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Physics of perpendicular magnetic recording: writing process
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Journal of Magnetism and Magnetic Materials 246 (2002) 335–344
Physics of perpendicular magnetic recording: writing process S. Khizroeva,*, Y. Liub, K. Mountfieldb, M.H. Krydera, D. Litvinova a
b
Seagate Research, Pittsburgh, PA, USA DSSC, Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA, USA Received 20 April 2001
Abstract Using 3D-boundary element modeling, guidelines for designing an ultra-high density perpendicular system were defined. Both geometrical and material influences were investigated. The theoretical principles developed to choose dimensions and materials were experimentally verified using a wafer test structure and spin-stand experiments with a 60 nm wide focused ion beam trimmed head. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Perpendicular recording; Magnetic recording; Focused ion beam (FIB)
1. Introduction Reducing the bit cell aspect ratio (BAR) is believed to be able to extend the superparamagnetic limit in conventional longitudinal recording only up to B100 Gbit/in2 [1,2]. If the current 100% compound annual growth rate (CAGR) in areal density continues, a 100 Gbit/in2 laboratory demonstration should be achieved within 1 yr. Therefore, a new technology should be chosen for the progress to continue. Besides the other closest candidates, AFM longitudinal recording and patterned media, perpendicular recording was predicted to be extendible to at least 500 Gbit/in2 due to a thicker recording layer and/or the use of a soft underlayer (SUL) [2]. Due to a SUL, a perpendicular recording system is capable of producing a significantly larger *Corresponding author. Fax: +1-412-918-7010. E-mail address: [email protected] (S. Khizroev).
recording field than an equivalent longitudinal system [3,4]. In longitudinal recording, writing is produced by the fringing field emanating from the gap region of a ring head, as shown in Fig. 1a. At densities exceeding 100 Gbit/in2, a typical longitudinal system suffers from the fringing field being insufficiently strong (B2pMs of the head material) to saturate the medium [5]. In perpendicular recording, however, writing is produced by the field in ‘‘the effective gap’’ (‘‘the effective gap’’ is created between the trailing pole of the write head and the trailing pole of the magnetic image due to a SUL), as shown in Fig. 1b. As a result, the perpendicular field can be significantly larger than 2pMs of the head material (approximately by a factor of two). The larger the recording field a head can generate, the larger anisotropy medium can be utilized, and the larger anisotropy medium, in turn, implies the higher density the superparamagnetic limit can be extended to. However, the capability of generating a strong field alone is not sufficient to achieve ultra-high densities.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 5 5 - 1
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2.1. Geometrical factor
Fig. 1. Schematic diagrams of a (a) longitudinal head and (b) a perpendicular head using a SUL.
Another critical requirement is the capability of producing a sufficiently large recording field gradient [6]. The field gradient, in turn, is strongly sensitive to the choice of the SUL. In this work, a detailed experimental and theoretical study of a perpendicular recording system using a medium with a SUL is presented. Fundamental guidelines for choosing optimum head/media parameters to operate at densities above 100 Gbit/in2 are defined [7].
2. Theory 3D-boundary element modeling (BEM) (using commercial software, Amperes) was used to find an optimum media/head yoke geometry.1 Although, micromagnetics become critical at relatively high densities when the bit cell dimensions are comparable to the characteristic exchange length of the soft materials the head and the SUL are made of, a macroscopic model, which assumes a non-linear magnetic behavior of the soft materials, can still be used to give a good insight into the physics and performance of a recording system at high areal densities [8]. This assumption is justified by the results of the previously published experiments which indicated that despite the micromagnetically governed processes, inside the magnetic head with dimensions o100 nm the magnetization switching takes place according to a substantially linear and sufficiently high permeability magnetic characteristics [9]. 1
Amperes, Integrated Engineering, Winnipeg, Canada.
