitch is potentially feasible, but clearly will be very challenging to achieve. A trend in writer design for both HDD and tape systems has been to reduce the yoke length to decrease eddy-current effects and hence to enable faster switching and higher data rates. We anticipate that it will be necessary to scale the yoke length from the about 25 mm currently used in tape writers to less than 10 mm as tape systems are scaled towards an areal density of 100 Gbit/in2. In the context of yoke length, pancake coil writer designs have the advantage of coil-turn stacking, but as described, a 14-mm pitch would accommodate at most two coil turns per layer. Thus, an 8-turn head would require at least four layers, which would be complex to fabricate and could still have a longer than desired yoke because of the height of the coils stack. In contrast, a helical coil writer design could potentially accommodate an arbitrary number of turns, at the cost of increased yoke length. An alternative to the current approach of using a linear array of writers is to build tiers of writers on two or more planes stacked on top of each other. For example, using two tiers would enable the fabrication of writers capable of writing tracks at half the pitch between writers in a given plane, that is, a 14-mm effective pitch could be achieved using two tiers of writers built on a 28-mm pitch. Unfortunately, this approach has device yield issues and tierto-tier alignment challenges, but does offer a route to continued scaling of the head span and the number of parallel channels. Another important consideration is that multiple transducer tiers result in a wider mechanical gap, that is, the gap between the hard ceramic portions of the head (Biskeborn and Eaton, 2003). Such an increased gap leads to greater susceptibility to wear-induced recession of the thin-film transducers, leading to increased magnetic spacing. The main challenge in scaling tape read transducers for operation at areal densities approaching 100 Gbit/in2 relates to maintaining sufficient output signal as the dimensions of the transducer are being shrunk. The issue of achieving tight centerline pitches, which is problematic for writers, is much less of a problem for readers because of the relatively smaller area occupied by a read sensor. In fact, readers with centerline pitches down to 9 mm (Tamakawa et al., 2006) have already been successfully demonstrated. In general, a read-head output of at least 1 mV is desirable to render electronic noise contributions to the overall SNR negligible. More
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quantitatively, a 1-mV output signal is roughly 10 larger than the nonmedium-related electronic noise, including the Johnson noise in the sensor itself, given by Vn [4kBTRDf]½. Here, kB is the Boltzmann constant, T is the temperature in Kelvin, R is the sensor resistance, and Df is the bandwidth of interest. At an areal density of 100 Gbit/in2, the width of the readers will have to be reduced to dimensions on the order of 0.08–0.1 mm. In addition, the shield-to-shield spacing will have to be optimized to match the linear density and is expected to be less than 0.08 mm. To better understand how head output scales with the sensor dimension, it is instructive to first consider an anisotropic magnetoresistive (AMR) head. In an AMR head, the sensing layer resistance change dR/R is typically on the order of 0.023. The sensor resistance, R, is proportional to WMR/tMRhMR, where WMR, tMR, and hMR denote the sensor width, thickness, and height, respectively. Hence, the reduction in head output that results from reader width scaling can be partially compensated by reducing the sensor free layer thickness tMR. However, below a thickness of about 13 nm, the output is no longer inversely proportional to the free layer thickness, but instead increases more slowly because the relative contribution of electron scattering at the free layer surfaces increases. That is, the resistance increases as the thickness is reduced; however, dR does not increase in the same proportion. The reduced head output that arises from reader width scaling can also be partially compensated by optimizing the sensor height, hMR. Continued linear density scaling will drive the need for thinner magnetic layers and reduced shield-to-shield spacing, both of which will contribute to a reduced signal amplitude. In principle, this amplitude reduction can be compensated by increasing the sensor bias current; however, this will raise the sensor temperature, which increases the Johnson noise and also leads to increased tape asperity cooling noise. Moreover, if the current is increased too much, the sensor lifetime and reliability will be degraded. GMR sensors have a sensitivity that is approximately 5 larger than that of AMR sensors, and the most recent linear tape drives use this technology. As mentioned, scaling to linear densities on the order of 100 Gbit/in2 will require reader widths on the order of 0.08–0.1 mm. Tunneling magnetoresistive (TMR) sensor technology provides an even higher sensitivity than GMR, with dR/R values of up to 80%. The HDD industry, which currently uses TMR sensors, made the transition from GMR to TMR technology around a sensor width of 0.15 mm. It is therefore likely that tape-drive manufacturers will make a similar transition. However, considering the relatively small volumes of tape heads produced each year, tape-drive manufacturers may transition to TMR technology earlier so that there is no need for head manufacturers to maintain a separate technology platform for the relatively small number of tape heads produced each year. In any case, it is likely that TMR reader technology will be required for areal densities approaching 100 Gbit/in2 and beyond.
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Another promising technology to enable the continued scaling of linear tape systems is the so-called planar head technology (Engelen et al., 2012), in which transducers are fabricated with the critical recording gaps orthogonal to the wafer surface. This is in contrast to conventional tape and disk heads, which are sliced and lapped to expose the recording gaps formed between the layers of the wafer. The main advantage of planar technology is that it enables the staggering of writer arrays to reduce the effective pitch between simultaneously written tracks and hence enables a reduced head span. In the extreme, planar heads could enable the simultaneous writing of bundles of adjacent tracks, which would minimize tape dimensional stability issues and eliminate track-squeezing effects that occur during shingle writing due to track-following errors. This means that adjacent track writing does not suffer from the wavy-track effect, that is, that residual shingled track widths vary down the length of the tape. Planar write heads have already been successfully demonstrated; however, planar read-head fabrication processes have yet to be developed. The lack of a planar reader technology gives rise to another challenge inhibiting the use of planar writers: the need for accurate track placement. Ideally, this is achieved using servo readers next to the write transducers. This could potentially be achieved using a hybrid of a planar writer head and a conventional read head complete with servo readers. However, in conventional heads, the position of the servo readers relative to the writers is very accurately controlled because of the accuracy of the lithographic processes used for fabrication. A hybrid head would therefore suffer from degraded position tolerance of the servo readers relative to the writers, leading to less accurate track placement. If the technologies to enable adjacent track recording can be developed, then there is an additional opportunity to improve the read process itself using arrays of readers to perform the so-called electronic track following. Here, the idea is to use arrays of readers that may bridge into neighboring tracks during readback. A multiplexer could then be used to select the reader that is most centered on the desired track at any given time. An alternative approach is to use the side information from readers aligned with the neighboring tracks to perform adjacent track interference cancellation. A final challenge of future tape heads arises from the read-while-write verification functionality of linear tape drives mentioned earlier. Historically, this function is enabled using an array of readers that are rigidly attached to the write head. These readers are positioned downstream and “in the shadow” of the active writers. As track widths are scaled to smaller dimensions, variations in the angle of the tape relative to the head, referred to as tape skew, make it increasingly more challenging to maintain the alignment of the read-verify head with the written tracks. The sensitivity to tape skew can be reduced by decreasing the distance between the writers and readers; however, there are limits to how much it can be reduced because of challenges in assembling the heads and the need for sufficient isolation of the reader from the active
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writers. A solution to this problem is to implement some form of dynamic skew compensation. One approach to achieve this is to implement a two degrees of freedom actuator (track following and skew following) and a second control loop used for skew cancellation. Another approach is to use two independent track-following actuators and control loops to control the position of the write and read-verify heads independently. A third approach is to build actuators into the tape guides upstream and downstream of the head and to perform active tape guiding to offset LTM and skew effects as will be discussed in Section 4.6.3.
4.3. Head–tape interaction As discussed in Section 3.3, the tape–head magnetic spacing will have to be continually reduced to continue scaling the areal density in magnetic recording systems. In addition to reducing the spacing, the head–tape interface must also be designed to minimize not only tape–head friction and wear but also chemical degradation of the magnetic elements. A critical problem in tape-recording systems is the signal degradation that occurs over the lifetime of a drive due to the increase in magnetic spacing as the magnetic elements wear down by as much as 40 nm below the plane of the tape-bearing surface. For operating points approaching 100 Gbit/in2, this head recession will have to be limited to 10 nm or less over the lifetime of the drive. In the past, diamond-like carbon (DLC) coatings have been applied to metal-evaporated tapes to improve their wear robustness. However, this is impractical and likely too expensive for use with MP tapes. In the HDD industry, ultrathin DLC coatings are used to protect the head and disk from occasional accidental contact. Currently, research efforts are ongoing to investigate the application of DLC coatings to tape heads (Rismani et al., 2011, 2012). However, the contact nature of tape recording and the associated high levels of wear combined with the need to minimize coating thickness to minimize spacing losses have limited progress in tape head coating. Moreover, because magnetic, chemical, and physical requirements limit the material choices for heads, designing for wear resistance has so far been relatively unfruitful. As mentioned, reducing the head closure gap can potentially reduce head recession, but the finite thickness of the stack of materials needed to build read and write transducers puts a limit on the minimum spacing. Finally, there is potential to reduce the abrasiveness of the media itself by making it smoother; however, this leads to increased start-up and running friction. If a head recession of 10 nm or less can be achieved, then a target total magnetic spacing of 20 nm is achievable by reducing the tape roughness to 10 nm or less. This target of 20-nm magnetic spacing is required for a target linear density of 800 kbpi. From a tape-manufacturing point of view, decreasing the tape roughness is relatively straightforward. However,
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decreasing the tape surface roughness increases the actual tape–head contact area, leading to increased adhesion and frictional forces (Bhushan, 2003). The static friction that has to be overcome to start moving tape over the head, called stiction, is generally much higher than the friction while the tape is in motion (referred to as running friction). When the tape is stopped, the tape deforms into local contact with the head and liquids condense or migrate into any narrow gaps in the tape–head contact, resulting in high stiction relative to the running friction. Clearly, friction and stiction must not be so high that they cause tape damage or stalling of the tape. However, even lower levels of friction can distort the data signal timing and degrade error rates, as discussed in Section 3.3. The challenges associated with high tape–head friction are further compounded by the trend to decrease tape thickness to improve the volumetric storage density for a given areal density. The 2012 INSIC tape roadmap (INSIC, 2012) predicts that tape thickness will be scaled from its current value of about 6 to 4 mm by 2022. This reduction will likely make tape more fragile and hence more susceptible to damage due to stiction and friction. These basic challenges have at least been partially addressed by the beveled head technology described in Section 3.3. Moreover, the effectiveness of this approach at reducing friction and stiction can be further improved by reducing the span of the head, which enables a larger fraction of the tape to be supported by an air bearing. In modern tape drives, the tape is in partial contact with the head, supported by the tips of the asperities of the tape roughness. Transport of the tape over the head and tape guides can generate small particles due to wear. The sharp leading edge of the head keeps particles from the tape–head interface, and loose particles that accumulate on the edge are brushed from the head during tape loading. Occasionally, a mildly abrasive cleaning tape is also used to remove any adherent debris from the head. Modern tapes also use lubricants to reduce tape–head friction. The tapes are fabricated with an excess of mobile lubricant molecules absorbed in the tape undercoat (between the base film and the magnetic coating), forming a reservoir for replenishing the tape surface with lubricant. Friction between the tape and head can be large even at zero applied normal load, because short-range surface forces pull the smooth flexible tape into contact with the head. Protruding asperities on the tape surface are designed to hold most of the tape surface away from the head; however, the overall attraction of the tape surface partially flattens the asperities, which in turn leads to an increase in the attractive force. Thus, adhesion and attraction increase quite rapidly as the roughness is decreased to maintain a low magnetic spacing. Mobile liquids on the tape surface or in the atmosphere can also contribute to attraction and increased friction through capillary condensation. Liquids with a large contact angle tend to wet the area around contacting asperities, bridging the narrow gaps between head and tape near the asperity.
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The tape is attracted to the head, because the total surface energy decreases as the surfaces move closer and wetting increases. This problem becomes worse in high-humidity environments. For example, for a pair of hydrophilic (high-contact angle) surfaces exposed to an 80% humidity environment, condensation can fill a 4.6-nm gap. This humidity sensitivity can be prevented by designing the tape surface, the head surface, and the lubricant systems to be hydrophobic, that is, with water contact angles >90 . In summary, friction scales poorly with the decreasing roughness and tape thickness needed for continued capacity scaling. The fundamental problem is that, as the surface roughness is reduced to decrease the magnetic spacing, the energy required (per nominal surface area) to deform the surface to flatness decreases, whereas the energy available from surface interactions is constant. One strategy to reduce the head–tape contact area and hence friction is to minimize the size of the head. In the lateral tape direction, this can be achieved using the beveling technique mentioned. In the tape-travel direction, the length of the head is already relatively small. For example, for the head described in Biskeborn and Eaton (2003), the entire tape contact length is only 1.2 mm. Unfortunately, this length cannot be reduced much further without disrupting the ability to maintain a small head–tape spacing. Thus, the microscopic design of the head–tape interface will be essential for controlling friction as the magnetic spacing is reduced in the future. The Young’s modulus of a particulate tape coating is about 1010 N/m2. This is sufficiently compliant for significant deformation to take place, leading to increased contact area and friction—even if the surfaces have low surface energies and in the absence of liquids. One promising route forward is to engineer the tape surface roughness on multiple length scales, whereby the long-wavelength roughness is minimized and a controlled level of short-wavelength roughness is generated using hard particles added to the magnetic coating (Cherubini et al., 2011). These particles protrude from an otherwise flat medium and thus can be used to create a desired magnetic spacing while minimizing the contact area and hence controlling friction. Until recently, the role of such particles was to continually clean the head and keep debris from accumulating. For this approach to be effective, the particles should be small enough so that their small radius results in a weak attraction to the head and at the same time large enough to distribute a large head force over the surrounding medium. In addition, the density of the particles needs to be sufficient to support the media, but they should not be too dense or else they may give rise to local magnetic dropouts. A second approach to controlling friction, which has yet to be investigated for tape recording, is to pattern the topography of the head in the regions remote from the transducers. Here, the idea is to provide structures or islands that support the tape and hence ensure good contact in the region
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of the read/write transducers while reducing the total contact area and hence friction. We conclude that using a combination of the above schemes, it should be feasible to design a head–tape interface that will support an areal density of 100 Gbit/in2 while providing low enough friction to ensure good runnability of the tape.
4.4. Data detection Achieving areal recording densities approaching 100 Gbit/in2 poses significant challenges for the design of the read channel. The main challenge is to ensure reliable operation of all front-end analog and digital signal-processing functions, including adaptive equalization, gain, and timing control, despite significant reductions in the available SNR that arise from the required dramatic reduction in the reader width. Under such reduced SNR conditions, the read channel will have to rely on powerful data-detection methods to guarantee sufficiently low postdetection symbol error rates. The general framework of NPML sequence detection (see Eleftheriou (2003) and the references therein) is well suited to address these performance requirements. For example, the class of NPML detectors with target polynomials (1 D2)W(D), where W(D) represents a noise whitening filter, has been implemented successfully in HDDs and recently also in tape-drive systems. At high linear densities, NPML schemes achieve a better match between the detector target and physical channel characteristics than, for example, EPR4 does. In addition, they whiten the stationary and nonstationary components of the noise process at the detector input, thereby also reducing its power. Another important aspect of tape systems is the inherent variability of the recording channel, which is due to a variety of reasons including cartridge exchange, variations in the read and write-head characteristics, recession of the head over time, nonstationary noise processes, variations in magnetic spacing, and coating thickness variations. Such aspects can be dealt with by using a detector target that automatically adapts itself to the current channel characteristics. Such a feature is enabled by the class of NPML targets described in the preceding text (Eleftheriou et al., 2010). Nonlinear distortion and/or transition jitter introduces a data-dependent component in the overall channel noise. As mentioned, NPML detection can be extended further to take into account the data-dependent distortion characteristics. For example, it is well known that surface roughness in tape media introduces a type of noise that is colored and data-dependent. Datadependent NPML detection allows one to achieve the best detection performance in the presence of such noise processes, which can be the predominant contributors to the total read-channel noise. For example,
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such a DD-NPML detection scheme was used in the 29.5-Gbit/in2 demonstration discussed in Section 3 (Cherubini et al., 2011). Finally, the noise-prediction concept in NPML and its various forms can work in tandem with advanced error-correcting codes (ECCs) that lend themselves to soft decoding, for example, low-density parity-check (LDPC) codes. For example, if noise-predictive detection is performed in conjunction with a maximum a posteriori (MAP) detection algorithm such as the BCJR algorithm (Bahl et al., 1974), then NPML and NPML-like detection allow the computation of soft reliability information on individual code symbols while retaining all the performance advantages associated with noise-prediction techniques. The soft information generated in this manner is used for soft decoding of the ECC. This, in turn, can be fed back to the soft detector to improve detection performance and ultimately to improve the error-rate performance at the decoder output in successive soft detection/decoding iterations.
4.5. Format efficiency To achieve reliable data readback while making efficient use of the magnetic recording channel, the bit stream written onto the magnetic medium includes redundancy and synchronization patterns. The specification of this overhead information is referred to as formatting or data format. The goal of an efficient format is to introduce as little overhead as possible while ensuring proper operation of the data acquisition and timing loops and achieving a user BER performance of 10–17 or better. Improvements in format efficiency directly lead to a higher cartridge capacity. The format efficiency of recent tape-recording systems such as LTO-4 is about 73% (LTO, 2006). The 27% overhead of LTO-4 can be broken down into roughly 16% for error-correcting code (ECC), 5.7% for modulation coding, and about 5% for sync patterns and data headers. There are several possible approaches to improve format efficiency. One is based on the use of longer ECCs of higher rates, resulting in a gain of about 4% without sacrificing error-correction performance. Even higher efficiency can be achieved by another approach that relies on reverse concatenation (RC), a technique that has been effectively implemented in HDD products. In a standard “forward concatenation scheme,” user data are first ECCencoded and then passed through a modulation encoder to enforce predetermined modulation constraints for timing and efficient data-detection purposes. In a RC scheme, the order of the ECC encoder and the modulation encoder are reversed (Bliss, 1981). This reversal provides three major benefits: (i) There is no error propagation through the modulation decoder; (ii) because error propagation is not an issue, the first modulation code can be very long, allowing the use of capacity-efficient and high-rate modulation codes and thereby resulting in code rate gains; and (iii) in the readback path, the ECC decoding block comes immediately after the channel detection
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block, which can readily pass soft information to the decoder on a bit-by-bit basis. This property creates the appropriate framework for using novel ECC techniques based on turbo and LDPC codes and hold the promise of large performance improvements (Dholakia et al., 2004). These benefits can also be exploited in tape-recording systems. However, the approach used in HDDs cannot be used directly because the structure of the ECC used in HDDs differs from that used in tape recording. Previously, RC schemes have been described for one-dimensional ECC architectures, where the ECC typically consists of a single code, such as a Reed–Solomon (RS) or an LDPC code (Blaum et al., 2007; van Wijngaarden and Immink, 2001). These RC architectures cannot be directly applied to the twodimensional ECC used in tape systems, which are based on RS product codes, with a C1 code along rows and a C2 code along columns. To overcome this problem, a novel RC scheme has been proposed, which is illustrated in Fig. 3.26 (Argumedo et al., 2008). The main steps in the write path are as follows: (i) The user data are reorganized into a stream of N2 rows by the serial-to-parallel block; (ii) modulation encoding is applied to each row by the first modulation encoder, ME-1; (iii) formatting is applied for partial symbol interleaving; (iv) C2-column-dependent encoding is applied; (v) C1 encoding is applied along rows; and (vi) modulation coding of the C1 parity is applied by a systematic modulation encoder ME-2. A key component in the proposed RC scheme is the first modulation code, for which one may select a very high-rate n/(n þ 1) Fibonacci code with, for example, n > 200 (Blaum et al., 2007). These codes have simple enumerative encoders and achieve very tight modulation constraints, which are comparable to those of the LTO-4 standard. Modulation constraints prevent the writing of sequences for which the overall detection process would be less reliable (they avoid unfavorable timing patterns, reduce the path-memory length in sequence detectors, and avoid quasicatastrophic error propagation). Another feature is the formatting block, which transforms the modulated user data array into an array with “empty” components in each column, which are the locations where the parity symbols of the C2 code will be introduced.