Both the pole tip and the SUL are modeled to be made of the same magnetic material, FeAlN, with a saturation moment, 4pMs ; of 20 kG and an anisotropy field, Hk ; of 10 Oe. The choice of the optimum head/media geometry is determined in a similar way as for a longitudinal ring head. The goal is to minimize the saturation write current and maximize the recording fields. Assuming perfect imaging by a SUL, a good estimate of parameters in a perpendicular magnetic system circuit can be obtained via a simple magnetic circuit expression, 1 I ¼ Hg Lg 1 þ ; ð1Þ ðRg =Rr Þ where I; Hg ; Lg and Rg =Rr are the write current, the deep gap field, the gap length (twice the distance between the air bearing surface (ABS) and the underlayer) and the reluctance ratio of the gap region and the rest of the magnetic path, respectively (see Fig. 1b) [10]. It is desired to keep the magnetic path (in this case, the path is the yoke length plus a region in the SUL) as short as possible for relatively good high frequency performance [11]. It can be seen that (to the approximation of Eq. 1) increasing the reluctance ratio of the gap region and the rest of the circuit and minimizing the effective gap length (in this case, the gap length is approximately twice the distance between the ABS and the SUL) is the approach to fulfill the goal. Although, this approximation is not sufficient to define a process according to which the magnetic flux propagates in the gap region itself, it is a good starting point to define the optimum system geometry. As mentioned above, the missing information on how the flux propagates in the gap region, which is necessary for determination of the recording field and the field gradient, was obtained using 3D-BEM. To take complete advantage of perpendicular recording (to have the recording field as close to 4pMs of the head material as possible), the ABS pole tip dimensions should be substantially larger than the ABS-to-SUL separation. Modeled vertical fields at saturation versus the distance down the track with a pole tip
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moment of 20 kG (same as for the SUL) with two values of the pole tip thickness, 0.5 and 0.1 mm, are shown in Fig. 2. The fields were calculated at a flying height of 10 nm for a 30-nm separation between the ABS and the SUL. These results show recording fields of B16 and 11 kOe, respectively. The values of the saturation current for these two cases are B75 and 85 mA, respectively. The exact values of the saturation currents obviously depend on the configuration of each specific magnetic flux system. Nevertheless, a reason to expect relatively small saturation currents can be understood with help of the magnetic image model: in comparison with an equivalent longitudinal system, there are
Fig. 2. Modeled vertical fields near the trailing edge for two values of the pole thickness, PT, of 0.5 and 0.1 mm, with the same trackwidth of 0.1 mm.
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effectively twice as many current sources due to the image in the SUL, as shown in Fig. 1b. The next approximation step is to take into consideration the finite thickness, magnetic moment and permeability of the SUL, which, in turn, makes the imaging non-perfect [12]. The vertical fields at saturation versus the distance down the track for two values of the underlayer thickness, 0.1 and 0.3 mm, are shown in Figs. 3a and b, respectively. The values of the corresponding saturation currents are B90 and 75 mA, respectively. The maximum fields at saturation are B16.3 and 17.7 kOe for thickness values of 0.1 and 0.3 mm, respectively. As mentioned above, besides the capability of generating a sufficiently strong field, to produce sufficiently sharp transitions necessary for achieving ultrahigh density recording, the recording head should be also capable of sufficiently large trailing field gradient. At this point, it is worth giving the most meaningful definition to the ‘‘trailing gradient.’’ Obviously, the most meaningful definition of the ‘‘trailing gradient’’ is strongly sensitive to every specific perpendicular magnetic system, which includes both a head and a medium. For example, if a system includes a perpendicular medium with a negligibly small distribution of the anisotropy field, the ‘‘trailing gradient’’ can be defined in a relatively local sense around the anisotropy field, Hk ; i.e. as the tangent of the slope at the point corresponding to Hk ; as shown in Fig. 4a. On the
Fig. 3. The vertical field versus the distance down the track for a system with (a) 0.1 and (b) 0.3 mm thick FeAlN as a SUL at different write current values.
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Fig. 4. Graphs helping understand how the ‘‘trailing gradient’’ is defined in (a) the local sense and (b) the integral sense.