Figure 3.26
Reverse concatenation architecture.
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Compared with the rate 16/17 code of the LTO-4 standard, this new RC scheme has a modulation scheme with less than 1% redundancy while maintaining essentially unaltered modulation constraints. This is an improvement in rate of more than 5% and, together with the 4% potential gain from longer C2 codes, leads to an overall format with an overhead of about 18% rather than the 27% of LTO-4, which represents a substantial reduction in redundancy. Furthermore, by using an LDPC code or turbo code for C1, the new format supports novel ECC techniques based on iterative decoding. The C1/C2-based ECC structure is an ideal setting for LDPC or turbo codes because the typical error floor issue of these codes is resolved by the C2 RS code, which can reduce the error rates to the desired 10–17 level. In addition, the use of iterative decoding techniques can lower the limit on SNR at the input to the detector that is required for reliable operation of the front-end signal-processing functions. For example, the DD-NPML scheme implemented for the 29.5-Gbit/in2 demonstration required an SNR at the input to the detector of >13.5 dB to ensure a raw BER of 104 at the output of the detector. The application of robust signalprocessing schemes, such as decision-aided timing recovery, equalization, and gain control, should allow reliable operation of the front-end adaptive function at a BER of 103 at the detector output, corresponding to a typical detection SNR of about 11.5 dB. An LDPC-coded channel can guarantee a C2 input byte erasure rate of about 106 at an SNR of approx. 11.5 dB and can operate at a significantly lower detection SNR while achieving the required erasure rate of 103 at the C2 decoder input. Moreover, iterative timing recovery that uses feedback from the detector/decoder could allow the operation of the read channel in the 10.5–11.5 dB SNR range while still maintaining reliable operation. Such performance enhancements of the data channel can provide an additional gain of 2–3 dB and hence relax somewhat the requirements for SNR improvements in the media that will be necessary to achieve a 100-Gbit/in2 operating point.
4.6. Track density limits 4.6.1. Track misregistration Linear tape drives typically operate with a reader that has about half the width of the written track. The difference between the written track width and the reader width is referred to as the tracking margin or the track misregistration budget. This margin is provided to account for effects such as tracking fidelity during write and read, tolerances in the dimensions of the write/read head, and tape dimensional stability (TDS) effects. Ultimately, the tracking margin limits the track pitch that can be written on tape and reliably read back. Hence, as the track width is being scaled down, the
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tracking margin must also be reduced by a similar factor. As mentioned, to achieve 100 Gbit/in2 with a linear density between 650 and 800 kbpi will require a written track width in the range of 165–200 nm. To a first-order approximation, if we assume the reader width to be 100 nm, then we require a tracking margin of 65–100 nm. The 2012 INSIC roadmap further assumes that the reader can “overhang” by up to 7% of its width into the adjacent track. If we adopt the same assumption here, then we require a tracking margin in the range of approx. 70–105 nm. Tape dimensional stability is a factor in the tracking margin because multiple heads are used in parallel, and hence, if the central elements are kept centered on track, lateral expansion and contraction of the tape cause the outermost elements to move off the track centerline. Three factors contribute to TDS: humidity, temperature, and tension variations. For example, the TDS specified for LTO-4 media is 900 ppm or less over the full environmental variation. Taking the LTO-4 head span of 2.5 mm, this results in a TDS contribution of 1.125 mm to the required tracking margin. Clearly, the tracking margin of 70–100 nm required for 100 Gbit/in2 imposes severe dimensional stability constraints. A major improvement in TDS to about 200 ppm can be achieved by using an Aramid substrate. Combining this with a reduction in the head span of a factor of three, as discussed in Section 4.2, leads to a TDS contribution of 83 nm, which clearly is still too large. TDS could be further reduced through additional improvements in the substrate or by improved tension control, as will be discussed in Section 4.6.5. Another option, albeit an undesirable one, would be to restrict the range of allowable environmental operating conditions. Ultimately, it will likely be necessary to actively compensate for TDS effects in a similar manner as LTM is compensated for today. For example, the effective distance between write and read transducers in the head can be changed by tilting the head relative to the tape. In addition, TDS can be measured by monitoring changes in the difference between the position signals provided by the two servo readers. Changes in the tape dimension could then be compensated using a feedback loop to control the angle of the head to adjust the effective channel pitch and thus keep the measured TDS signal constant. In general, the time constants of TDS effects are rather slow, and hence, the tape dimension can potentially be measured very accurately by averaging the position signals. Moreover, if a low noise level in the TDS measurement can be achieved, combined with the slow nature of the variations, the TDS following error could potentially be reduced to a very small, or even negligible, value. 4.6.2. Servo channel and servo pattern Timing-based servo (TBS) is a technology developed specifically for linear tape drives in the mid-1990s (Barrett et al., 1998) and is currently used in all commercial linear tape drives. In TBS systems, recorded patterns to aid
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track-following servo consist of transitions with two different azimuthal slopes as described in Section 3.5. The lateral position is derived from the relative timing of pulses generated by a narrow head reading the pattern. TBS patterns also enable the encoding of additional LPOS information without affecting the generation of the transversal PES. This is achieved by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). In tape systems, two dedicated servo channels are normally available from which LPOS information and PES can be derived. By combining the information from these two servo channels, it is possible to generate information about the relative skew angle between the tape and head. To achieve small values of sPES, advances in the servo-channel architecture are required, which include the following three main functions: (i) optimum matched-filter detection of servo bursts, (ii) optimum demodulation of LPOS symbols, and (iii) generation of a fixed number of signal samples per unit of tape length, irrespective of tape velocity. Section 3.6 described an experimental synchronous servo channel (Cherubini et al., 2007) in which all of these functions have been implemented. The performance of the track-following and reel-to-reel servo systems is ultimately limited by the resolution and bandwidth of the velocity and position signals provided by the servo channel, which, in turn, are related to the geometry of the servo pattern. Section 3.5 described previous work to optimize the servo-pattern geometry to maximize the accuracy of the resulting position estimates. The resolution, or noise floor, of this pattern in combination with the synchronous servo channel and the particulate BaFe medium described in Sections 2 and 3 is less than 10 nm (Lantz et al., 2011, 2012). Recently, an analytic framework/model has been developed to characterize how the servo format parameters, the servo read-head geometry, and the magnetic media properties affect the servo readback signal (Furrer et al., 2012), with the aim of being able to optimize the system parameters for future operating points. By adding electronics and transition jitter noise, the readback signal expressions can be extended to a servochannel model suitable for TBS performance prediction by means of Monte Carlo simulations and/or bounds on the variance of the PES. The servopattern parameters (such as azimuth angle, transition width, and period) can then be optimized jointly with the read-head geometry (e.g., shield-toshield distance, magnetoresistive sensor width, and reader width) and the magnetic media characteristics to optimize system performance. Using such an approach, combined with further improvements in the SNR of the media, it should be possible to reduce the noise floor of the position estimates to values on the order of a few nanometers. Another approach that offers potential for further improving the servo performance lies in improving the write-head technology that is used to preformat media with the servo pattern. Two important factors determine the quality of a magnetic transition written to tape: the write gap width and
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the current switching time. A narrower write gap results in a sharper transition because the gradient in the magnetic write field produced by the write gap is larger. The accuracy of a TBS servo channel is limited in part by the accuracy with which the relative timing of the transitions can be estimated. Sharper transitions therefore result in better timing estimates and hence a more accurate position estimate. Conventional servo write heads for servo-formatting magnetic tape have a large inductance and require a large write current (>5 A). This makes it challenging to obtain the nanosecond-scale switching times required for writing sharp transitions at high tape velocities. Moreover, it is challenging to fabricate these conventional heads with very narrow write gaps. Recently, a novel planar servo write head has been reported (Engelen et al., 2012): microfabrication technology makes it possible to create a servo write head with much smaller dimensions, thus overcoming the limitations of conventional servo writers. Current rise times of 3 ns have been obtained, and preliminary results show that the rise time can be further reduced. A second advantage of this technology is that heads having a much narrower write gap than conventional servo write heads can be fabricated. The reduced switching time and the narrower write gap improve the quality of the transitions written to tape and also facilitate an increase in the tape-formatting velocity. The improved quality of the transitions should lead to a further reduction in the noise floor of the position signal generated from patterns written with a planar head. 4.6.3. Tape paths for high track density In modern high-performance tape drives, rolling elements, referred to as roller guide bearings, transport the tape from one reel to the other over the write/read head. In many tape drives, flanges on the roller guides are used to limit the lateral motion of the tape as it is transported from one reel to the other. The use of flanged rollers results in the accumulation of debris on the roller flanges, which strike the tape edges, exciting high-frequency LTM. These high-frequency components of the LTM cannot be effectively suppressed by the track-following controller if they exceed the bandwidth and slewing capability of the track-following actuator. In general, debris accumulates because the spacing between the two reel flanges is much larger than the roller flange spacing. Therefore, tape tends to stack against the reel flanges, so that when it is transported from a reel to the first roller, a large force develops between the tape edge and the roller flange, causing wear and, over time, debris accumulation. A straightforward solution to this problem is to remove the roller guide flanges altogether, but this introduces other challenges. For example, without the constraint of the flanges, the amplitude of the LTM increases because the tape moves up or down between the widely spaced reel flanges. This increases the range-of-motion requirement for the track-following actuator. Second, the angle of the tape
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with respect to the head can become skewed, which can interfere with the read-while-write verify functionality of the drive. These additional challenges can be addressed by constructing a more advanced actuator, capable of following a larger LTM and of servoing its rotation angle to keep the head perpendicular to the tape. A second option to address these two challenges is to actively control tilting elements elsewhere in the path to reduce the skew and lateral excursion of the tape relative to the head. For example, miniature actuators can be built into the tape guide rollers to actively tilt the roller and thus steer the tape as it is transported through the tape path (Pantazi et al., 2010b). In this concept, optical sensors are used to measure the position of the tape edge, and this information is exploited by a feedback controller to drive the tilting rollers to compensate for LTM and/or tape skew. Lateral tape motion arises primarily from imperfections in the tape guide rollers and reels, such as runouts, eccentricities, and other tape path imperfections. Even if the flanges are removed from the tape guides, imperfections in the mechanical bearings give rise to high-frequency disturbances that are difficult for the track-following controller to suppress. The performance of the track-following controller can therefore be improved by improving the tape path design. One approach to reducing the amplitude of the LTM is hard-edge guiding such as that used in the tape path described in Section 3.4 (MEII path), which was used for the areal density demonstration. This approach leads to a large reduction in the amplitude of the LTM; however, the mechanical contract between the tape edge and the guide generates high-frequency LTM, which is difficult to compensate. A second approach is to reduce the magnitude of the disturbances in a conventional roller guide path through improvements in the components and path design. Figure 3.27 shows a photograph of an experimental path in which LTM is constrained only by the reel flanges, resulting in a rather large LTM. However, the high-frequency components of the LTM are significantly lower than with a more conventional, heavily constrained tape path. The tape is guided by rollers that use pressurized air bearings rather than ball bearings to reduce periodic disturbances that typically result from ball-bearing imperfections and wear. The rollers are grooved to quench the air bearing that would otherwise form between the tape and roller surface (Argumedo et al., 2008). The elimination of this air bearing results in an increase in the frictional forces that oppose LTM. The two rollers closest to the reel motors use front-side guiding, whereas the two rollers adjacent to the head use backside guiding, which allows the span of tape running over the head to be reduced. All four rollers are of a flangeless design. The tape path uses the head actuator and electronics card from an IBM TS1130 tape drive. Tape transport is performed using motors taken from an IBM LTO-3 drive, which were fitted with high-tolerance bearings and with balanced precision-machined aluminum hubs to minimize once-around effects. The
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Photograph of the prototype flangeless tape path.
Figure 3.28 Time and frequency domain plots of open-loop LTM of the MEII (black) and the flangeless (green) tape path. Reprinted with permission from Lantz et al. (2011).
flanges of the tape reels are not tapered and have a spacing that is approx. 50 mm larger than the nominal tape width. With this tape path, periodic LTM is typically observed with a frequency determined by the rotation frequency of the reel motors and a peak-to-peak amplitude of 10–20 mm at tape transport velocities of 2 m/s. Figure 3.28 presents comparisons of time and frequency domain plots of captures of LTM in the two tape paths at transport velocities of 2 m/s. Although the amplitude of LTM in the experimental flangeless tape path is roughly an order of magnitude larger
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than that of the MEII path, most of the motion occurs at relatively low frequencies. In contrast, in the frequency range from 0.5 to 5 kHz, the flangeless tape path exhibits significantly lower LTM. This reduction in high-frequency LTM results in improved track-following fidelity, as discussed in Section 4.6.4. 4.6.4. Track-following control Reducing LTM by optimizing the tape path is desirable, as it reduces the disturbances that enter the track-following control system. It is then the task of the track-following controller to follow the LTM and position the head read/write elements at the centerline of the tracks (Pantazi et al., 2010a). The bandwidth of the actuator, the frequency characteristics of the LTM disturbance, and the noise performance of the medium-derived PES determine the achievable closed-loop bandwidth and thereby the trackfollowing performance. Achieving reliable tape-drive operation with track widths in the range of 165–200 nm requires that the track-following servocontrol system achieves a significantly higher positioning accuracy than in current commercial tape drives. The H1 robust control approach is a promising technique to design the controller for a tape track-following system. This approach uses experimentally obtained system models and takes into account the LTM disturbance characteristics. Its main advantage is that performance objectives, such as bandwidth and tracking error, are incorporated in the formulation of the optimization problem. The track-following control problem can be cast as an optimization problem using the general control configuration shown in Fig. 3.29a. In this configuration, the exogenous input is the LTM disturbance w ¼ [d] and the error signal is z ¼ [z1z2z3]T. The error signals, z1, z2, and z3, are defined as indicated in Fig. 3.29b, for example, z2 is the product of Wu and u. The performance requirements are specified by different weighting transfer functions. To achieve them, these weighting transfer functions are used to shape the closed-loop transfer functions of the system.
Figure 3.29
H1 control formulation block diagram.
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Figure 3.29b depicts the closed-loop system with the weighted outputs. For the track-following feedback loop, the transfer function that relates the disturbance d to the error e is S ¼ 1/(1 þGK). The capabilities of the system to follow the LTM can then be captured using the weight Ws, which can be chosen such that the magnitude of S is low in the low-frequency range, where LTM predominates. The transfer function T ¼ GK/(1 þGK) measures the impact of the measurement noise on the output. The weight WT can then be used to shape T in such a way that the controller rolls off at high frequencies. This approach was used to design a 7th-order controller for the experimental flangeless tape path described earlier. The bandwidth of the system derived from the sensitivity transfer function was approx. 700 Hz. This controller was implemented in the electronics hardware platform described in Section 3.4. Track-following experiments were performed using perpendicularly oriented BaFe medium (tape A in Section 3) formatted with the 18 servo pattern described in Section 3.5. Figure 3.30 presents time and frequency domain plots of the closed-loop PES measured at 2.28 m/s with this setup. The data exhibit a standard deviation of 13.0 nm, almost a factor of two lower than that achieved with the MEII tape path in the areal density demonstration. However, even further improvements in track-following performance will be required to achieve a tracking margin in the range of 70–105 nm necessary to achieve an areal density of 100 Gbit/in2. If we divide the tracking margin equally between contributions from TDS, head tolerances, and tracking errors, then the total allowable tracking error is 23–35 nm. Following the approach of the 2012 INSIC roadmap, the contribution of the sPES to the tracking margin is 4.24sPES, leading to a tolerable trackfollowing error of sPES ¼ 5.4–8.2 nm. It may be possible to relax this requirement somewhat if the TDS component can be made negligible using a TDS compensation scheme such as that presented above. As discussed, the achievable track-following performance is ultimately limited by the bandwidth of the actuator, the frequency characteristics of the LTM disturbance, and the noise performance of the medium-derived PES. Further improvements in the track-following performance can therefore be achieved by further reducing the noise floor in the position estimates as discussed in Section 4.6.2. Significant improvements in the track-following actuator bandwidth have also been demonstrated (Kartik et al., 2009, 2010). By combining these approaches, it should be possible to achieve the required 5–8 nm sPES. 4.6.5. Reel-to-reel control One of the main advantages of tape-based storage systems is the very high volumetric density achieved by winding a very long, thin tape onto a single reel. To further increase the tape cartridge capacity, both the areal and
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Figure 3.30 (a) Time domain signal and (b) power spectral density of PES for a ¼ 18 , v ¼ 2.28 m/s.
volumetric storage densities have to be improved. The latter can be improved by reducing the tape thickness while simultaneously increasing the tape length. The 2012 INSIC tape roadmap predicts a reduction in tape thickness to approx. 4 mm and an increase in length to 1357 m by 2022. High areal densities require excellent tape motion and tension control because the quality of the tape transport directly affects the data write and read performance. Improved tension control reduces the tension contribution to the TDS component of the tracking margin and hence leads to higher track densities. Moreover, higher volumetric densities require thinner magnetic coatings and substrates, which in turn may lead to increased susceptibility to tape damage. To counteract these effects, the design of the reel-to-reel servo system for tape velocity and tension control becomes increasingly important. A promising route to achieve the required velocity and tension control is to depart from standard PID controllers and introduce state space-based methods. The main advantage of a state space-based control system is its
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suitability for the design of a multi-input, multi-output (MIMO) control system. A MIMO control design allows multiple inputs, as is necessary if information from multiple sensors and several estimated parameters need to be used. Another important feature is its capability of handling designs in which the rate of measurements from the sensors is not commensurate with the sampling frequency of the digital controller. The notion of a MIMO control system for tape transport was introduced in Franklin et al. (1997) and has been applied to a prototype tape transport system in Mathur and Messner (1998). A MIMO architecture enables simultaneous control of tension and velocity, which, in conjunction with an optimized tape path, can substantially reduce lateral tape motion. Important steps in defining the reel-to-reel control architecture include accurate plant characterization and proper design of controller and estimator. One of the main challenges in using a MIMO system is the provision of reliable sensor measurements to determine the state of the system. There are various approaches that include embedded sensors and signalprocessing techniques to gather the necessary information to provide tension and velocity feedback.
5. Summary and Conclusions The areal density demonstration described in Section 3 provides clear experimental evidence that there is potential to continue scaling particulatebased magnetic tape technology for many more years. It is also important to note that the 29.5-Gbit/in2 areal density achieved does not represent the physical limit of the scaling potential of BaFe media, but rather, it represents a snapshot of the state of the art of the technology at the time that the demonstration was performed (Cherubini et al., 2011). There remains considerable potential to continue scaling BaFe technology well beyond this point. While it is clear that there will be significant challenges in scaling commercial linear magnetic tape drives to operate at areal densities in the range of several tens of Gbit/in2, there is also significant experimental evidence that these challenges can be overcome by means of skillful engineering. Controlling the tape–head interaction combined with continued advances in tape media technology will be key to achieving high linear density with ultranarrow readers, whereas improvements in track-following and reel-to-reel servo mechanisms and improvements in TDS and reducedspan heads will be key to achieving high track densities. In addition, advanced head and data-detection technologies and improved lateral tape motion control will impact both the linear density and the track pitch and therefore be key enablers to achieving ultrahigh areal densities in a linear magnetic tape system. Through the combination of these technologies, the
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roughly 50 Gbit/in2 areal density predicted by the INSIC roadmap to be needed for a 128-TB cartridge appears achievable. Pushing these technologies further, a linear density of 650–800 kbit/in and a track pitch in the range of 0.165–0.2 mm may also be feasible, leading to an achievable areal density of 100 Gbit/in2.
ACKNOWLEDGMENTS The material presented in this chapter is primarily an updated and extended version of the material originally presented in Argumedo et al. (2008) and Cherubini et al. (2011). We would like to thank our coauthors of those original manuscripts: A. J. Argumedo, D. Berman, R. G. Biskeborn, G. Cherubini, R.D. Cideciyan, L. Dellmann, W. Haeberle, T. Harasawa, D. J. Hellman, R. Hutchins, W. Imaino, J. Jelitto, P.-O. Jubert, K. Judd, V. Kartik, P. Koeppe, ¨ lc¸er, H. Ohtsu, G. McClelland, Y. Murata, A. Musha, C. Narayan, H. Noguchi, S. O A. Pantazi, H. Rothuizen, P. J. Seger, O. Shimizu, R. Suzuki, and K. Tsuruta. In addition, we are grateful to C. Bolliger for careful proofreading of the manuscript.
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T H R E E
Future Scaling Potential of Particulate Media in Magnetic Tape Recording☆ Mark A. Lantz* and Evangelos Elefteriou Contents 1. Introduction 2. Media Considerations 2.1. Metal particle media 2.2. BaFe particulate media 2.3. Future scaling potential of BaFe media 3. 29.5-Gbit/in2 Areal Recording Demonstration 3.1. Introduction 3.2. Media 3.3. Low-friction head technology 3.4. Tape path and hardware platform 3.5. Servo pattern design 3.6. Synchronous servo channel 3.7. Design of the track-following controller 3.8. Track-following performance 3.9. Read channel and recording performance 3.10. Summary of demo results 4. Continued Future Scaling Potential of Magnetic Tape Technology 4.1. Introduction 4.2. Write and read-head technology 4.3. Head–tape interaction 4.4. Data detection
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*Corresponding author: [email protected] IBM Research - Zurich, Sa¨umerstrasse, Ru¨schlikon, Switzerland ☆
This chapter is an updated and expanded version of the information published in the following two papers: “Scaling tape recording areal densities to 100 Gb/in2” by A. J. Argumedo, D. Berman, R. G. Biskeborn, G. Cherubini, R. D. Cideciyan, E. Eleftheriou, et al., IBM J. Res. Develop., Vol. 52, No. 4/5, pp. 513–527, July/September 2008, and “29.5-Gb/in2 recording areal density on barium ferrite tape” by G. Cherubini, R.D. Cideciyan, L. Dellmann, E. Eleftheriou, W. Haeberle, J. Jelitto et al., IEEE Trans. Magnet., Vol. 47, No. 1, pp. 137–147, January 2011.
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4.5. Format efficiency 4.6. Track density limits 5. Summary and Conclusions Acknowledgments References
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1. Introduction The volume of digital data produced each year is growing at an overwhelming pace. According to a recent International Data Corporation (IDC) study (IDC, 2011), 1.2 zettabytes (1.2 1021) of data were created and replicated in 2010. In the future, this staggering volume of data is projected to grow at a compound annual growth rate (CAGR) of 50% or more, significantly faster than the expected growth rate of storage capacity. New regulatory requirements imply that an ever-larger fraction of these data will have to be preserved for much longer periods of time. All of this translates into a growing need for cost-effective digital archives, in which tape plays an integral part in current tiered storage infrastructure. Tape systems are particularly well suited for low-cost, long-term storage of data and for backup and disaster recovery purposes. Tape technology offers several important advantages, including energy savings, security, lifetime, reliability, and cost. Moreover, in such applications, the main drawback of tape, namely, its slow access time, does not have a major impact on the system performance. Once data have been recorded in tape systems, the medium is passive: it simply sits in a rack and no power is consumed. Compared with similar HDD (hard disk drive)-based systems, a tape-based archive consumes roughly 290 less power (Reine and Kahn, 2008). Tape systems also offer additional security in that once data have been recorded and the cartridge has been removed from the tape drive, the data are inaccessible until the cartridge is reinstalled in a drive. Security is further enhanced by drive-level encryption, which was introduced in Linear Tape-Open1 generation-4 (LTO-4) drives, and is also standard in enterprise-level tape drives. The tape medium has a lifetime of 30þ years (Fujifilm, 2011; Watson et al., 2008); however, this is rarely used because the rapid advances in tape hardware lead to significant cost savings associated with migration to higher-capacity newer-format cartridges. In terms of reliability, the latest LTO tape drives have a bit error rate (BER) that is several orders of magnitude better than that of a SAS (serial attached scsi) 1
Linear Tape-Open and LTO are trademarks of HP, IBM, and Quantum in the United States and other countries. IBM is a trademark of International Business Machines Corporation, registered in many jurisdictions worldwide. Other product and service names might be trademarks of IBM or other companies.
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HDD. Moreover, the fact that tape media are removable and interchangeable means that, in contrast to HDDs, mechanical failure of a tape drive does not lead to data loss because a cartridge can simply be mounted in another drive. All of the earlier-mentioned advantages contribute to the two net advantages of tape: cost and reliability. Estimates of the savings of the total cost of ownership of a tape backup system, relative to HDD backup, range from a factor of three to more than 20, even if technologies such as data deduplication are taken into account. In archival applications, where data deduplication is ineffective, the cost savings are even higher. Historically, the cost and the size of the media recording area of both tape cartridges and disk drives have remained relatively constant. Reductions in the cost per Gbyte, which are fundamental to the continued viability of these technologies, have primarily resulted from increases in the areal recording density. Figure 3.1 compares the evolution over time of the areal densities of commercially available tape drives and HDDs and also shows recent tape areal density demonstrations. The figure illustrates that historically, tape drives have operated at an areal density roughly two orders of magnitude lower than that of HDDs. However, tape drives maintain a lower cost/Gbyte relative to disk because tape media can be produced at a very low cost per unit area and because the media are removable, allowing the cost of the drive to be amortized over many cartridges. The cost/Gbyte difference is the key reason that tape technology has remained successful, even though the investment into research and development in tape drives is far lower than that for HDDs. From another perspective, the areal density
Figure 3.1 Recording areal density of hard disk drive and tape products and tape demonstrations. Adapted from Berman et al. (2007).
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difference between tape and disk systems presents a great opportunity for achieving a much lower cost/Gbyte for tape if the areal density of tape can be brought closer to that of HDDs by skillful engineering. Moreover, Fig. 3.1 also indicates that even though the gap between the areal densities of HDD and tape products has remained essentially constant in recent years, tape areal density demonstrations exhibit a slope of about 60% CAGR, indicating the potential to reduce the areal density gap between tape and HDD. We can gain insight into how this rapid improvement in tape areal density demonstrations was achieved by comparing the bit aspect ratios (BARs) of tape drives and HDDs. The typical BAR (bit width to bit length) in recent HDD technology demonstrations is on the order of 6:1 (Bandic and Victoria, 2008). In contrast, LTO generation-5 (LTO-5) tape drives operate at an areal density of 1.2 Gbit/in2 with a BAR of about 123:1 (IBM, 2010). This comparison indicates that from a magnetic recording physics perspective, there is considerable room to reduce the track width in tape technologies. In the past, one of the obstacles to a more widespread use of tape technology has been the difficulty of using tape in a general or standalone context. Hard disks provide random access to data and generally contain a file index managed by a file system. These files can be accessed using standard sets of application programming interfaces (APIs) via various operating systems and applications. Tape, in contrast, is written in a linear, sequential fashion using a technique called “shingling,” which not only provides backward-write functionality but also imposes the restriction that new data can only be appended and that previously written areas can only be reclaimed if the entire cartridge is reclaimed. In traditional tape systems, an index of the files recorded on a given cartridge is typically kept only in an external database managed by an application such as a proprietary backup application. The requirement to access an external database to retrieve data renders data on tape much less portable and accessible than data on alternative storage methods, such as a HDD or a USB flash drive. To address these deficiencies, a new long-term file system (LTFS) has recently been introduced into the LTO-5 tape-drive systems and recent enterprise tape systems to enable efficient access to tape using standard system tools and interfaces (Pease et al., 2010). LTFS is implemented using the dual-partition capabilities supported in the LTO-5 format initially and now also in recent enterprise products. A so-called index partition is used for writing the index, and the second, much larger partition for the data itself. With this new file system, files and directories show up on the desktop with a directory listing. Users can “drag and drop” files to and from tape and can run applications developed for disk systems. All of these features help to reduce the costs associated with using tape and eliminate the dependency on a middleware layer. In addition, tape becomes
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cross-platform portable (Linux, Mac, and Windows2), enabling and facilitating the sharing of data between platforms. These features render significant new use cases for tape such as video archives and storage of medical images possible. Considering the cost advantages of tape over other storage solutions and the increasing usability of tape provided by advances such as LTFS, tape is set to play an important role in the rapidly expanding market for archival data storage solutions. How large a role tape will play—and for how long— will depend on the continued scaling of tape to higher areal densities at a constant cost so that the cost per Gbyte advantage of tape over other technologies either remains constant or increases. In this chapter, we will explore the future scaling potential of magnetic tape technology based on low-cost particulate media. In Section 2, we discuss the state of the art of particulate media. In Section 3, we review the achievements and the technologies behind the recent 29.5 Gbit/in2 areal density demonstration using low-cost particulate BaFe media. In Section 4, we examine the potential for further scaling, posing the questions: “Can magnetic tape be scaled to 100 Gbit/in2?” and “What technologies are required to do so?” (Argumedo et al., 2008). Finally, in Section 5, a summary and conclusions are presented.
2. Media Considerations 2.1. Metal particle media Most of the present and legacy commercial magnetic tape media use iron– cobalt-based metal particles (MPs). In this section, we will discuss various particulate media considerations to achieve areal densities beyond 50 Gbit/in2, which according to the INSIC 2012 roadmap (INSIC, 2012) will bring us to cartridge capacities of 128 TB by 2022. In addition, we will examine the potential to scale particulate tape beyond this point towards 100 Gbit/in2. The following parameters limit the areal recording density that can be achieved on tape: the magnetic stability of the medium, the maximum achievable write-head field, the minimum bit length that can be recorded in the medium, and the signal-to-noise ratio (SNR). The broadband SNR varies with the number of particles per bit volume, and model calculations have shown that to achieve reasonable SNR, about 100 particles per bit are required (Mallinson, 1974). Thus, particles must become smaller as the areal density increases. However, to preclude 2
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thermally induced switching of the particles, the anisotropy energy KUV of each particle must be much larger than the thermal energy in the environment, kBT. Here, KU is the uniaxial anisotropy energy density, V the volume of the particle, kB the Boltzmann constant, and T the absolute temperature. A widely used rule for data retention is that KUV > 60 kBT. Particles with a higher anisotropy also require larger magnetic fields for magnetization reversal. Unfortunately, head fields are limited by the saturation magnetization (Bs) of the head-pole materials, and thus, there is a limit to how much the particle volume can be reduced. Current tape drives use write heads with a saturation magnetization on the order of 16–18 kG, and more than 20 kG is considered large. The limit of existing materials is on the order of about 24 kG (e.g., Co35Fe65 has a Bs of 24 kG). Following Weller and Moser (1999) and Zhou and Bertram (1999), the minimum bit length Bmin in a thin longitudinal media is approximately sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 D B min p 0:35a2WC þ 2
where D is the particle diameter and aWC the Williams–Comstock transition width parameter. In SI units, aWC can be expressed as (Bertram, 1994; Williams and Comstock, 1971) ð1 S Þðd þ t=2Þ þ aWC pQ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 S Þðd þ t=2Þ 2 Mr t ðd þ t=2Þ : þ pQ pQHC
Here, d is the head/medium spacing, t the medium thickness, Mr the remanent magnetization, S the hysteresis squareness, and Q the head field gradient. HC denotes the medium coercivity, which is proportional to the medium anisotropy field Ha. For imperfectly oriented medium at typical recording timescales, it can be approximated by 0.5Ha or KU/Ms, where Ms is the saturation magnetization (Sharrock, 1999). Using these equations, the minimum bit length for longitudinal recording can be estimated as a function of medium thickness (i.e., thickness of the magnetic layer on the tape), head/medium spacing, coercive field, and medium saturation magnetization. Figure 3.2 presents the calculated minimum bit length achievable, assuming a maximum coercive field of 5300 Oe (hence, a Bs of 20 kG, 80% usable deep gap field, and HC ¼ 1/3 of the deep gap field to account for head/medium spacing) and a saturation magnetization of 550 emu/cc. These values correspond to a required particle anisotropy constant of 2.9 106 erg/cc and a particle diameter of at least 12 nm. These calculations also show that at a head/medium spacing of, for example, 20 nm, longitudinal media can sustain a linear density of 800 kbpi if the medium is no thicker than 15 nm.
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Figure 3.2 Minimum bit length versus medium thickness calculated for three different head/medium spacings. Hc ¼ 5300 Oe, Ms ¼ 550 emu/cc, Q ¼ 0.8, S ¼ 0.8. With these values, the maximum particle anisotropy equals of 2.9 106 erg/cc, and the minimum particle diameter for magnetic stability is 12 nm. Reprinted with permission from Argumedo et al. (2008).
For such a linear density and a medium thickness of 15 nm, the minimum bit area is 32 nm 380 nm if we assume 100 particles of 12 nm diameter per bit with a 50% volume packing fraction. To account for servoing error and dimensional stability, a 50% margin should be added to the track pitch, which yields a track density of 45 ktpi and a maximum areal density of 36 Gbit/in2, in agreement with earlier projections (Charap et al., 1997). To achieve an areal density of 100 Gbit/in2 with a linear density of 800 kbpi, the track density needs to be 125 ktpi, which corresponds to a minimum bit area of 32 nm 135 nm (note that 50% of the track width has been removed to account for track-following error) or 36 particles of 12 nm diameter with a 50% volume packing fraction. This corresponds to a loss in SNR of 4.5 dB compared with the SNR in the 100-particle case, which has been defined as 20 log(N1/2). To compensate for this loss in SNR, the signal processing would have to be improved. Other routes to achieving 100 Gbit/in2 with longitudinal media have been proposed, but these put even more constraints on the track density or the head/medium spacing (Bertram and Williams, 2006; Dee, 2006; Mallinson, 1974). In summary, achieving a linear density of 800 kbpi, although theoretically possible, appears extremely challenging with longitudinal recording. A more attractive alternative for achieving such high linear densities is the migration to perpendicular recording. In perpendicular recording,
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demagnetizing fields actually favor high linear densities. Moreover, thicker films could be used for better thermal stability. For example, linear densities of 1500 kbpi have been demonstrated for a HDD using pole heads and perpendicular media with a soft underlayer (Seagate, 2006). On tape, achieving 800 kbpi linear density will require very thin coatings, probably containing only a monolayer of particles. To achieve low noise for low bit sizes, the particles must be magnetically isolated but packed closely and uniformly with their neighbors. Packing would be easiest for uniformly sized spherical particles, and recently, nanoparticles have been successfully synthesized and arranged uniformly on surfaces (Lin and Samia, 2006; Pyun, 2007). To render the use of these particles attractive for magnetic tape will require inexpensive synthesis and a rapid method for coating them uniformly on the tape, especially as typical tape lengths are approaching 1 km (some products have even exceeded this length). An alternative to particles would be the formation of thin magnetic films using vacuum deposition. Evaporated metal films have already been introduced in tape products (Motohashi et al., 2007); they have the advantages that they can be deposited at high rates and that inexpensive metals can be used. Sputtered metal films, which have been successfully developed for HDD, have the potential for much larger areal density; however, the cost of the raw materials required to produce sputtered magnetic tapes would likely result in a significant increase in cartridge cost. While it may be possible to transfer that technology in an economically viable solution to tape media, this remains challenging (Lee et al., 2005; Moriwaki et al., 2005). In summary, sputtered magnetic films are promising for tape, but optimized particulate and evaporated metal films may still have the potential to achieve 100 Gbit/in2. In the next section, we will focus on low-cost particulate media, specifically BaFe media, which hold the promise to bring us to areal densities beyond 50 Gbit/in2 and may even be extendible to 100 Gbit/in2.