Fig. 5. (a) Trailing gradient at saturation and (b) saturation current versus the underlayer thickness.
other hand, if a perpendicular medium with a relatively large distribution of Hk is utilized, the ‘‘trailing gradient’’ should be defined in a more integral (or global) sense, including the whole region of the Hk distribution, Hk distr ; as shown in Fig. 4b. Because the intention of this paper is to explore the writing process in perpendicular recording and, therefore, a medium is not a subject of this investigation, the authors used an arbitrary definition of the ‘‘trailing gradient’’ as of the maximum field gradient in the local sense at a point on the trailing slope, for which the field gradient reaches maximum. The maximum ‘‘trailing gradient’’ of the recording field at saturation and the saturation current versus the underlayer thickness are shown in Figs. 5a and b, respectively. Although, the saturation current significantly changes as the thickness is reduced to 0.1 mm, the maximum field remains
approximately the same. The other essential effect to observe is the fact that the trailing gradient for the case of a 0.1 mm thick SUL, B175 Oe/nm, is significantly less than the trailing gradient for the case of a 0.3 mm thick SUL, B550 Oe/nm. (It is worth mentioning that at 100 Gbit/in2, assuming a 4:1 BAR, the trailing gradient is required to be larger than B300 Oe/nm to provide good recordings onto a medium with an anisotropy field of 10 kOe.) The fact that the value of the saturation current practically does not depend on the thickness value for thickness larger than 0.1 mm indicates that saturation occurs first in the pole tip rather than in the SUL. If the saturation current were sensitive to the SUL thickness, it would have indicated that the SUL starts to saturate before the pole tip does, thus distorting the magnetic imaging. As can be seen, the latter is the case for a SUL thickness oB0.1 mm, thus
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indicating that the SUL starts to saturate before the pole tip does for a SUL thickness oB0.1 mm. This is in agreement with the result that the trailing gradient at saturation is the smallest in the thinnest underlayer case. In this case, the magnetic field tends to fringe through the edges of the pole tip, because the SUL gets saturated right under the pole tip center, thus reducing the trailing field gradient. The thickness cross-over can be approximately predicted as the thickness, TSUL ; at which the amount of the magnetic flux coming from a pole tip is equal to the amount of the magnetic flux which can ‘‘freely’’ go into the underlayer. Assuming that both the pole tip and the SUL are made of the same soft material, to the first order approximation, the cross-over condition is satisfied when the pole tip cross-section area, Apt ; is approximately equal to the circumference of the pole tip cross-section, Spt ; multiplied by the underlayer thickness, TSUL ; i.e., Apt BSpt TSUL :
ð2Þ
To some degree, the effect of using too thin SUL is similar to that due to a non-optimum material used as a SUL and will be described below from the materials perspective (see Fig. 6). This effect can, also, be looked at from a very different perspective. With the SUL saturated, the perpendicular head turns into a regular longitudinal ring head with an extremely large gap length. In turn, a ring head with an extremely large gap length drastically suffers in the field gradient, not to mention the intrinsic inability of a ring head in combination with a medium without a SUL to produce a sufficiently large recording field [6].
Fig. 6. Diagrams showing flux propagation in the case of the SUL moment (a) less and (b) larger than the pole tip moment. The saturated region is highlighted in black.
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2.1.1. Material factor Another critical parameter for designing the optimum recording system is the combination of materials used for the pole tip and the SUL. From the magnetic image model, described above, the purpose of the SUL is to provide sufficiently good magnetic imaging so that sufficiently large recording field and field gradient can be generated. However, theoretically, the magnetic imaging can be substantially distorted by having the SUL not sufficiently thick (see Section 2). Image distortion can also occur if the SUL material has a significantly lower moment than the pole tip, regardless of the SUL thickness. When the write current is below any saturation point, i.e. both materials are relatively soft and, therefore, there is no significant field, H; in either material, according to the Maxwell’s equation, div B ¼ 0; the vertical moments should be approximately the same at the interface. This implies that as soon as the vertical moment reaches the moment of the lower moment material (underlayer in this case) the SUL gets saturated right under the pole tip, thus enhancing the flux fringing from the pole tip edge regions, which, in turn, deteriorates the trailing field gradient. To help understand this phenomenon, schematic diagrams showing the magnetic flux propagation in the two cases, with the SUL moment lower and larger than the head moment, are shown in Figs. 6a and b, respectively. To demonstrate the importance of the right choice of the SUL material, three materials were explored, permalloy with a Bs of 10 kG and a Hk of 5 Oe, FeAlN with a Bs of 20 kG and a Hk of 10 Oe, and an FeCo compound with a Bs of 24 kG and a Hk of 100 Oe. The head was always assumed to be made of FeAlN, and the SUL material was varied. The vertical fields versus the distance down the track at different write current values with a 0.5 thick SUL made of each of the three materials are shown in Figs. 7a, b and c, respectively. One can see that the curves look similar for FeAlN and FeCo, despite the fact that they have different values of the saturation moment and the anisotropy field. Both these materials have at least one similar property: contrary to permalloy, the magnetic moment of each of the two materials is
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Fig. 8. (a) The trailing gradient and (b) the maximum vertical field versus the write current for permalloy and FeAlN.