2.2. BaFe particulate media As discussed in the preceding text, one approach to increase the recording capacity of magnetic tape is to reduce the volume of the magnetic particles used in the recording layer. In this section, we investigate the use of BaFe particles as a potential replacement of MPs in future tape media. As the coercivity of MP media originates in the shape anisotropy, it is difficult to maintain a high coercivity when the particle volume is reduced. In contrast, the coercivity of BaFe particles arises from the crystalline magnetic anisotropy, rendering the scaling of BaFe particles more favorable. Another issue with traditional MP media results from the oxidation of the iron–cobalt that is typically used. To prevent this, the FeCo particles are covered with a
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protective, nonmagnetic “shell,” which is a further issue in scaling to finer particle sizes. BaFe, in contrast, is already an oxide and therefore does not need a protective shell. Another advantage of BaFe particles is their unique platelet shape, which makes it easier to orient the easy axis in the perpendicular direction of the medium surface. Note that perpendicular orientation results not only in an increase in signal amplitude but also in a reduction of the demagnetizing field at high linear densities (Thompson, 1997). Together, these properties make the scaling of BaFe to finer particle sizes much more favorable than MP technologies. Therefore, the use of BaFe particles (Berman et al., 2007; Harasawa et al., 2010; Kubo et al., 1982; ¨ lc¸er Matsumoto et al., 2010; McClelland et al., 2009; Nagata et al., 2006; O et al., 2009; Shimizu et al., 2010; Watson et al., 2008) in future generations of magnetic recording media has been investigated extensively. To improve the performance of BaFe media even further relative to that of previous media generations (Berman et al., 2007; Harasawa et al., 2010; Kubo et al., 1982; Matsumoto et al., 2010; McClelland et al., 2009; Nagata ¨ lc¸er et al., 2009; Shimizu et al., 2010; Watson et al., 2008), et al., 2006; O three new technologies were applied in developing the medium. First, the average particle volume was reduced to 1600 nm3. Second, the easy axis of the particles was oriented in the perpendicular direction during the coating process, resulting in a squareness ratio of 0.86 in the perpendicular direction. Finally, the surface waviness was reduced, achieving a surface roughness (Ra) of 0.7 nm, as measured with optical interferometry, which neglects the small asperities, and an Ra of 2.1 nm measured using atomic force microscopy (AFM). The properties of this new perpendicular BaFe tape (tape A) are summarized in Table 3.1. For comparison, we have also included the properties of the BaFe medium used in an earlier areal density demonstration of 6.7 Gbit/in2 (tape B) (Berman et al., 2007) and the properties of commercial MP media (LTO-5). Note that LTO-5 operates at an areal density of 1.2 Gbit/in2 with a cartridge capacity of 1.5 TB (IBM, 2010). The magnetic layer of all three media types is approx. 60 nm thick. 2.2.1. Fine-particle BaFe In general, as the size of a magnetic particle is reduced, also the thermal stability factor KUV/kBT will be reduced. For BaFe particles, the anisotropy can be adjusted by substitution of Fe with other elements (Kubo et al., 1982). To compensate for the reduced particle volume of tape A, the type and amount of substitution elements used in the BaFe particles were adjusted to increase KU. By optimizing the formulation of the 1600 nm3 particles, a thermal stability factor of 75, measured as described in Matsumoto et al. (2010), was achieved. This provides sufficient stability for long-term archival applications.
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Table 3.1 Properties of magnetic particles and tapes using barium ferrite (BaFe) and metal particle (MP) media (Cherubini et al., 2011) Medium type
Tape A
Tape B
LTO
Magnetic particle material Volume (nm3) Coercivity (Oe) Saturation magnetization (emu/g) Orientation direction
BaFe 1600 2250 48 Perpendicular
BaFe 2100 2300 52 Nonoriented
MP 2800 2400 103 Longitudinal
1970 0.40 0.31
2730 0.86 1.39
2660 0.61
– –
0.40
–
2.0
2.0
2.0
2.5
Magnetic properties measured in longitudinal direction Hc (Oe) 1480 SQ 0.22 Mrt (memu/cm2) 0.18 Magnetic properties measured in perpendicular direction Hc (Oe) 2940 SQ (with demagnetization 0.86 compensation) Mrt (memu/cm2) 0.56 Surface roughness Ra (nm) Optical interferometry (measured 0.7 with a WYKO HD2000 instrument) AFM 2.1
2.2.2. Perpendicular orientation Figure 3.3 presents transmission electron microscopy (TEM) images of tapes A and B. Note that in tape A, the platelet-shaped BaFe particles are well aligned, whereas in tape B, they appear randomly oriented. The more regular orientation of the particles in tape A results in an increased squareness ratio and improved performance (Shimizu et al., 2010). This improved squareness ratio was achieved through the use of intensive dispersion prior to coating and the application of a magnetic field in the drying zone of the coating process. The M—H loops of these tapes are shown in Fig. 3.4. The M—H loop in the perpendicular direction of tape A is very similar to that of the LTO-5 tape in the longitudinal direction, indicating a similar degree of orientation. 2.2.3. Low-waviness surface To reduce the head/medium spacing, the surface of the medium should be as smooth as possible. However, the fundamental dilemma in contact magnetic recording is that such an increase in medium smoothness may result in increased friction and hence may reduce the durability and
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Figure 3.3 TEM images of cross sections of (a) tape A (perpendicular orientation) and (b) tape B (non-oriented). Scale bars are 20 nm. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Figure 3.4 M—H loop in the perpendicular direction of tapes A and B, and M—H loop in the longitudinal direction of LTO-5 tape. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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runnability of the medium. To resolve this dilemma, the long-range surface roughness, referred to as “waviness,” was reduced. Simultaneously, a moderate roughness when measured over a shorter length scale was maintained. Figure 3.5 presents 180 mm 240 mm surface profiles of tapes A and B, measured with optical interferometry. The data show a waviness of Ra ¼ 0.7 nm for tape A and an Ra ¼ 0.2 nm for tape B. Figure 3.6 presents 40 mm 40 mm surface profiles of both tapes measured with AFM. Note that compared with tape B, tape A has many small asperities. This combination of low surface waviness and moderate short-range roughness in tape A results in an increased signal from the medium while maintaining excellent durability and runnability.
2.3. Future scaling potential of BaFe media The perpendicularly oriented BaFe tape described previously (tape A) was used to perform a 29.5-Gbit/in2 areal density recording demonstration (Cherubini et al., 2011). The details of this demonstration are described in
Figure 3.5 Optical interferometry measurements of surface roughness (waviness): (a) tape A and (b) tape B. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Figure 3.6 AFM images of tape surface profiles: (a) tape A and (b) tape B. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Section 3. This areal density achievement does not, however, represent the limit of the scaling potential of BaFe media, but rather is a snapshot of the state of the art of the technology at the time the demonstration was performed. There remains considerable potential to continue scaling BaFe technology beyond this point. For example, the particle volume of 1600 nm3 used in tape A could be reduced to significantly smaller volumes, leading to even better SNR values. However, the coercivity of the particles would then have to be increased to ensure the archival stability of the medium. This in turn necessitates the development of write heads with a higher saturation magnetization than that of the heads used for the demonstration. Second, the magnetic spacing could be further reduced in future generations of the media if the tape–head friction can be controlled through technologies such as those discussed in Sections 3 and 4. Third, further improvements in the orientation of the particles will result in additional SNR gains. Finally, the media noise can be further reduced through decreasing the variations in particle size and magnetic properties and by improving the dispersion of the particles. We believe that with such improvements, it should be possible to scale BaFe-based tape to areal densities of 50 Gbit/in2 and beyond, which are required for a 128-TB cartridge. To achieve an areal density of 100 Gbit/in2 will require major advances in all of the areas discussed in the preceding text. However, even when BaFe reaches its scaling limits, there is potential to continue scaling magnetic tape technology through the development of new particle technologies or by transitioning to sputtered media.
3. 29.5-Gbit/in2 Areal Recording Demonstration 3.1. Introduction We can gain insight into the state of the art of magnetic tape-recording technology and the experimentally established potential for future areal density scaling by reviewing the results of a recent single-channel areal recording demonstration of 29.5 Gbit/in2 (Cherubini et al., 2011). This was achieved by increasing the track density by more than 18-fold relative to that of LTO-5 and by dramatically improving the performance of the track-following servo system. The servo improvements were realized through the use of an optimized servo pattern in combination with a new servo channel, an H1-based track-following controller, and a new low-friction head technology. This aggressive scaling of the track density necessitates the use of much narrower read elements, leading to a reduced readback amplitude. To compensate for this reduction, a new magnetic tape based on ultrafine, perpendicularly oriented BaFe magnetic particles was employed that enables high density without the need for expensive
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vacuum-coating methods and provides a much better SNR than standard MP tape media. Finally, an advanced noise-predictive maximum-likelihood (NPML) detection method was implemented that supported linear densities of up to 518 kbpi with an ultranarrow, 0.2 mm wide data reader, leading to an additional gain in areal density. In this section, each of these technologies will be discussed, followed by a description of how they can be combined to achieve an areal density of 29.5 Gbit/in2. This areal density corresponds to approx. 25 the areal density of LTO-5. However, it is important to note that this was a single-channel demonstration, in which effects such as tape dimensional stability and head fabrication tolerances, which are important for multichannel parallel tape systems, were not taken into account. The impact of such effects and the potential to scale tape beyond 29.5 Gbit/in2 towards 100 Gbit/in2 are discussed in Section 4.
3.2. Media A detailed discussion of particulate tape media was presented in Section 2 of this chapter. The properties of the two types of tape investigated in the 29.5 Gbit/in2 demonstration were summarized in Table 3.1, in which also the properties of LTO-5 tape were included for comparison.
3.3. Low-friction head technology The tape magnetic spacing, which is defined as the distance between the magnetic coating on the tape and the read/write transducers in the head, is a critical parameter for scaling the linear density in magnetic recording systems. The 2012 INSIC roadmap (INSIC, 2012) predicts that, in order to scale the linear density of a tape system from the current value of 400–500 kbpi to around 1000 kbpi by the 2022 time frame, the magnetic spacing will have to be reduced from the current (2012) value of approx. 36 nm to about 19 nm by 2022. Unlike in HDDs, there is no air bearing between the head and the storage media in tape systems. Instead, tape heads are designed with a sharp leading “skiving” edge that prevents air from entering the tape–head interface. This results in a reduced air pressure below the tape, such that the higher air pressure above the tape pushes the tape into contact with the head (Biskeborn and Eaton, 2002). In conventional tape head designs, the head surface is smoother than the tape, such that the head–tape contact area is determined primarily by the tape roughness. In addition, the magnetic spacing is determined by the roughness of the tape and any recession of the pole tips in the head, which can occur over the lifetime of the head because of wear. Several factors contribute to the tape surface roughness, including the base film roughness, coating uniformity, and the effect of asperities engineered to control the head–tape contact area and for head cleaning.
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To reduce the magnetic spacing, the tape roughness must be reduced. In a conventional flat-head geometry, this results in an increase in contact area, which in turn causes an increase in friction and stiction forces (Biskeborn and Eaton, 2002). Variations in the tape velocity caused by varying friction forces can degrade the performance of the timing recovery loop in the read channel, leading to a degradation in detector performance and an increase in the raw error rate at the output of the detector. In extreme cases, stiction can cause the tape transport to stall and even result in tape damage. The dynamic effect of head–tape friction is primarily to excite a compression wave in the tape, which reflects back and forth between the two rollers on either side of the head at a frequency determined by the mechanical properties of the tape and the distance between the rollers. Typical frequencies are on the order of several tens of kilohertz. The effect of the resulting timing variation is largest at low tape speeds, at which the timescale is fastest compared with the single-channel data rate. Although a low magnetic spacing is required at the read/write and servo elements, these elements span slightly less than one-quarter of the tape width, that is, one out of four data bands. In the LTO 1–5 tape formats, these elements’ span is only 2.9 mm out of the 12.65-mm tape width. Although the entire width of the tape must be supported by the head, the regions distant from the read/write elements can be just as effectively supported by an air bearing as by the hard contact required in the central region of the head. A simple and elegant method to produce an air bearing at the periphery of the head while maintaining intimate contact in the central region occupied by the read/write transducers is to bevel or round the skiving edge of a conventional head in the regions distant from the read/ write transducers. This approach was used in the 29.5-Gbit/in2 areal density demonstration to enable the use of smoother media with reduced magnetic spacing while maintaining tolerable levels of friction and stiction. In addition, the beveled head technology has been adopted in the latest IBM tape drives. The basic head design is illustrated in Fig. 3.7: The skiving edge appears only near the write/read elements, whereas elsewhere, the contour is approximately that of a circle with a radius of 6 mm into which a straight section has been inserted. The edge slope is 2.2 . For a cylindrical bearing of radius R, the flying height h at velocity v is given by h ¼ 0.643 (6 mv/ (T/w))2/3R (Eshel and Elrod, 1965), where m is the air viscosity and T and w are the tension and width of the tape, respectively. Thus, for velocities between 1.4 and 7 m/s, the fly height varies between 0.75 and 2.2 mm. The portions of the head that are rounded during the beveling process are intentionally roughened to exhibit a roughness with an rms value of about 12 nm. This roughness is well below the air-bearing thickness such that there is no contact with the tape flying over the air bearing during tape motion. However, the roughness is much larger than the tape roughness and therefore reduces the contact area between the tape and head at zero velocity and
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Figure 3.7 Schematics (not to scale) and photograph of the beveled head. (a) Illustration of a head attached to mounting beam (brown). (b) Photograph of the central section of a beveled head, showing the location of the cross sections in (c) and (d). (c) Illustration of a cross section of the flat area of the head, with the tape in contact. (d) Illustration of a cross section of the beveled region of the head, illustrating the air bearing between the tape and the head. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
hence the static friction at start-up and when the tape velocity goes through zero when the tape direction is reversed. Because the tape contacts the rough area only while starting and stopping, the tape is not damaged by the rough surface, and the rough areas of the head will not be polished smooth. Compared with a conventional flat tape head, such a beveled head reduces running friction on smooth prototype BaFe media from 0.3 to 0.1 N and start-up stiction from >2 to <0.8 N. The reduction in running friction results in a reduction in high-frequency variations in tape velocity, which in turn leads to an improvement in error-rate performance, particularly at low tape speed. Figure 3.8 compares the recording quality and readback performance of a commercial tape drive fitted with a beveled and a nonbeveled head on a smooth formulation of BaFe tape (Berman et al., 2007) written at 2 m/s at a linear density of 480 kbpi and read with a 3.8-mm wide giant magnetoresistance (GMR) reader. At high velocities, the error rate with the beveled head is reduced by a factor of 2–4, whereas at very low tape velocities, the error rate is reduced by up to a factor of 20 relative to the conventional head (McClelland et al., 2009).
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Figure 3.8 Recording quality versus velocity for a conventional flat and a beveled head. The recording quality (MbTF) is the mean number of bits between error events, that is, the inverse of the error rate. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
3.4. Tape path and hardware platform As discussed, aggressive track-density scaling is the key to the continued scaling of tape areal densities. An important tool to investigate the potential for track-density scaling, which was used in the 29.5-Gbit/in2 demonstration, was an experimental platform developed for closed-loop track-following experiments. The setup consists of a commercially available experimental tape transport (manufactured by Mountain Engineering II Inc., Longmont, CO, United States), the main electronics card and track-following actuator from an IBM TS1130 tape drive, a prototype beveled GMR tape head equipped with 2.5-mm wide servo readers, an FPGA (field-programmable gate array) board, a FPGA/DSP (digital signal processor) board, and a host computer. A photograph of the tape path, electronics card, and head-actuator assembly and a block diagram of the experimental platform are shown in Fig. 3.9. The tape path is composed of 10 porous ceramic externally pressurized air-bearing tape guides that use hard-edge guiding and two precision aluminum reels. The low-friction air-bearing guides in combination with a long tape path result in a low magnitude of lateral tape motion (LTM) during tape transport. With this tape-guiding approach, the typically observed LTM is strongly influenced by the edge roughness of the tape. For the prototype BaFe samples used in the demonstration, an LTM with a standard deviation of 1 mm was typically observed for tape transport velocities of 2 m/s. The tape transport is also equipped with two tension sensors and performs velocity and
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Figure 3.9 (a) Photograph of the tape path, TS1130 electronics card, and the headactuator assembly. (b) Block diagram of the prototype experimental setup for trackfollowing experiments. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
tension control without the need to read and decode servo information written on the tape. The TS1130 electronics card functions as the analog front-end for the read/write head and provides bias currents to the head and amplification and analog-to-digital conversion of the signals from two servo and 16 data readers in each of the two head modules. Digital samples of the readback signal from the two servo readers are fed from the TS1130 card to the FPGA board in which the servo channel, described in Section 3.6, has been implemented. The servo channel decodes the servo readback waveform and provides estimates of the lateral position of the head to the DSP/FPGA board, which in turn runs the track-following controller described in Section 3.7. The track-following controller operates in a synchronous mode in which the FPGA board provides an interrupt to the DSP board whenever a new estimate of the head lateral position is available. The DSP/FPGA board also provides a digital-to-analog converter and a voltage-to-current amplifier that are used to produce a control signal that is fed back to the head actuator. Data and control parameters are transferred between a host computer and the DSP/FPGA board using a FireWire interface, and user parameters are entered using a graphical user interface implemented in MatLabTM.
3.5. Servo pattern design In modern tape drives, encoded position information is prewritten onto the tape during manufacturing in dedicated areas, referred to as servo bands. During tape-drive operation, the encoded position information is read back continuously by dedicated servo read elements. The servo readback signal is decoded by a servo channel that generates estimates of the tape velocity and
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Figure 3.10 Illustration of the servo pattern used in the 29.5-Gbit/in2 demonstration. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
the lateral position of the head relative to the tape. These estimates are in turn used for closed-loop feedback control of the position of the head to compensate for and track lateral tape motion. The geometry of the servo pattern has a major influence on the overall servo system performance and hence on the achievable track density. In the 29.5-Gbit/in2 demo, the basic servo principle described in the LTO standard (ECMA, 2001) was adopted; however, the pattern geometry was modified to enhance the performance of the servo system as illustrated in Fig. 3.10. The influence of the individual servo-pattern parameters on the overall servo performance was analyzed using a model of the servo readback signal that includes the servo reader width W, the azimuth angle a, and the pulse width at 50% amplitude P as parameters (Cherubini et al., 2008, 2009). The primary design goal was to enhance the track-following performance by improving the quality of the position and velocity estimates and by increasing the update rate of the estimates, which enables higher bandwidth control and/or averaging of the estimates. As illustrated in Fig. 3.10, the geometry of the servo pattern can be described by six parameters: the azimuth angle a, the servo stripe distance s, the servo stripe width t, the servo subframe distance d, the number of servo bursts (four in the LTO standard, designated A, B, C, and D), and the number of stripes per servo burst. The geometry of the pattern influences the shape of the readback waveform and the amount of energy and the frequency content contained in the signal. In addition, the shape of the servo readback waveform is influenced by the tape medium properties such as orientation and by the servo reader width W. The lateral position of the head is determined by measuring the distance x between adjacent servo bursts along the longitudinal tape direction and then estimating the lateral position y using the relationship (Barrett et al., 1998): yest ¼ cxest
1 ; 2 tan ðaÞ
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where xest is the estimated distance between two adjacent servo bursts at a lateral position y and c is a pattern-dependent constant. The term 1/tan(a) describes the amplification of the measurement error associated with the measurement of x that results from the relatively shallow azimuth angle a. The quality of the parameter estimates can therefore be improved by increasing a. For example, by increasing a from 6 to 12 , the error amplification is reduced by a factor of about 2. Increasing a, however, has additional implications on the system performance. First, if the other parameters are kept fixed, a larger azimuth angle results in a longer servo frame length and hence a lower update rate and a longer position estimation delay. To partially compensate this pattern extension, the pattern can be compressed by reducing the servo stripe width t and spacing s. Second, increasing the azimuth angle also results in an increase in the dispersion of the readback waveform. This can be compensated for by reducing the servo reader width, as depicted in Fig. 3.11. A narrow servo reader width results in a smaller servo signal amplitude and hence in a lower SNR. Increasing the azimuth angle, reducing the servo stripe width, and reducing the servo reader width result in an energy loss in the servo readback waveform. This energy loss limits the performance gains that can be achieved from the reduced error amplification associated with an increased azimuth angle. To find a good performance trade-off between azimuth angle, servo reader width, and servo stripe width, extensive VHDL level simulations
Figure 3.11 Simulated dibit readback waveform shape plotted as a function of the azimuth angle, reader width, and servo stripe width. The dibit shapes have been calculated using the dibit response model described in Cherubini et al. (2008) and Cherubini et al. (2009). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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of the servo-channel and track-following controller were performed in combination with extensive experiments. This work indicated that despite the energy loss, a significant improvement in track-following performance can be obtained by using an azimuth angle of 18 and a servo pattern that was compressed by a factor of 5/3 relative to the standard LTO pattern. Compression of the servo pattern by a factor of 5/3 does not suffice to compensate for the pattern-length increase due to increasing the azimuth angle to 18 . To compensate for the larger angle and to further increase the servo update rate relative to the standard LTO pattern, the pattern height was reduced to 23.35 mm, a reduction by a factor of 8 relative to the LTO specification. This servo-pattern height enables a reduction of the servo frame length from 200 mm in LTO to 100 mm for the demo pattern, hence doubling the update rate of the estimates. The final step in the servo-pattern optimization process was to adapt the servo-formatting process for the new servo-pattern geometry and the new tape medium. One aspect of this was selecting a suitable pre-erase method to control the magnetization of the media before the servo pattern is written to the perpendicularly oriented BaFe medium, which also determines the orientation of the medium in the regions of the pattern between the stripes. Additional aspects included optimizing the write gap width of the servo format head to achieve the desired stripe width and optimizing the applied servo write current in combination with the pre-erase optimization to obtain a symmetric readback waveform with high SNR. An optical micrograph of the servo pattern written on perpendicular BaFe tape and decorated with ferrofluid is presented in Fig. 3.12. The details of the overall geometry were summarized in Fig. 3.10.