Fig. 7. The vertical field at different write current versus the distance down the track for (a) permalloy, (b) FeAlN, and (c) FeCo as a 0.5 mm thick underlayer.
equal or larger than the moment of the head material, FeAlN. The ‘‘trailing gradient’’ and the maximum field versus the write current are shown for permalloy and FeAlN in Figs. 8a and b, respectively. The magnetic field dependencies for the two materials look similar. If permalloy is used as the SUL, however, the ‘‘trailing gradient’’ starts to significantly degrade as the recording field increases. It can be noticed that for permalloy at the write current value when the maximum field is achieved, the ‘‘trailing gradient’’ significantly
drops. On the other hand, for FeAlN, the maximum of the ‘‘trailing gradient’’ is achieved at the same write current value as the maximum field is achieved. This is consistent with the scenario described above, indicating that the lower moment SUL gets saturated under the pole tip, thus reducing the ‘‘trailing gradient’’. It is worth noticing that FeAlN and FeCo show no difference, despite the fact that they have different values for permeability, 2000 and 240, respectively. To explore in more detail the effect of permeability on the recording process, the recording fields at a write current value of 25 mA for FeAlN as the SUL with a 0.3 mm thickness for 3 values of permeability, 2000, 240, and 20, were calculated, as shown in Fig. 9. The write current value was chosen to be small to avoid the
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Fig. 9. The vertical field versus the distance down the track at a 25 mA write current for 3 cases: SUL permeability of 2000, 240 and 20. The maximum recording field versus permeability is shown in the inset.
possibility of saturation, so that only the effect of permeability could be observed. Naturally, identical results are expected for the other two cases of the SUL materials, permalloy and FeCo and, therefore, are not shown to avoid the redundancy. A graph of the recording field at 25 mA versus permeability for a 0.3 mm thick SUL is shown in the inset of Fig. 9. One can see that only when permeability is oB100, does it start to affect efficiency of the system. This result is consistent with the previous conclusion derived for the playback process in perpendicular recording [4]. It should be noticed that the knowledge of the lower boundary value for the SUL permeability is critical for different aspects of the future perpendicular recording implementation, for example, such as the flexibility in controlling the domain wall noise in the SUL and biasing the SUL for the playback process [12].
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tion. The other top layers represent a write coil and the single pole head. The inductance of this structure was measured as a function of the dimensions of the layers and the set of layer materials. The head material used in this test structure was Ni/Fe (45/55) with a 4pMs of 16 kG and a Hk of 5 Oe. The SUL was made of one of the following three materials: permalloy (4pMs ¼ 10 kG and Hk ¼ 5 Oe), FeAlN (4pMs ¼ 20 kG and Hk ¼ 10 Oe), or FeCo (4pMs ¼ 24 kG and Hk ¼ 100 Oe). The inductance versus the write current for the test structures with the three materials used as a 0.3 mm thick underlayer is shown in Fig. 11. One can see that the dependencies are very similar for the FeAlN and FeCo structures. However, for permalloy, the inductance roll-off region starts at a different write current value and is significantly broader than for the other two cases. This is in agreement with the previous conclusion that in this case, the SUL
Fig. 10. A diagram showing a test structure imitating a perpendicular recording system with a SUL.
3. Experiment 3.1. Test structure A diagram of a test structure developed for a study of a perpendicular recording system with a SUL is shown in Fig. 10. A soft material imitating the SUL is deposited as the bottom layer. The next insulating layer imitates the ABS-to-SUL separa-
Fig. 11. Inductance versus the write current for the test structures with the three materials used as a 0.3 mm thick underlayer, permalloy, FeAlN, and FeCo.
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saturates before the pole tip does, while in the other two cases the pole tip saturates first. 3.2. Spin-stand measurements A focused ion beam (FIB) trimmed single pole head made of Ni/Fe 45/55 material with a Bs of B16 kG was used for the spin-stand measurements. A FIB image of a trimmed head with a 60nm wide pole tip is shown in Fig. 12 [5]. An MFM image of two adjacent tracks with a 65-nm trackpitch recorded using the described head onto a 25 nm thick CoCr based perpendicular medium with an anisotropy field, Hk ; of B7 kOe in combination with a 300 nm thick FeAlN SUL is shown in Fig. 13. Relatively well-defined ultranarrow tracks and sharp transitions indicate recording onto a medium with a sufficiently small distribution of the anisotropy field, as explained in Section 2. The roll-off curves at saturation, obtained using a recording head of the type described above with
a trackwidth value of 100 nm and a perpendicular medium with a 25 nm thick hard layer and biased FeAlN SULs of different thicknesses, 50 and 300 nm, are shown in Fig. 14. In these experiments, a conventional longitudinal GMR shielded head with a 70-nm separation between the soft magnetic shields at a flying height of B25 nm was utilized. Although, it is obvious that the roll-off curve is specific to each head/media combination not only from the dimensions but also from the materials perspective, the drastic difference between the two cases of the SUL thickness clearly indicates the importance of the proper choice of the SUL. The deteriorated with the thickness reduction roll-off curve is in agreement with the theoretical conclusions (see Section 2) which indicate the ‘‘trailing gradient’’ degradation with the SUL thickness reduction. At the same time, it should be well remembered that the particular roll-curves are made not only of the writing capability of the perpendicular system, but also of complex dependencies on the density of properties of the perpendicular medium and the perpendicular playback process. For example, the demagnetization and stray fields in a perpendicular medium is relatively strongly sensitive to the recording density: unlike in a longitudinal medium, in a perpendicular medium the dominant contribution into the demagnetization and stray fields is not
Fig. 12. A FIB image of a FIB trimmed perpendicular head with a 60 nm trackwidth and a 1.5 mm gap length.
Fig. 13. A MFM image of two tracks with a 65 nm trackpitch.
Fig. 14. Roll-off curves for the two different cases of the underlayer thickness, 50 and 300 nm.
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from the ‘‘magnetic charges’’ in the magnetization transitions but rather from the ‘‘magnetic charges’’ at the top and effective bottom (defined by the SUL properties) surfaces of the recording medium [12,13]. As to the playback process, previously it was shown that the magnetic imaging by the SUL in perpendicular recording gives rise to a fundamentally new property, which does not exist in longitudinal recording: the roll-curve is relatively sensitive to the quality of the SUL [12,14,15]. Therefore, it should be remembered that each of the roll-curves we see in Fig. 14 is the result of the superposition of the contribution due to the writing quality of the perpendicular system with the contributions due to the complex dependencies on the density of the perpendicular medium and the playback process. The saturation current versus the SUL thickness for permalloy and FeAlN as the SUL material is shown in Fig. 15. It can be seen that for FeAlN, the saturation current remains approximately constant at 4 mA down to approximately a 0.1 mm thickness. This is in agreement with the previous modeling conclusion that down to a 0.1 mm thickness the pole tip saturates before the SUL does regardless of the SUL thickness. When the thickness is further reduced, the saturation current starts to relatively rapidly increase. This is a direct indication that now the SUL saturates before the pole tip does. As expected from the theoretical model, in this case, we expect only the geometrical factor to influence the saturation current value. Otherwise, in the case of permalloy,
Fig. 15. Saturation current versus the underlayer thickness with permalloy and FeAlN as a SUL.
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it can be seen that the transition from the constant saturation current region at relatively high thickness values to the relatively large saturation currents at low thickness values is significantly broader, thus indicating a different nature of saturation. According to the modeling, in this case, due to a lower moment, saturation occurs first in the SUL. As a result, both the geometrical and material factors influence the saturation process.