3.6. Synchronous servo channel A typical servo waveform resulting from the readback of the demo servo pattern written on perpendicular BaFe tape and captured with 2.5-mm wide servo reader is shown in Fig. 3.12b. The current tape velocity and lateral position of the head reading the servo patterns are calculated from the relative timing of the servo bursts in the readback waveform. The position and velocity are computed by a digital servo channel that processes samples of the servo signal obtained by an analog-to-digital converter (ADC) operating at a fixed clock frequency. The position measurements are subsequently fed into the track-following servo controller. In this work, a synchronous architecture was adopted for the servochannel implementation (Cherubini et al., 2007). Optimum filtering of the servo signal is achieved using a digital matched-filter interpolator/correlator to minimize measurement noise in the readback signal. As illustrated in the block diagram of Fig. 3.13, the interpolator/correlator is implemented in the synchronous servo channel prior to calculation of the servo parameters,
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Figure 3.12 (a) Optical micrograph of demo servo pattern written on perpendicular BaFe tape and decorated with ferrofluid. (b) Servo readback waveform of demo servo pattern written on perpendicular BaFe medium and readback with a 2.5-mm wide servo reader. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
that is, tape velocity, head lateral position, and detection of the longitudinal position (LPOS) information that is encoded in the servo pattern. Interpolated servo samples are generated at a predetermined rate that is independent of the tape velocity and defined in terms of samples per unit of tape length. This rate is different from the ADC sampling rate, which is specified in terms of samples per unit of time. A timing-base reference unit provides the interpolation instants for a desired interpolation distance. Hence, the matched-filter interpolator/correlator achieves a close approximation of the optimum filter for the detection of the servo signal. The time instants at which the output signal of the correlator exhibits maximum values are used to compute the tape velocity and head lateral position. This correlation method results in a significant improvement in the measurement quality compared with measurements based on the peak arrival times of the servo bursts (aka peak detection). The improved performance can be intuitively understood by noting that the matchedfiltering technique exploits all of the available signal energy or information encoded in the servo waveform rather than only the positive or negative peaks of the signal.
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Figure 3.13 Block diagram of the digital synchronous servo channel. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
The resolution of the synchronous servo channel is on the order of 1 nm for an ideal (noise-free) servo signal and for the finite arithmetic precision of the detection algorithms implemented. By further considering the effects of the finite SNR of the servo readback signal, the noise floor of the position measurements can be estimated to be less 10 nm by simulating closed-loop track-following control, assuming negligible LTM, a proportional-integralderivative (PID) controller, a tape formatted with the 18 demo pattern described in Section 3.5, and an SNR of 28 dB measured at the servochannel input (as measured on the perpendicular BaFe used for the demo).
3.7. Design of the track-following controller The primary function of the tape drive’s track-following controller is to ensure that the write/read elements in the head are positioned on or near the centerline of the data tracks during write and read operations. The performance of the system is typically measured in terms of the position error signal (PES) or the standard deviation of the PES, where the PES is specified as the difference between the desired and the measured head position. The primary disturbance that the control system has to compensate for originates from LTM during tape transport, which causes a misalignment of the read/write transducers relative to the track locations (Cherubini et al., 2007; Pantazi et al., 2010a). The track-following feedback
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controller utilizes the position measurements generated by the servo channel to control the lateral position of the head via a voice coil actuator to follow the lateral tape motion and to keep the read/write transducers centered on a specified track location. A one-degree-of-freedom track-following controller, described by the block diagram shown in Fig. 3.14, was implemented for the demo. In the block diagram, G is a transfer function that describes the dynamics of the head actuator in the lateral direction and is determined by measuring the frequency response of the actuator, which is obtained by applying a chirp excitation to the actuator coil and then determining the position of the head using a laser Doppler vibrometer (Polytec GmbH, Waldbronn, Germany). The controller K was designed using numerical optimization and weighting functions that specify the desired requirements or attributes of the closedloop transfer functions. These results were confirmed by simultaneously measuring the longitudinal head position via the decoded servo pattern at a tape velocity of 2 m/s. Both experimentally obtained frequency responses are presented in Fig. 3.15. The dynamics of the actuator are dominated by the fundamental resonance mode and can be accurately described by a second-order model. The additional delay in the phase response data measured using the servo pattern, relative to the vibrometer data, primarily arises from the position measurement delay, which depends on the servopattern format and the tape speed. The bandwidth requirements for the track-following control system depend on the frequency and amplitude characteristics of the LTM. An approximate measurement of the LTM can be obtained by capturing the decoded position information while maintaining a fixed lateral position of the actuator. Such a measurement also contains effects arising from measurement noise, which is denoted as n in Fig. 3.14. Several factors contribute to the measurement noise in the decoded position signal, including the format of the servo pattern, the method used for detecting and decoding the position information, the characteristics of the servo reader, the analog
Figure 3.14 Schematic diagram of the track-following control system. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Figure 3.15 Head-actuator frequency response measured using a laser Doppler vibrometer (black) and using the decoded servo pattern (blue). A second-order model fit to the data is shown in red. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
front-end of the servo reader, the magnetic properties of the medium including medium noise, defects in the written servo pattern, and imperfections in the servo-formatting process such as written-in velocity noise. Figure 3.16 presents a plot of the power spectral density of open-loop head-position data captured using an LTO-4 tape formatted with the standard 6 azimuth angle LTO servo pattern and using the new perpendicular BaFe medium formatted with the 18 demo pattern described in Section 3.5. Both data sets were captured using a tape speed of 2 m/s. The low-frequency regions of both spectra are dominated by LTM, which is primarily determined by the mechanical component of the tape transport system, the roughness of the tape edge, and the tape speed. Comparing the two spectra in the high-frequency regime, it can be seen that the data captured from the demo pattern exhibit a higher resolution or a lower noise floor because of the larger azimuth angle of the pattern combined with the higher SNR of the new BaFe medium. The H1 control framework (Skogestad and Postletwaithe, 1996) was used to design the track-following control used for the demo. The weighting functions used in the design of controller K were specified to balance the system requirements in terms of disturbance-rejection capabilities and sensitivity to measurement noise and to minimize the track-following error
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Figure 3.16 Power spectral density of open-loop head-position data captured using an LTO-4 tape formatted with the standard 6 azimuth angle LTO servo pattern (blue) and using the new perpendicular BaFe medium formatted with the 18 demo pattern (red). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
(PES) under the experimental conditions of the demo. For the one-degreeof-freedom feedback loop of Fig. 3.14, the transfer function that serves as a measure of the system’s capability to follow the LTM disturbance, that is, that relates the disturbance d to the error e, is given by S¼
1 : ð1 þ GK Þ
The performance weight wp was specified such that the magnitude of S is small in the low-frequency range, where the LTM is largest. The transfer function T¼
GK ð1 þ GK Þ
relates the noise signal n to the output y and hence is a measure of the effect of measurement noise on the output of the controller. The weight wT was specified to shape the transfer function T such that the controller rolls off sufficiently fast at high frequencies and hence to reduce performance degradation arising from measurement noise. An additional weight wu was specified to impose a constraint on the maximum magnitude of the control
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signal u. The objective of the H1 control design methodology is to find a stabilizing controller K by solving the problem: min K kN ðK Þk1
in which N(K) is defined as 2
3 wp S N ðK Þ ¼ 4 w T T 5 : wu KS
In practice, the specified requirements will be met if a controller is found such that kN ðK Þk1 < 1:
For the areal density demonstration, a 7th-order controller was designed using H1 synthesis. The magnitude responses of the controller’s closedloop transfer functions S and T are plotted in Fig. 3.17. The closed-loop transfer function that relates the LTM to the PES exhibits a bandwidth measured at the 3 dB point of approx. 650 Hz.
Figure 3.17 Magnitude responses of the closed-loop transfer functions S and T. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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3.8. Track-following performance Track-following experiments were performed at a tape transport velocity of 2 m/s using the same perpendicular BaFe tape medium as used in the recording experiments described below. The track-following controller described in Section 3.7 was implemented for synchronous operation using the DSP of the experimental platform. The 40-kHz sampling frequency of the controller was determined by the 2-m/s tape speed and the 50-mm distance between position estimates derived from the demo servo pattern. A 7.3-s long capture of the closed-loop PES that is representative of the performance achieved is plotted in Fig. 3.18a. Two metrics were used to assess the fidelity of the track-following performance: (1) the standard deviation of the PES (sPES) and (2) the 99.9% cumulative distribution function (CDF99.9) of the PES. The CDF99.9 is a measure of the distance
Figure 3.18 (a) Plot of a 7.3-s capture of the closed-loop PES demonstrating a standard deviation of 23.4 nm. (b) CDF99.9 of the PES data plotted in (a). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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within which 99.9% of the magnitude of the position error estimates fall. For a Gaussian distribution, CDF99.9 ¼ 3.3s. Figure 3.18b presents a plot of the CDF99.9 of the data plotted in (a). The data exhibit a CDF99.9 of 87 nm and a standard deviation of 23.4 nm. For single-channel recording, in which head tolerance and tape dimensional stability effects can be neglected, the minimum track width (TW) that can be supported with a reader width RW and the achieved track-following fidelity can be estimated using the model described by the Information Storage Industry Consortium (INSIC, 2008): pffiffiffi TW ¼ 2 2 CDF99:9 þ RW
or pffiffiffi TW ¼ 2 2 3:3sPES þ RW
if the PES data exhibit a Gaussian distribution. If we now assume a reader width of 0.2 mm, as used in the recording experiments described in Section 3.9, and track-following performance exhibiting a CDF99.9 of 87 nm, as measured from the data of Fig. 3.18, we can estimate the minimum reliable track width to be 0.446 mm and an achievable track density of 57 ktpi.
3.9. Read channel and recording performance Magnetic-recording channels that use BaFe particulate tape exhibit distinct characteristics in their system responses and noise properties relative to those based on MP tape. For example, the dibit response of a tape channel that uses longitudinally oriented MP tape is plotted in Fig. 3.19a and can be compared with the dibit responses of two recording channels using a non-oriented BaFe tape (tape B) and a perpendicularly oriented BaFe tape (tape A) that are plotted in Fig. 3.19b. Compared with the MP dibit response, the non-oriented BaFe medium exhibits a relatively asymmetric dibit shape and the perpendicular BaFe medium exhibits a strong initial undershoot, both of which are features that result from the specific orientation of the BaFe particles. It can also be observed in Fig. 3.19 that the dibit responses from the BaFe tape samples decay more rapidly than that from the MP media and thus have more high-frequency content. The pulse can be further characterized by the PW50, defined as the pulse width at 50% peak amplitude. Fitting a Lorentzian approximation to the BaFe dibit response results in a best match of PW50/T 2.1 for both BaFe particle orientations at a linear density of 343 kbpi, where T is the bit duration. Note that only uncoded data were used in the experiments presented in this section and that the linear densities have therefore been normalized using the LTO-4 channel linear density
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Figure 3.19 Dibit responses of (a) an LTO-4 recording channel with MP tape captured using a GMR read head with 3.8 mm reader width and 0.18 mm shield-to-shield gap length and (b) the dibit responses of recording channels using non-oriented and perpendicularly oriented BaFe tape, both captured using a GMR read head with 0.2 mm reader width and 0.08 mm shield-to-shield gap length. For all three dibit responses, the linear density was 343 kbpi and no write equalization was employed. On the scale of the y-axis, T denotes the bit duration. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
of 343 kfci (after modulation coding) rather than the user linear density of 323 kbpi. Increasing the linear density to 600 kbpi results in a best fit of PW50/T 3.6 for both BaFe particle orientations. Also, the write equalization scheme typically used with MP tape (a spectral shaping technique that employs preemphasis to boost the high-frequency content in the MP recording channel) is not well matched to the channel response of BaFe tape and hence is not appropriate for this medium. In fact, the use of write equalization with BaFe tape leads to a loss of signal energy through the recording channel and hence to a performance degradation. As mentioned, MP and BaFe particulate media also exhibit differences in the noise components in their readback signals. Specifically, the ratios of stationary electronics and medium noise to nonstationary data-dependent noise vary significantly between the MP tape and the two tapes based on BaFe particles. The differences can be quantified using a noise decomposition technique described in Pozidis (2004). The results of such a study are presented in Fig. 3.20, in which the linear density has been normalized relative to the LTO-4 linear density of 343 kbpi. At the relative linear density of 1, nonstationary noise for the LTO-4 MP medium contributes about 70% of the total noise power, whereas for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe), nonstationary noise contributes only about 25% and 30%, respectively, to the total noise power.
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Figure 3.20 Contribution of data-dependent noise to the total noise power for the MP (LTO-4) and the non-oriented and perpendicular BaFe tapes. The recording conditions were the same as for Fig. 3.19. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
Although the fraction of nonstationary noise exhibited by the two BaFe tapes is relatively smaller than that for the MP tape, the nonnegligible contribution to the total noise power still warrants the use of datadependent noise-predictive detection techniques, as discussed later. Note that the increase in data-dependent noise power at higher frequencies observed for the BaFe may result partly from increased jitter in the timing phase of the signals used for noise decomposition. The power spectral densities of the total noise for tape B and tape A at two linear recording densities are plotted in Fig. 3.21. Compared with tape B, tape A exhibits a significantly lower total noise power owing to the combination of the perpendicular orientation of the particles, the reduced particle volume, the improved particle dispersion, and the reduced surface roughness of the tape. To determine the maximum useful linear recording density that can be achieved with the most advanced formulation of the BaFe medium (tape A), a single track of data was recorded in a loop tester using a write transducer having a saturation magnetic flux density of 18 kG and write gap length of 0.17 mm. Readback of the data was performed using an ultranarrow 0.2-mm GMR reader having a shield-to-shield gap length of 0.08 mm. No tracking control of the read head was applied during the readback process; however, the written track was wide enough relative to the reader to ensure that there
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Figure 3.21 Power spectra of the total noise power of the readback signals for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe) at (a) 343 kbpi and (b) 600 kbpi. T denotes the bit duration and f is the frequency. The dip observed at 0.8 and the peak between 0.8 and 1 are artifacts due to aliasing that results from the 5/4 sampling rate used to capture the readback signals, which were then resampled at a fivefold higher rate. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
was no signal degradation due to track misregistration during readback. The written data consisted of a repeating pseudorandom binary sequence of length 255, which was recorded at linear densities ranging from 343 to 600 kbpi. A software read channel that implements all functions of an actual indrive read channel was used to process the captured readback waveforms. Note, however, that unlike an in-drive hardware read channel, the software channel uses full precision for the signal and coefficient representations. The data-detection schemes were optimized to minimize the BER at the output of the detector, despite the relatively low channel SNR values that resulted from the ultranarrow 0.2-mm reader and the high linear recording densities. To maximize the linear recording density and still achieve our target BER of 104 at the output of the detector, it was necessary to move away from the traditional approach of extended partial-response class 4 (EPR4) signal shaping and detection and implement a read-channel and detector design based on “generalized” partial-response signaling. In this approach, the target response of the overall channel can be chosen such that a good match to the actual response of the recording channel is ensured while effectively whitening the noise process and reducing its power at the input of the detector. This objective can be achieved using NPML detection
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(Chevillat, Eleftheriou and Maiwald, 1992; Coker et al., 1998; Eleftheriou, 2003; Eleftheriou and Hirt, 1996). The detection performance can be further improved by extending the noise-prediction concept implemented in NPML sequence detection to take into account the data-dependent nature of some of the distortion components. The notion of handling time-varying nonlinear and linear distortion, which encompasses data-dependent distortion, was introduced by Eleftheriou and Hirt (1996), Coker et al. (1998), and Eleftheriou (2003). In this approach, a whitening filter is implemented in the forward path and as input to the sequence detector, whereas the data-dependent distortion is handled in a RAM-based NPML detector by updating the contents of the RAM lookup table with known adaptation techniques. Past decisions taken from the RAM-based NMPL detector form a binary address for the RAM, with the addressed value being used to compute the branch metric. In this way, together with predictor coefficients, the contents of the RAM can be updated to provide near-optimal performance in the presence of linear, nonlinear, and/or data-dependent distortion. Caroselli et al. (1997) proposed an extension of NMPL that uses a plurality of noise-prediction filters that are obtained conditioned on data patterns taken from the path memory of the NMPL detector. In this approach, also referred to as data-dependent NPML (DD-NPML), the branch-metric computation of the NPML sequence detector involves data-dependent noise prediction such that the predictor coefficients and prediction error both depend on the local data pattern. This approach was developed further and analyzed by Kavcic and Moura (2000) and Moon and Park (2001). An overview of the generic NPML read-channel architecture and its various instantiations can be found in Eleftheriou (2003). For the NPML and DD-NPML detectors used in this work, a target polynomial of the form F ð D Þ ¼ 1 D 2 1 p1 D p 2 D 2
was used. For the DD-NPML case, the coefficients p1 and p2 of the noiseprediction filter are data-dependent. Equalization of the readback signal towards the (1 D2) class-4 PR target has the advantage of placing a spectral null and hence mitigating noise components at both dc and the Nyquist frequency. The 4th-order target polynomial F(D) leads to an NPML detector with 16 states. For the DD-NPML detector, the length of the data pattern on which noise prediction is conditioned determines the number of prediction filters needed. Here, it was assumed that data patterns consist of 5 consecutive channel bits, and therefore, a separate noise-prediction filter was implemented for each of the 32 branches in the detector trellis. Although it is possible to further improve the performance of DD-NPML detection by
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using even higher-order target polynomials, this approach was not pursued in order to limit the evaluation to detectors that are realistic in their complexity of hardware implementation for multichannel parallel recording. In Fig. 3.22, the SNR at the detector input for an EPR4 read-channel is plotted as a function of the relative linear density for tape B (non-oriented BaFe) and tape A (perpendicularly oriented BaFe). Comparing the two data sets indicates that tape A provides a substantial improvement of up to 2.9 dB in available SNR relative to tape B. In Fig. 3.23, the BER performance data versus the normalized linear density for the two BaFe tapes are plotted for 8-state EPR4, 16-state NPML, and 16-state DD-NPML detection. Using EPR4 detection and a relative linear density of 1.4, the BER of tape A (perpendicularly oriented BaFe) is more than 40 lower than that of tape B (non-oriented medium). The detection performance is significantly improved by using 16-state NPML and even more so when using 16-state DD-NPML. For the postdetection BER target of 10–4 used in this demonstration, 16-state DD-NPML allows operation at a relative linear density of up to 1.51, corresponding to a linear density of 518 kbpi. Combining this result with the potential for operation at a track density of 57 ktpi demonstrated in Section 3.8 leads to the conclusion at an areal recording density of up to 29.5 Gbit/in2 can be supported by the perpendicular BaFe media.
Figure 3.22 SNR versus relative linear density of tapes A and B at the output of an EPR4 read channel. Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
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Figure 3.23 BER performance versus relative linear density for tapes B and A obtained using 8-state EPR4 (circles), 16-state NPML (diamonds), and 16-state DD-NPML detection (squares). Reprinted with permission from Cherubini et al. (2011) © 2011 IEEE.
3.10. Summary of demo results Using flexible tape media based on ultrafine, perpendicularly oriented BaFe particles, an areal recording density of 29.5 Gb/in2 was demonstrated on a single recorded channel. Specifically, using a 0.2-mm wide GMR reader, a postdetection BER target of 104 was achieved at a linear density of 518 kbpi using a 16-state DD-NPML read channel. In addition, a state space-based track-following controller, a novel synchronous servo channel, and a new servo pattern were described, which in combination with the improved SNR of the BaFe media allowed a track-following performance with a 23.4-nm standard deviation of the track-following error to be achieved. The minimum track width supported by this degree of trackfollowing fidelity and a 0.2-mm wide reader is 0.446 mm, corresponding to a track density of 57 ktpi. The combination of these two results leads to an areal density of up to 29.5 Gb/in2. Note that this was a single-channel recording demonstration that focused on the performance of the head/ medium/read-channel combination and on the performance of the trackfollowing servo system. Effects, such as erase bands between tracks and the side-reading behavior of the read sensor that will likely become important at submicron track widths, were not considered. Moreover, head tolerances and tape dimensional stability, which are important in parallel-channel tape recording, were not considered.
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4. Continued Future Scaling Potential of Magnetic Tape Technology 4.1. Introduction In this section, we examine the feasibility and technologies required to continue scaling parallel-channel linear magnetic tape-recording systems towards a target operating point of 100 Gbit/in2. Extrapolating from the latest 2012 INSIC Magnetic Tape Storage Roadmap (INSIC, 2012), such an operating point would enable the continued scaling of tape cartridge capacities at historical rates of doubling the capacity every 2 years, up to the 2024–2025 time frame. The current state of HDD technology and demonstrations of tape areal densities including both helical scan and linear tape drives (Berman et al., 2007; Childers et al., 2003; Dee, 2006; Matsumoto et al., 2006; Matsunuma et al., 2011; Motohashi et al., 2007; Nagata et al., 2006; Ozue et al., 2002; Xiao et al., 2006) provide important insight into critical technology choices and highlight key parameters that must be considered for further advances in areal density. In contrast to many linear tape areal density demonstrations reported in the literature that used only a single channel, here, we will also consider the effects on the achievable areal density that arise from the parallel recording nature of linear tape drives, such as from tape dimensional stability. To establish potential operating points to achieve an areal density of 100 Gbit/in2, we take as a starting point the operating point of the 29.5-Gbit/in2 demo, which used a linear density of 518 kbpi and track density of 57 ktpi corresponding to a track width of 446 nm. To achieve 100 Gbit/in2, we need to scale the linear and the track density by a combined factor of 3.4. The simplest approach would be to divide the required scaling equally between the track and the linear density. However, in this areal density range, such an approach is not optimal because the penalty in terms of SNR loss of linear density scaling is much larger than that of track-density scaling. For example, doubling the track density can be achieved by reducing the reader width by a factor of two and also reducing the tracking margin by a factor of two. For a given medium, reducing the reader width by a factor of two results in a loss in SNR of 3 dB, whereas doubling the linear density results in a much larger loss in SNR (see Fig. 3.23). This loss in SNR must be compensated by improvements in the read head and the tape medium and by improvements in the data channel that facilitate reliable operation at lower levels of SNR. It is therefore more favorable to scale the track density more aggressively, despite the additional need for scaling the tracking margin this entails. Here, we consider two linear density options: (1) a modest linear density increase to 650 kbpi and (2) a more aggressive increase to 800 kbpi. If we
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assume a reader width of 100 nm for both cases, we can use the data of Fig. 3.23 to estimate the required uplift in SNR over the head/medium combination used for the 29.5-Gbit/in2 demo to be 6 dB and 11 dB for option 1 and 2, respectively. At these linear densities, an areal density of 100 Gbit/in2 then requires a track pitch of 165 nm for option 1 and 200 nm for option 2. The first option requires more aggressive scaling of the tracking margin; however, the required uplift in SNR is more likely to be achievable using low-cost BaFe-based media than with the second option. The second option, which somewhat relaxes the requirements on tracking margin, requires a much larger performance improvement in the SNR of the medium and hence would likely require a transition to another media type such as sputtered media.
4.2. Write and read-head technology Unlike HDDs, which operate a single write/read head at a time, the recording heads in modern linear tape drives use arrays of write and read transducers operating simultaneously to enable not only high-speed writing and reading but also read-while-write verification of the written data. During write operations, the data are immediately read and verified by a set of read transducers placed downstream of and aligned with the write elements. Operating multiple parallel write/read channels in tape systems is necessary to achieve data rates that are competitive with HDDs because, compared with disk drives, tape moves past the head at a lower velocity (4–6 m/s for tape vs. more than 14 m/s for disk) and data are recorded to tape at 3–4 lower linear density. The use of multiple parallel transducers also limits the time required to write or read an entire half-inch tape cartridge to a few hours. In addition, it also reduces the number of passes required to write or read back a full cartridge, which in turn reduces media wear. The time and number of passes required to fill a cartridge has not changed much since tape cartridges were introduced more than 25 years ago, because of the gradual increase in the number of parallel channels used. Hence, a future paradigm in which the data rate does not scale with capacity will result in longer fill times. In addition, keeping the number of transducers fixed combined with aggressive track-density scaling, which will likely be needed to achieve future capacity increases, will result in a large increase in the number of passes to fill a tape and hence an increase in tape durability requirements and wear issues. The large investment in cartridges and tape library automations and the need for maintaining a cost advantage over disk drive-based libraries provide strong incentives to preserve the existing halfinch tape and cartridge formats. Thus, a challenge in the design of future tape systems and operating points is to increase the areal density and tape cartridge capacity while maintaining the current cartridge form factors and
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preserving or adapting the expectations of tape user in terms of backup and restore times. A cartridge containing a 1200-m long tape recorded at 100 Gbit/in2 will hold approx. 180 TB of data, that is, more than 80 the capacity of an LTO-5 cartridge, assuming the same system overheads and coding efficiencies as in LTO-5. Having 32 active write heads—the maximum today in a commercially available drive—and 60,000 data tracks, a simple calculation shows that 1875 one-way passes would be required to fill this cartridge. At 6 m/s tape speed, this translates into more than 100 h! This could force a change in backup management strategy. Beyond data-rate considerations, there may also be challenges associated with managing a library composed of such large individual objects. Doubling both tape speed and the number of channels reduces the fill time to 25 h, a value still much in excess of what is currently likely to be acceptable. Increasing the tape speed is challenging because of media-handling issues, such as the need to improve tension control as tapes become thinner, and air entrainment effects, which render reel-to-reel control more challenging at high speeds. In addition, LTM, which limits track-following fidelity and hence the achievable track density, also tends to increase with tape speed. Moreover, there remains an ongoing need to increase the number of active channels regardless of the tape speed to limit increases in the number of passes required to fill a cartridge. However, simply increasing the number of active transducers will be challenging and is not sufficient for scaling to 100 Gbit/in2. As will be discussed below, transducer dimensions will have to be shrunk to enable writing smaller tracks at higher linear density. In addition, the span occupied by the transducers must be reduced to reduce the effects of tape dimensional stability (TDS), which may improve only to about 100–200 ppm from about 800 ppm in current LTO tape cartridges. This final requirement for reducing the head span may potentially be relaxed if a scheme to measure and actively compensate TDS effects can be implemented. In any case, in the future, more transducers will have to fit into the current or a smaller span, leading to the need for smaller transducer dimensions. To illustrate the future scaling requirements of write and read transducers, we first consider the span of transducers in an LTO-5 head, which consists of two symmetrical halves, called “modules.” As mentioned, two such modules are required to read-verify data in real time as they are written. Each module consists of 16 write, 16 data read, and 2 or more dedicated servo read transducers. The write and data read transducers are piggybacked, as in disk heads, and the pitch (distance between the centerlines of adjacent write/read pairs) is 166.5 mm. The distance or span between the outermost pairs is (16 1) 166.5 mm or about 2.5 mm. A schematic cross section of a pair of write transducer is shown in Fig. 3.24. The spacing between the elements in Fig. 3.24 has been reduced
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Figure 3.24 Schematic of a cross-sectional (half-symmetry) view of a pair of magnetic write transducers. The yokes (green, purple, and blue) are sectioned to show front and back gaps and the yoke length. The coils (red) are shown nonsectioned for clarity. The spacing in this example schematic has been reduced relative to the LTO-5 spacing of 166.5 mm so that two transducers can be shown at a reasonable scale on the same page. Reprinted with permission from Argumedo et al. (2008).
relative to that of an LTO-5 head so that two writers can be displayed at a reasonable scale and to highlight a key difference between tape and HDD heads, which have only a single write transducer. An environmentally induced 200 ppm change in tape width between writing and subsequent readback could produce up to 0.25 mm of misregistration between an ideal head with a span of 2.5 mm and the tape, assuming tracking is done in reference to the center of the head span. This is more than the entire projected track width for the 100-Gbit/in2 operating point. To reduce this to a reasonable fraction of the tracking budget, an at least threefold reduction in the head span will be needed. Considering further that at least 64 active channels will be required to support continued data-rate scaling, the transducer centerline spacing must shrink to 14 mm, as shown schematically in Fig. 3.25b and (c). At this spacing, writers may interfere, or “cross talk,” a phenomenon in which the magnetic field produced by one write transducer alters the magnetic fields produced by its nearest neighbors (Biskeborn et al., 2008). To minimize such effects, the writer must be designed such that the yokes do not saturate, particularly in the portion wound by the coils, referred to as the “back-gap” region, as this can divert stray flux into neighboring write
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Figure 3.25 Schematic illustrations comparing the layouts and geometries of current and potential future write heads. All dimensions, including the transducer pitch, write pole width, yoke length, and back-gap width, are shown in microns. (a) Schematic of an exemplary LTO-5 write transducer consisting of two coil layers (only 1 shown), with seven turns in each layer. The pitch between write transducers is 166.5 mm; thus, only one transducer fits into this figure at the scale used. (b) Schematic of a future write transducer at the same scale having three coil layers (only 1 shown), with two turns in each layer. The pitch between transducers is 14 mm, which enables 64 writers (4 LTO-5) to fit in 1/3 of the LTO-5 span. (c) Cross section of the layout shown in (b). Reprinted with permission from Argumedo et al. (2008).
gaps. This can be achieved by using high-Bs pole materials and wide back gaps (wider than top pole). A target residual track width of 0.2 mm can be achieved by shingle writing with a front pole width in the range of 0.5–1.0 mm. Such a writer requires a back gap of at least 2 mm to avoid the saturation effects mentioned earlier. This leaves a distance of approximately 12 mm between the edges of neighboring write yokes, which is sufficient to accommodate two coil turns on a 1.75-mm pitch, assuming a conventional pancake coil design. These coil dimensions are within the capabilities of current thin-film microfabrication technology (see Fig. 3.25b and c). Crosstalk effects due to coil proximity are expected to be small compared with effects arising from magnetic stray field coupling between yokes. Therefore, traditional design methodology would suggest that the coils in tape write transducers at this pitch will be limited to having between 2 (1 layer) and 6 (3 layers) turns.
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Scaling future tape writers to dimensions that limit the coil design to a maximum of only 2–6 turns creates additional challenges. Although modern HDD write transducers are typically designed with 3–5 coil turns, current tape writers typically use 8–14 turns. This discrepancy arises from a combination of factors. In disk drives, current-mode write drivers are mounted on the actuator arm in close proximity to the head, and therefore, cable impedance is not critical. In addition, HDD write drivers, which supply only a single channel at a time, can supply more than 100 mA to the writer coil, which in turn enables a sufficient write field to be produced with fewer turns (write field is proportional to the number of turns times the current). In tape drives, the write driver chips are located on a printed circuit board (PCB) with the other drive electronics at the fixed end of the flex cables, because of heat dissipation issues, space requirements of multiple parallel channels, row-access-strobe latency, and other restrictions. The flex cables connecting the head to the PCB must be long enough to enable a sufficient range of motion such that the heads can access the entire half-inch width of the tape. Typical flex cable lengths are greater than 10 cm, and hence, cable impedance becomes critical. As a result, voltage-mode write drivers, in which the steady-state write current is limited by a series resistance, are typically used for tape. This in turn reduces the maximum available write current and increases the minimum required number of coil turns. Reducing the operating voltage range of the write process can potentially enable the use of series write resistors in combination with a lower cable impedance, which ultimately affects the minimum acceptable turn count. Impedance matching the driver, flex cable, and head can also be beneficial to minimize the 10–90 rise time. However, up to 50% overshoot has been used in both tape and disk systems, although for different reasons. In tape, the relationship between overshoot and error rates, especially for thinner magnetic coatings and non-oriented particulate tapes, is a subject of active research and debate (INSIC, 2005). Another important consideration is that lowering the write voltage results in less power dissipation in the write driver and series resistors. This is of particular importance for 32 or potentially even 64 simultaneously active writers. Even without this benefit, lower-voltage operation may become a necessity as higher-voltage mixed analog–digital CMOS technologies approach the end of life and are replaced by lower-voltage and higher-integration technologies. One potential solution to all of these design challenges would be to develop a write driver that can be mounted on the flex cable near the head, as is done in HDD. Here, the main challenge is in designing a sufficiently low-power write driver such that the waste heat from 32 or more parallel channels can be dissipated by means of innovative cooling solutions. Another factor that can potentially limit write-head coil turns is electromigration, which imposes a limit on the maximum acceptable current owing to reliability concerns arising from the small cross-sectional area of the coil, which leads
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to high current densities. Continued scaling of the areal density imposes the need for media with higher coercivity, which in turn require larger head fields, leading to the need for more amp turns for a given head design. However, as the linear density is scaled, media coatings are expected to become thinner, enabling the use of smaller write gaps. This in turn results in an improved head efficiency, which partially alleviates the increased current requirements. In summary, six coil turns appear to be an aggressive but potentially workable lower bound. Hence, a 14-mm scaled transducer pitch is potentially feasible, but clearly will be very challenging to achieve. A trend in writer design for both HDD and tape systems has been to reduce the yoke length to decrease eddy-current effects and hence to enable faster switching and higher data rates. We anticipate that it will be necessary to scale the yoke length from the about 25 mm currently used in tape writers to less than 10 mm as tape systems are scaled towards an areal density of 100 Gbit/in2. In the context of yoke length, pancake coil writer designs have the advantage of coil-turn stacking, but as described, a 14-mm pitch would accommodate at most two coil turns per layer. Thus, an 8-turn head would require at least four layers, which would be complex to fabricate and could still have a longer than desired yoke because of the height of the coils stack. In contrast, a helical coil writer design could potentially accommodate an arbitrary number of turns, at the cost of increased yoke length. An alternative to the current approach of using a linear array of writers is to build tiers of writers on two or more planes stacked on top of each other. For example, using two tiers would enable the fabrication of writers capable of writing tracks at half the pitch between writers in a given plane, that is, a 14-mm effective pitch could be achieved using two tiers of writers built on a 28-mm pitch. Unfortunately, this approach has device yield issues and tierto-tier alignment challenges, but does offer a route to continued scaling of the head span and the number of parallel channels. Another important consideration is that multiple transducer tiers result in a wider mechanical gap, that is, the gap between the hard ceramic portions of the head (Biskeborn and Eaton, 2003). Such an increased gap leads to greater susceptibility to wear-induced recession of the thin-film transducers, leading to increased magnetic spacing. The main challenge in scaling tape read transducers for operation at areal densities approaching 100 Gbit/in2 relates to maintaining sufficient output signal as the dimensions of the transducer are being shrunk. The issue of achieving tight centerline pitches, which is problematic for writers, is much less of a problem for readers because of the relatively smaller area occupied by a read sensor. In fact, readers with centerline pitches down to 9 mm (Tamakawa et al., 2006) have already been successfully demonstrated. In general, a read-head output of at least 1 mV is desirable to render electronic noise contributions to the overall SNR negligible. More
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quantitatively, a 1-mV output signal is roughly 10 larger than the nonmedium-related electronic noise, including the Johnson noise in the sensor itself, given by Vn [4kBTRDf]½. Here, kB is the Boltzmann constant, T is the temperature in Kelvin, R is the sensor resistance, and Df is the bandwidth of interest. At an areal density of 100 Gbit/in2, the width of the readers will have to be reduced to dimensions on the order of 0.08–0.1 mm. In addition, the shield-to-shield spacing will have to be optimized to match the linear density and is expected to be less than 0.08 mm. To better understand how head output scales with the sensor dimension, it is instructive to first consider an anisotropic magnetoresistive (AMR) head. In an AMR head, the sensing layer resistance change dR/R is typically on the order of 0.023. The sensor resistance, R, is proportional to WMR/tMRhMR, where WMR, tMR, and hMR denote the sensor width, thickness, and height, respectively. Hence, the reduction in head output that results from reader width scaling can be partially compensated by reducing the sensor free layer thickness tMR. However, below a thickness of about 13 nm, the output is no longer inversely proportional to the free layer thickness, but instead increases more slowly because the relative contribution of electron scattering at the free layer surfaces increases. That is, the resistance increases as the thickness is reduced; however, dR does not increase in the same proportion. The reduced head output that arises from reader width scaling can also be partially compensated by optimizing the sensor height, hMR. Continued linear density scaling will drive the need for thinner magnetic layers and reduced shield-to-shield spacing, both of which will contribute to a reduced signal amplitude. In principle, this amplitude reduction can be compensated by increasing the sensor bias current; however, this will raise the sensor temperature, which increases the Johnson noise and also leads to increased tape asperity cooling noise. Moreover, if the current is increased too much, the sensor lifetime and reliability will be degraded. GMR sensors have a sensitivity that is approximately 5 larger than that of AMR sensors, and the most recent linear tape drives use this technology. As mentioned, scaling to linear densities on the order of 100 Gbit/in2 will require reader widths on the order of 0.08–0.1 mm. Tunneling magnetoresistive (TMR) sensor technology provides an even higher sensitivity than GMR, with dR/R values of up to 80%. The HDD industry, which currently uses TMR sensors, made the transition from GMR to TMR technology around a sensor width of 0.15 mm. It is therefore likely that tape-drive manufacturers will make a similar transition. However, considering the relatively small volumes of tape heads produced each year, tape-drive manufacturers may transition to TMR technology earlier so that there is no need for head manufacturers to maintain a separate technology platform for the relatively small number of tape heads produced each year. In any case, it is likely that TMR reader technology will be required for areal densities approaching 100 Gbit/in2 and beyond.
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Another promising technology to enable the continued scaling of linear tape systems is the so-called planar head technology (Engelen et al., 2012), in which transducers are fabricated with the critical recording gaps orthogonal to the wafer surface. This is in contrast to conventional tape and disk heads, which are sliced and lapped to expose the recording gaps formed between the layers of the wafer. The main advantage of planar technology is that it enables the staggering of writer arrays to reduce the effective pitch between simultaneously written tracks and hence enables a reduced head span. In the extreme, planar heads could enable the simultaneous writing of bundles of adjacent tracks, which would minimize tape dimensional stability issues and eliminate track-squeezing effects that occur during shingle writing due to track-following errors. This means that adjacent track writing does not suffer from the wavy-track effect, that is, that residual shingled track widths vary down the length of the tape. Planar write heads have already been successfully demonstrated; however, planar read-head fabrication processes have yet to be developed. The lack of a planar reader technology gives rise to another challenge inhibiting the use of planar writers: the need for accurate track placement. Ideally, this is achieved using servo readers next to the write transducers. This could potentially be achieved using a hybrid of a planar writer head and a conventional read head complete with servo readers. However, in conventional heads, the position of the servo readers relative to the writers is very accurately controlled because of the accuracy of the lithographic processes used for fabrication. A hybrid head would therefore suffer from degraded position tolerance of the servo readers relative to the writers, leading to less accurate track placement. If the technologies to enable adjacent track recording can be developed, then there is an additional opportunity to improve the read process itself using arrays of readers to perform the so-called electronic track following. Here, the idea is to use arrays of readers that may bridge into neighboring tracks during readback. A multiplexer could then be used to select the reader that is most centered on the desired track at any given time. An alternative approach is to use the side information from readers aligned with the neighboring tracks to perform adjacent track interference cancellation. A final challenge of future tape heads arises from the read-while-write verification functionality of linear tape drives mentioned earlier. Historically, this function is enabled using an array of readers that are rigidly attached to the write head. These readers are positioned downstream and “in the shadow” of the active writers. As track widths are scaled to smaller dimensions, variations in the angle of the tape relative to the head, referred to as tape skew, make it increasingly more challenging to maintain the alignment of the read-verify head with the written tracks. The sensitivity to tape skew can be reduced by decreasing the distance between the writers and readers; however, there are limits to how much it can be reduced because of challenges in assembling the heads and the need for sufficient isolation of the reader from the active
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writers. A solution to this problem is to implement some form of dynamic skew compensation. One approach to achieve this is to implement a two degrees of freedom actuator (track following and skew following) and a second control loop used for skew cancellation. Another approach is to use two independent track-following actuators and control loops to control the position of the write and read-verify heads independently. A third approach is to build actuators into the tape guides upstream and downstream of the head and to perform active tape guiding to offset LTM and skew effects as will be discussed in Section 4.6.3.
4.3. Head–tape interaction As discussed in Section 3.3, the tape–head magnetic spacing will have to be continually reduced to continue scaling the areal density in magnetic recording systems. In addition to reducing the spacing, the head–tape interface must also be designed to minimize not only tape–head friction and wear but also chemical degradation of the magnetic elements. A critical problem in tape-recording systems is the signal degradation that occurs over the lifetime of a drive due to the increase in magnetic spacing as the magnetic elements wear down by as much as 40 nm below the plane of the tape-bearing surface. For operating points approaching 100 Gbit/in2, this head recession will have to be limited to 10 nm or less over the lifetime of the drive. In the past, diamond-like carbon (DLC) coatings have been applied to metal-evaporated tapes to improve their wear robustness. However, this is impractical and likely too expensive for use with MP tapes. In the HDD industry, ultrathin DLC coatings are used to protect the head and disk from occasional accidental contact. Currently, research efforts are ongoing to investigate the application of DLC coatings to tape heads (Rismani et al., 2011, 2012). However, the contact nature of tape recording and the associated high levels of wear combined with the need to minimize coating thickness to minimize spacing losses have limited progress in tape head coating. Moreover, because magnetic, chemical, and physical requirements limit the material choices for heads, designing for wear resistance has so far been relatively unfruitful. As mentioned, reducing the head closure gap can potentially reduce head recession, but the finite thickness of the stack of materials needed to build read and write transducers puts a limit on the minimum spacing. Finally, there is potential to reduce the abrasiveness of the media itself by making it smoother; however, this leads to increased start-up and running friction. If a head recession of 10 nm or less can be achieved, then a target total magnetic spacing of 20 nm is achievable by reducing the tape roughness to 10 nm or less. This target of 20-nm magnetic spacing is required for a target linear density of 800 kbpi. From a tape-manufacturing point of view, decreasing the tape roughness is relatively straightforward. However,
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decreasing the tape surface roughness increases the actual tape–head contact area, leading to increased adhesion and frictional forces (Bhushan, 2003). The static friction that has to be overcome to start moving tape over the head, called stiction, is generally much higher than the friction while the tape is in motion (referred to as running friction). When the tape is stopped, the tape deforms into local contact with the head and liquids condense or migrate into any narrow gaps in the tape–head contact, resulting in high stiction relative to the running friction. Clearly, friction and stiction must not be so high that they cause tape damage or stalling of the tape. However, even lower levels of friction can distort the data signal timing and degrade error rates, as discussed in Section 3.3. The challenges associated with high tape–head friction are further compounded by the trend to decrease tape thickness to improve the volumetric storage density for a given areal density. The 2012 INSIC tape roadmap (INSIC, 2012) predicts that tape thickness will be scaled from its current value of about 6 to 4 mm by 2022. This reduction will likely make tape more fragile and hence more susceptible to damage due to stiction and friction. These basic challenges have at least been partially addressed by the beveled head technology described in Section 3.3. Moreover, the effectiveness of this approach at reducing friction and stiction can be further improved by reducing the span of the head, which enables a larger fraction of the tape to be supported by an air bearing. In modern tape drives, the tape is in partial contact with the head, supported by the tips of the asperities of the tape roughness. Transport of the tape over the head and tape guides can generate small particles due to wear. The sharp leading edge of the head keeps particles from the tape–head interface, and loose particles that accumulate on the edge are brushed from the head during tape loading. Occasionally, a mildly abrasive cleaning tape is also used to remove any adherent debris from the head. Modern tapes also use lubricants to reduce tape–head friction. The tapes are fabricated with an excess of mobile lubricant molecules absorbed in the tape undercoat (between the base film and the magnetic coating), forming a reservoir for replenishing the tape surface with lubricant. Friction between the tape and head can be large even at zero applied normal load, because short-range surface forces pull the smooth flexible tape into contact with the head. Protruding asperities on the tape surface are designed to hold most of the tape surface away from the head; however, the overall attraction of the tape surface partially flattens the asperities, which in turn leads to an increase in the attractive force. Thus, adhesion and attraction increase quite rapidly as the roughness is decreased to maintain a low magnetic spacing. Mobile liquids on the tape surface or in the atmosphere can also contribute to attraction and increased friction through capillary condensation. Liquids with a large contact angle tend to wet the area around contacting asperities, bridging the narrow gaps between head and tape near the asperity.
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The tape is attracted to the head, because the total surface energy decreases as the surfaces move closer and wetting increases. This problem becomes worse in high-humidity environments. For example, for a pair of hydrophilic (high-contact angle) surfaces exposed to an 80% humidity environment, condensation can fill a 4.6-nm gap. This humidity sensitivity can be prevented by designing the tape surface, the head surface, and the lubricant systems to be hydrophobic, that is, with water contact angles >90 . In summary, friction scales poorly with the decreasing roughness and tape thickness needed for continued capacity scaling. The fundamental problem is that, as the surface roughness is reduced to decrease the magnetic spacing, the energy required (per nominal surface area) to deform the surface to flatness decreases, whereas the energy available from surface interactions is constant. One strategy to reduce the head–tape contact area and hence friction is to minimize the size of the head. In the lateral tape direction, this can be achieved using the beveling technique mentioned. In the tape-travel direction, the length of the head is already relatively small. For example, for the head described in Biskeborn and Eaton (2003), the entire tape contact length is only 1.2 mm. Unfortunately, this length cannot be reduced much further without disrupting the ability to maintain a small head–tape spacing. Thus, the microscopic design of the head–tape interface will be essential for controlling friction as the magnetic spacing is reduced in the future. The Young’s modulus of a particulate tape coating is about 1010 N/m2. This is sufficiently compliant for significant deformation to take place, leading to increased contact area and friction—even if the surfaces have low surface energies and in the absence of liquids. One promising route forward is to engineer the tape surface roughness on multiple length scales, whereby the long-wavelength roughness is minimized and a controlled level of short-wavelength roughness is generated using hard particles added to the magnetic coating (Cherubini et al., 2011). These particles protrude from an otherwise flat medium and thus can be used to create a desired magnetic spacing while minimizing the contact area and hence controlling friction. Until recently, the role of such particles was to continually clean the head and keep debris from accumulating. For this approach to be effective, the particles should be small enough so that their small radius results in a weak attraction to the head and at the same time large enough to distribute a large head force over the surrounding medium. In addition, the density of the particles needs to be sufficient to support the media, but they should not be too dense or else they may give rise to local magnetic dropouts. A second approach to controlling friction, which has yet to be investigated for tape recording, is to pattern the topography of the head in the regions remote from the transducers. Here, the idea is to provide structures or islands that support the tape and hence ensure good contact in the region
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of the read/write transducers while reducing the total contact area and hence friction. We conclude that using a combination of the above schemes, it should be feasible to design a head–tape interface that will support an areal density of 100 Gbit/in2 while providing low enough friction to ensure good runnability of the tape.
4.4. Data detection Achieving areal recording densities approaching 100 Gbit/in2 poses significant challenges for the design of the read channel. The main challenge is to ensure reliable operation of all front-end analog and digital signal-processing functions, including adaptive equalization, gain, and timing control, despite significant reductions in the available SNR that arise from the required dramatic reduction in the reader width. Under such reduced SNR conditions, the read channel will have to rely on powerful data-detection methods to guarantee sufficiently low postdetection symbol error rates. The general framework of NPML sequence detection (see Eleftheriou (2003) and the references therein) is well suited to address these performance requirements. For example, the class of NPML detectors with target polynomials (1 D2)W(D), where W(D) represents a noise whitening filter, has been implemented successfully in HDDs and recently also in tape-drive systems. At high linear densities, NPML schemes achieve a better match between the detector target and physical channel characteristics than, for example, EPR4 does. In addition, they whiten the stationary and nonstationary components of the noise process at the detector input, thereby also reducing its power. Another important aspect of tape systems is the inherent variability of the recording channel, which is due to a variety of reasons including cartridge exchange, variations in the read and write-head characteristics, recession of the head over time, nonstationary noise processes, variations in magnetic spacing, and coating thickness variations. Such aspects can be dealt with by using a detector target that automatically adapts itself to the current channel characteristics. Such a feature is enabled by the class of NPML targets described in the preceding text (Eleftheriou et al., 2010). Nonlinear distortion and/or transition jitter introduces a data-dependent component in the overall channel noise. As mentioned, NPML detection can be extended further to take into account the data-dependent distortion characteristics. For example, it is well known that surface roughness in tape media introduces a type of noise that is colored and data-dependent. Datadependent NPML detection allows one to achieve the best detection performance in the presence of such noise processes, which can be the predominant contributors to the total read-channel noise. For example,
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such a DD-NPML detection scheme was used in the 29.5-Gbit/in2 demonstration discussed in Section 3 (Cherubini et al., 2011). Finally, the noise-prediction concept in NPML and its various forms can work in tandem with advanced error-correcting codes (ECCs) that lend themselves to soft decoding, for example, low-density parity-check (LDPC) codes. For example, if noise-predictive detection is performed in conjunction with a maximum a posteriori (MAP) detection algorithm such as the BCJR algorithm (Bahl et al., 1974), then NPML and NPML-like detection allow the computation of soft reliability information on individual code symbols while retaining all the performance advantages associated with noise-prediction techniques. The soft information generated in this manner is used for soft decoding of the ECC. This, in turn, can be fed back to the soft detector to improve detection performance and ultimately to improve the error-rate performance at the decoder output in successive soft detection/decoding iterations.
4.5. Format efficiency To achieve reliable data readback while making efficient use of the magnetic recording channel, the bit stream written onto the magnetic medium includes redundancy and synchronization patterns. The specification of this overhead information is referred to as formatting or data format. The goal of an efficient format is to introduce as little overhead as possible while ensuring proper operation of the data acquisition and timing loops and achieving a user BER performance of 10–17 or better. Improvements in format efficiency directly lead to a higher cartridge capacity. The format efficiency of recent tape-recording systems such as LTO-4 is about 73% (LTO, 2006). The 27% overhead of LTO-4 can be broken down into roughly 16% for error-correcting code (ECC), 5.7% for modulation coding, and about 5% for sync patterns and data headers. There are several possible approaches to improve format efficiency. One is based on the use of longer ECCs of higher rates, resulting in a gain of about 4% without sacrificing error-correction performance. Even higher efficiency can be achieved by another approach that relies on reverse concatenation (RC), a technique that has been effectively implemented in HDD products. In a standard “forward concatenation scheme,” user data are first ECCencoded and then passed through a modulation encoder to enforce predetermined modulation constraints for timing and efficient data-detection purposes. In a RC scheme, the order of the ECC encoder and the modulation encoder are reversed (Bliss, 1981). This reversal provides three major benefits: (i) There is no error propagation through the modulation decoder; (ii) because error propagation is not an issue, the first modulation code can be very long, allowing the use of capacity-efficient and high-rate modulation codes and thereby resulting in code rate gains; and (iii) in the readback path, the ECC decoding block comes immediately after the channel detection
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block, which can readily pass soft information to the decoder on a bit-by-bit basis. This property creates the appropriate framework for using novel ECC techniques based on turbo and LDPC codes and hold the promise of large performance improvements (Dholakia et al., 2004). These benefits can also be exploited in tape-recording systems. However, the approach used in HDDs cannot be used directly because the structure of the ECC used in HDDs differs from that used in tape recording. Previously, RC schemes have been described for one-dimensional ECC architectures, where the ECC typically consists of a single code, such as a Reed–Solomon (RS) or an LDPC code (Blaum et al., 2007; van Wijngaarden and Immink, 2001). These RC architectures cannot be directly applied to the twodimensional ECC used in tape systems, which are based on RS product codes, with a C1 code along rows and a C2 code along columns. To overcome this problem, a novel RC scheme has been proposed, which is illustrated in Fig. 3.26 (Argumedo et al., 2008). The main steps in the write path are as follows: (i) The user data are reorganized into a stream of N2 rows by the serial-to-parallel block; (ii) modulation encoding is applied to each row by the first modulation encoder, ME-1; (iii) formatting is applied for partial symbol interleaving; (iv) C2-column-dependent encoding is applied; (v) C1 encoding is applied along rows; and (vi) modulation coding of the C1 parity is applied by a systematic modulation encoder ME-2. A key component in the proposed RC scheme is the first modulation code, for which one may select a very high-rate n/(n þ 1) Fibonacci code with, for example, n > 200 (Blaum et al., 2007). These codes have simple enumerative encoders and achieve very tight modulation constraints, which are comparable to those of the LTO-4 standard. Modulation constraints prevent the writing of sequences for which the overall detection process would be less reliable (they avoid unfavorable timing patterns, reduce the path-memory length in sequence detectors, and avoid quasicatastrophic error propagation). Another feature is the formatting block, which transforms the modulated user data array into an array with “empty” components in each column, which are the locations where the parity symbols of the C2 code will be introduced.
Figure 3.26
Reverse concatenation architecture.
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Compared with the rate 16/17 code of the LTO-4 standard, this new RC scheme has a modulation scheme with less than 1% redundancy while maintaining essentially unaltered modulation constraints. This is an improvement in rate of more than 5% and, together with the 4% potential gain from longer C2 codes, leads to an overall format with an overhead of about 18% rather than the 27% of LTO-4, which represents a substantial reduction in redundancy. Furthermore, by using an LDPC code or turbo code for C1, the new format supports novel ECC techniques based on iterative decoding. The C1/C2-based ECC structure is an ideal setting for LDPC or turbo codes because the typical error floor issue of these codes is resolved by the C2 RS code, which can reduce the error rates to the desired 10–17 level. In addition, the use of iterative decoding techniques can lower the limit on SNR at the input to the detector that is required for reliable operation of the front-end signal-processing functions. For example, the DD-NPML scheme implemented for the 29.5-Gbit/in2 demonstration required an SNR at the input to the detector of >13.5 dB to ensure a raw BER of 104 at the output of the detector. The application of robust signalprocessing schemes, such as decision-aided timing recovery, equalization, and gain control, should allow reliable operation of the front-end adaptive function at a BER of 103 at the detector output, corresponding to a typical detection SNR of about 11.5 dB. An LDPC-coded channel can guarantee a C2 input byte erasure rate of about 106 at an SNR of approx. 11.5 dB and can operate at a significantly lower detection SNR while achieving the required erasure rate of 103 at the C2 decoder input. Moreover, iterative timing recovery that uses feedback from the detector/decoder could allow the operation of the read channel in the 10.5–11.5 dB SNR range while still maintaining reliable operation. Such performance enhancements of the data channel can provide an additional gain of 2–3 dB and hence relax somewhat the requirements for SNR improvements in the media that will be necessary to achieve a 100-Gbit/in2 operating point.
4.6. Track density limits 4.6.1. Track misregistration Linear tape drives typically operate with a reader that has about half the width of the written track. The difference between the written track width and the reader width is referred to as the tracking margin or the track misregistration budget. This margin is provided to account for effects such as tracking fidelity during write and read, tolerances in the dimensions of the write/read head, and tape dimensional stability (TDS) effects. Ultimately, the tracking margin limits the track pitch that can be written on tape and reliably read back. Hence, as the track width is being scaled down, the
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tracking margin must also be reduced by a similar factor. As mentioned, to achieve 100 Gbit/in2 with a linear density between 650 and 800 kbpi will require a written track width in the range of 165–200 nm. To a first-order approximation, if we assume the reader width to be 100 nm, then we require a tracking margin of 65–100 nm. The 2012 INSIC roadmap further assumes that the reader can “overhang” by up to 7% of its width into the adjacent track. If we adopt the same assumption here, then we require a tracking margin in the range of approx. 70–105 nm. Tape dimensional stability is a factor in the tracking margin because multiple heads are used in parallel, and hence, if the central elements are kept centered on track, lateral expansion and contraction of the tape cause the outermost elements to move off the track centerline. Three factors contribute to TDS: humidity, temperature, and tension variations. For example, the TDS specified for LTO-4 media is 900 ppm or less over the full environmental variation. Taking the LTO-4 head span of 2.5 mm, this results in a TDS contribution of 1.125 mm to the required tracking margin. Clearly, the tracking margin of 70–100 nm required for 100 Gbit/in2 imposes severe dimensional stability constraints. A major improvement in TDS to about 200 ppm can be achieved by using an Aramid substrate. Combining this with a reduction in the head span of a factor of three, as discussed in Section 4.2, leads to a TDS contribution of 83 nm, which clearly is still too large. TDS could be further reduced through additional improvements in the substrate or by improved tension control, as will be discussed in Section 4.6.5. Another option, albeit an undesirable one, would be to restrict the range of allowable environmental operating conditions. Ultimately, it will likely be necessary to actively compensate for TDS effects in a similar manner as LTM is compensated for today. For example, the effective distance between write and read transducers in the head can be changed by tilting the head relative to the tape. In addition, TDS can be measured by monitoring changes in the difference between the position signals provided by the two servo readers. Changes in the tape dimension could then be compensated using a feedback loop to control the angle of the head to adjust the effective channel pitch and thus keep the measured TDS signal constant. In general, the time constants of TDS effects are rather slow, and hence, the tape dimension can potentially be measured very accurately by averaging the position signals. Moreover, if a low noise level in the TDS measurement can be achieved, combined with the slow nature of the variations, the TDS following error could potentially be reduced to a very small, or even negligible, value. 4.6.2. Servo channel and servo pattern Timing-based servo (TBS) is a technology developed specifically for linear tape drives in the mid-1990s (Barrett et al., 1998) and is currently used in all commercial linear tape drives. In TBS systems, recorded patterns to aid
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track-following servo consist of transitions with two different azimuthal slopes as described in Section 3.5. The lateral position is derived from the relative timing of pulses generated by a narrow head reading the pattern. TBS patterns also enable the encoding of additional LPOS information without affecting the generation of the transversal PES. This is achieved by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). In tape systems, two dedicated servo channels are normally available from which LPOS information and PES can be derived. By combining the information from these two servo channels, it is possible to generate information about the relative skew angle between the tape and head. To achieve small values of sPES, advances in the servo-channel architecture are required, which include the following three main functions: (i) optimum matched-filter detection of servo bursts, (ii) optimum demodulation of LPOS symbols, and (iii) generation of a fixed number of signal samples per unit of tape length, irrespective of tape velocity. Section 3.6 described an experimental synchronous servo channel (Cherubini et al., 2007) in which all of these functions have been implemented. The performance of the track-following and reel-to-reel servo systems is ultimately limited by the resolution and bandwidth of the velocity and position signals provided by the servo channel, which, in turn, are related to the geometry of the servo pattern. Section 3.5 described previous work to optimize the servo-pattern geometry to maximize the accuracy of the resulting position estimates. The resolution, or noise floor, of this pattern in combination with the synchronous servo channel and the particulate BaFe medium described in Sections 2 and 3 is less than 10 nm (Lantz et al., 2011, 2012). Recently, an analytic framework/model has been developed to characterize how the servo format parameters, the servo read-head geometry, and the magnetic media properties affect the servo readback signal (Furrer et al., 2012), with the aim of being able to optimize the system parameters for future operating points. By adding electronics and transition jitter noise, the readback signal expressions can be extended to a servochannel model suitable for TBS performance prediction by means of Monte Carlo simulations and/or bounds on the variance of the PES. The servopattern parameters (such as azimuth angle, transition width, and period) can then be optimized jointly with the read-head geometry (e.g., shield-toshield distance, magnetoresistive sensor width, and reader width) and the magnetic media characteristics to optimize system performance. Using such an approach, combined with further improvements in the SNR of the media, it should be possible to reduce the noise floor of the position estimates to values on the order of a few nanometers. Another approach that offers potential for further improving the servo performance lies in improving the write-head technology that is used to preformat media with the servo pattern. Two important factors determine the quality of a magnetic transition written to tape: the write gap width and
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the current switching time. A narrower write gap results in a sharper transition because the gradient in the magnetic write field produced by the write gap is larger. The accuracy of a TBS servo channel is limited in part by the accuracy with which the relative timing of the transitions can be estimated. Sharper transitions therefore result in better timing estimates and hence a more accurate position estimate. Conventional servo write heads for servo-formatting magnetic tape have a large inductance and require a large write current (>5 A). This makes it challenging to obtain the nanosecond-scale switching times required for writing sharp transitions at high tape velocities. Moreover, it is challenging to fabricate these conventional heads with very narrow write gaps. Recently, a novel planar servo write head has been reported (Engelen et al., 2012): microfabrication technology makes it possible to create a servo write head with much smaller dimensions, thus overcoming the limitations of conventional servo writers. Current rise times of 3 ns have been obtained, and preliminary results show that the rise time can be further reduced. A second advantage of this technology is that heads having a much narrower write gap than conventional servo write heads can be fabricated. The reduced switching time and the narrower write gap improve the quality of the transitions written to tape and also facilitate an increase in the tape-formatting velocity. The improved quality of the transitions should lead to a further reduction in the noise floor of the position signal generated from patterns written with a planar head. 4.6.3. Tape paths for high track density In modern high-performance tape drives, rolling elements, referred to as roller guide bearings, transport the tape from one reel to the other over the write/read head. In many tape drives, flanges on the roller guides are used to limit the lateral motion of the tape as it is transported from one reel to the other. The use of flanged rollers results in the accumulation of debris on the roller flanges, which strike the tape edges, exciting high-frequency LTM. These high-frequency components of the LTM cannot be effectively suppressed by the track-following controller if they exceed the bandwidth and slewing capability of the track-following actuator. In general, debris accumulates because the spacing between the two reel flanges is much larger than the roller flange spacing. Therefore, tape tends to stack against the reel flanges, so that when it is transported from a reel to the first roller, a large force develops between the tape edge and the roller flange, causing wear and, over time, debris accumulation. A straightforward solution to this problem is to remove the roller guide flanges altogether, but this introduces other challenges. For example, without the constraint of the flanges, the amplitude of the LTM increases because the tape moves up or down between the widely spaced reel flanges. This increases the range-of-motion requirement for the track-following actuator. Second, the angle of the tape
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with respect to the head can become skewed, which can interfere with the read-while-write verify functionality of the drive. These additional challenges can be addressed by constructing a more advanced actuator, capable of following a larger LTM and of servoing its rotation angle to keep the head perpendicular to the tape. A second option to address these two challenges is to actively control tilting elements elsewhere in the path to reduce the skew and lateral excursion of the tape relative to the head. For example, miniature actuators can be built into the tape guide rollers to actively tilt the roller and thus steer the tape as it is transported through the tape path (Pantazi et al., 2010b). In this concept, optical sensors are used to measure the position of the tape edge, and this information is exploited by a feedback controller to drive the tilting rollers to compensate for LTM and/or tape skew. Lateral tape motion arises primarily from imperfections in the tape guide rollers and reels, such as runouts, eccentricities, and other tape path imperfections. Even if the flanges are removed from the tape guides, imperfections in the mechanical bearings give rise to high-frequency disturbances that are difficult for the track-following controller to suppress. The performance of the track-following controller can therefore be improved by improving the tape path design. One approach to reducing the amplitude of the LTM is hard-edge guiding such as that used in the tape path described in Section 3.4 (MEII path), which was used for the areal density demonstration. This approach leads to a large reduction in the amplitude of the LTM; however, the mechanical contract between the tape edge and the guide generates high-frequency LTM, which is difficult to compensate. A second approach is to reduce the magnitude of the disturbances in a conventional roller guide path through improvements in the components and path design. Figure 3.27 shows a photograph of an experimental path in which LTM is constrained only by the reel flanges, resulting in a rather large LTM. However, the high-frequency components of the LTM are significantly lower than with a more conventional, heavily constrained tape path. The tape is guided by rollers that use pressurized air bearings rather than ball bearings to reduce periodic disturbances that typically result from ball-bearing imperfections and wear. The rollers are grooved to quench the air bearing that would otherwise form between the tape and roller surface (Argumedo et al., 2008). The elimination of this air bearing results in an increase in the frictional forces that oppose LTM. The two rollers closest to the reel motors use front-side guiding, whereas the two rollers adjacent to the head use backside guiding, which allows the span of tape running over the head to be reduced. All four rollers are of a flangeless design. The tape path uses the head actuator and electronics card from an IBM TS1130 tape drive. Tape transport is performed using motors taken from an IBM LTO-3 drive, which were fitted with high-tolerance bearings and with balanced precision-machined aluminum hubs to minimize once-around effects. The
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Photograph of the prototype flangeless tape path.
Figure 3.28 Time and frequency domain plots of open-loop LTM of the MEII (black) and the flangeless (green) tape path. Reprinted with permission from Lantz et al. (2011).
flanges of the tape reels are not tapered and have a spacing that is approx. 50 mm larger than the nominal tape width. With this tape path, periodic LTM is typically observed with a frequency determined by the rotation frequency of the reel motors and a peak-to-peak amplitude of 10–20 mm at tape transport velocities of 2 m/s. Figure 3.28 presents comparisons of time and frequency domain plots of captures of LTM in the two tape paths at transport velocities of 2 m/s. Although the amplitude of LTM in the experimental flangeless tape path is roughly an order of magnitude larger
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than that of the MEII path, most of the motion occurs at relatively low frequencies. In contrast, in the frequency range from 0.5 to 5 kHz, the flangeless tape path exhibits significantly lower LTM. This reduction in high-frequency LTM results in improved track-following fidelity, as discussed in Section 4.6.4. 4.6.4. Track-following control Reducing LTM by optimizing the tape path is desirable, as it reduces the disturbances that enter the track-following control system. It is then the task of the track-following controller to follow the LTM and position the head read/write elements at the centerline of the tracks (Pantazi et al., 2010a). The bandwidth of the actuator, the frequency characteristics of the LTM disturbance, and the noise performance of the medium-derived PES determine the achievable closed-loop bandwidth and thereby the trackfollowing performance. Achieving reliable tape-drive operation with track widths in the range of 165–200 nm requires that the track-following servocontrol system achieves a significantly higher positioning accuracy than in current commercial tape drives. The H1 robust control approach is a promising technique to design the controller for a tape track-following system. This approach uses experimentally obtained system models and takes into account the LTM disturbance characteristics. Its main advantage is that performance objectives, such as bandwidth and tracking error, are incorporated in the formulation of the optimization problem. The track-following control problem can be cast as an optimization problem using the general control configuration shown in Fig. 3.29a. In this configuration, the exogenous input is the LTM disturbance w ¼ [d] and the error signal is z ¼ [z1z2z3]T. The error signals, z1, z2, and z3, are defined as indicated in Fig. 3.29b, for example, z2 is the product of Wu and u. The performance requirements are specified by different weighting transfer functions. To achieve them, these weighting transfer functions are used to shape the closed-loop transfer functions of the system.
Figure 3.29
H1 control formulation block diagram.
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Figure 3.29b depicts the closed-loop system with the weighted outputs. For the track-following feedback loop, the transfer function that relates the disturbance d to the error e is S ¼ 1/(1 þGK). The capabilities of the system to follow the LTM can then be captured using the weight Ws, which can be chosen such that the magnitude of S is low in the low-frequency range, where LTM predominates. The transfer function T ¼ GK/(1 þGK) measures the impact of the measurement noise on the output. The weight WT can then be used to shape T in such a way that the controller rolls off at high frequencies. This approach was used to design a 7th-order controller for the experimental flangeless tape path described earlier. The bandwidth of the system derived from the sensitivity transfer function was approx. 700 Hz. This controller was implemented in the electronics hardware platform described in Section 3.4. Track-following experiments were performed using perpendicularly oriented BaFe medium (tape A in Section 3) formatted with the 18 servo pattern described in Section 3.5. Figure 3.30 presents time and frequency domain plots of the closed-loop PES measured at 2.28 m/s with this setup. The data exhibit a standard deviation of 13.0 nm, almost a factor of two lower than that achieved with the MEII tape path in the areal density demonstration. However, even further improvements in track-following performance will be required to achieve a tracking margin in the range of 70–105 nm necessary to achieve an areal density of 100 Gbit/in2. If we divide the tracking margin equally between contributions from TDS, head tolerances, and tracking errors, then the total allowable tracking error is 23–35 nm. Following the approach of the 2012 INSIC roadmap, the contribution of the sPES to the tracking margin is 4.24sPES, leading to a tolerable trackfollowing error of sPES ¼ 5.4–8.2 nm. It may be possible to relax this requirement somewhat if the TDS component can be made negligible using a TDS compensation scheme such as that presented above. As discussed, the achievable track-following performance is ultimately limited by the bandwidth of the actuator, the frequency characteristics of the LTM disturbance, and the noise performance of the medium-derived PES. Further improvements in the track-following performance can therefore be achieved by further reducing the noise floor in the position estimates as discussed in Section 4.6.2. Significant improvements in the track-following actuator bandwidth have also been demonstrated (Kartik et al., 2009, 2010). By combining these approaches, it should be possible to achieve the required 5–8 nm sPES. 4.6.5. Reel-to-reel control One of the main advantages of tape-based storage systems is the very high volumetric density achieved by winding a very long, thin tape onto a single reel. To further increase the tape cartridge capacity, both the areal and
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Figure 3.30 (a) Time domain signal and (b) power spectral density of PES for a ¼ 18 , v ¼ 2.28 m/s.
volumetric storage densities have to be improved. The latter can be improved by reducing the tape thickness while simultaneously increasing the tape length. The 2012 INSIC tape roadmap predicts a reduction in tape thickness to approx. 4 mm and an increase in length to 1357 m by 2022. High areal densities require excellent tape motion and tension control because the quality of the tape transport directly affects the data write and read performance. Improved tension control reduces the tension contribution to the TDS component of the tracking margin and hence leads to higher track densities. Moreover, higher volumetric densities require thinner magnetic coatings and substrates, which in turn may lead to increased susceptibility to tape damage. To counteract these effects, the design of the reel-to-reel servo system for tape velocity and tension control becomes increasingly important. A promising route to achieve the required velocity and tension control is to depart from standard PID controllers and introduce state space-based methods. The main advantage of a state space-based control system is its
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suitability for the design of a multi-input, multi-output (MIMO) control system. A MIMO control design allows multiple inputs, as is necessary if information from multiple sensors and several estimated parameters need to be used. Another important feature is its capability of handling designs in which the rate of measurements from the sensors is not commensurate with the sampling frequency of the digital controller. The notion of a MIMO control system for tape transport was introduced in Franklin et al. (1997) and has been applied to a prototype tape transport system in Mathur and Messner (1998). A MIMO architecture enables simultaneous control of tension and velocity, which, in conjunction with an optimized tape path, can substantially reduce lateral tape motion. Important steps in defining the reel-to-reel control architecture include accurate plant characterization and proper design of controller and estimator. One of the main challenges in using a MIMO system is the provision of reliable sensor measurements to determine the state of the system. There are various approaches that include embedded sensors and signalprocessing techniques to gather the necessary information to provide tension and velocity feedback.
5. Summary and Conclusions The areal density demonstration described in Section 3 provides clear experimental evidence that there is potential to continue scaling particulatebased magnetic tape technology for many more years. It is also important to note that the 29.5-Gbit/in2 areal density achieved does not represent the physical limit of the scaling potential of BaFe media, but rather, it represents a snapshot of the state of the art of the technology at the time that the demonstration was performed (Cherubini et al., 2011). There remains considerable potential to continue scaling BaFe technology well beyond this point. While it is clear that there will be significant challenges in scaling commercial linear magnetic tape drives to operate at areal densities in the range of several tens of Gbit/in2, there is also significant experimental evidence that these challenges can be overcome by means of skillful engineering. Controlling the tape–head interaction combined with continued advances in tape media technology will be key to achieving high linear density with ultranarrow readers, whereas improvements in track-following and reel-to-reel servo mechanisms and improvements in TDS and reducedspan heads will be key to achieving high track densities. In addition, advanced head and data-detection technologies and improved lateral tape motion control will impact both the linear density and the track pitch and therefore be key enablers to achieving ultrahigh areal densities in a linear magnetic tape system. Through the combination of these technologies, the
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roughly 50 Gbit/in2 areal density predicted by the INSIC roadmap to be needed for a 128-TB cartridge appears achievable. Pushing these technologies further, a linear density of 650–800 kbit/in and a track pitch in the range of 0.165–0.2 mm may also be feasible, leading to an achievable areal density of 100 Gbit/in2.
ACKNOWLEDGMENTS The material presented in this chapter is primarily an updated and extended version of the material originally presented in Argumedo et al. (2008) and Cherubini et al. (2011). We would like to thank our coauthors of those original manuscripts: A. J. Argumedo, D. Berman, R. G. Biskeborn, G. Cherubini, R.D. Cideciyan, L. Dellmann, W. Haeberle, T. Harasawa, D. J. Hellman, R. Hutchins, W. Imaino, J. Jelitto, P.-O. Jubert, K. Judd, V. Kartik, P. Koeppe, ¨ lc¸er, H. Ohtsu, G. McClelland, Y. Murata, A. Musha, C. Narayan, H. Noguchi, S. O A. Pantazi, H. Rothuizen, P. J. Seger, O. Shimizu, R. Suzuki, and K. Tsuruta. In addition, we are grateful to C. Bolliger for careful proofreading of the manuscript.
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