4. Conclusions Theoretical arguments based on 3D-BEM indicate that for achieving areal densities above 100 Gbit/in2 using perpendicular recording with a SUL, it is critical to have the SUL with the magnetic moment equal or larger than the moment of the recording pole tip. Besides, having a relatively high moment, the SUL should be thick enough to provide a sufficiently large gradient. For example, assuming a recording head made of FeAlN with a 20 kG moment, a 300 nm thick SUL made of the same material, FeAlN, or even a higher moment material, such as FeCo with a 24 kG moment, provides a trailing field gradient of more than 550 Oe/nm (sufficient for more than 100 Gbit/in2 density). For comparison, if the same SUL were made of permalloy with a 10 kG moment, the trailing gradient would have been only B200 Oe/nm. FeCo did not show any difference from FeAlN despite the fact that relative permeability of FeCo, B240, was significantly smaller than the relative permeability of FeAlN. These conclusions were experimentally confirmed with inductance measurements of a test structure imitating a perpendicular system with a SUL and by spin-stand measurements. Both theoretically and experimentally, FeCo did not show any difference from FeAlN despite the fact that relative permeability of FeCo, B240, was significantly smaller than the relative permeability of FeAlN, indicating that a relatively high anisotropy material can be used as the SUL if required, for example, for the playback process.
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Acknowledgements The authors thank K. Howard, R. Gustafson, J. Wolfson, D. Karns, J. McCarthy, and C. Bowman for helpful discussions and maintenance of the experimental equipment.
[8]
[9] [10]
References [1] J. George, Y. Fan, J. Appl. Phys. 31 (5) (1960) 402S. [2] H.N. Bertram, M. Williams, IEEE Trans. Magn. 36 (1) (2000) 4. [3] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. 13 (1977) 1272. [4] S. Khizroev, J.A. Bain, M.H. Kryder, IEEE Trans. Magn. 33 (5) (1997) 2893. [5] S. Khizroev, M.H. Kryder, Y. Ikeda, K. Rubin, P. Arnett, M. Best, D.A. Thompson, IEEE Trans. Magn. 35 (5) (1999) 2544. [6] M.H. Kryder, W.-Y. Lai, IEEE Trans. Magn. 30 (6) (1994) 1297. [7] S. Khizroev, R.M. Chomko, Y. Liu, K. Mountfield, M.H. Kryder, D. Litvinov, Perpendicular systems above
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100 Gbit/in2 density, Presented at Intermag’2000, Toronto, Canada, April 2000. D. Litvinov, R. Chomko, G. Chen, L. Abelmann, K. Ramstock, S. Khizroev, Micromagnetics of a Soft Underlayer, Presented at Intermag’2000, Toronto, Canada, IEEE Trans. Magn. 36(5) (2001) 2483. S. Khizroev, J.A. Bain, W. Jayasekara, R. Jones Jr., M.H. Kryder, IEEE Trans. Magn. 34 (4) (1998) 2030. R.E. Jones Jr., W. Nystrom, Thin film inductive transducer, US Patent 4, 190, 872, 1979. I. Tagawa, K. Koshikawa, Y. Sasaki, IEEE Trans. Magn. 36 (1) (1999) 177. D. Litvinov, M. Kryder, S. Khizroev, JMMM 232 (2001) 84. D. Litvinov, M. Kryder, S. Khizroev, Recording physics of perpendicular media: recording layer, Presented at the Eighth Joint MMM-Intermag Conference, January 2001, to be published. D. Litvinov, M. Kryder, S. Khizroev, Physics of perpendicular recording: playback, Presented at the Eighth Joint MMM-Intermag Conference, January 2001, to be published. S. Khizroev, M. Kryder, D. Litvinov, IEEE Trans. Magn. 37 (4) (2001) 1922.
Journal of Magnetism and Magnetic Materials 246 (2002) 335–344
Physics of perpendicular magnetic recording: writing process S. Khizroeva,*, Y. Liub, K. Mountfieldb, M.H. Krydera, D. Litvinova a
b
Seagate Research, Pittsburgh, PA, USA DSSC, Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA, USA Received 20 April 2001
Abstract Using 3D-boundary element modeling, guidelines for designing an ultra-high density perpendicular system were defined. Both geometrical and material influences were investigated. The theoretical principles developed to choose dimensions and materials were experimentally verified using a wafer test structure and spin-stand experiments with a 60 nm wide focused ion beam trimmed head. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Perpendicular recording; Magnetic recording; Focused ion beam (FIB)
1. Introduction Reducing the bit cell aspect ratio (BAR) is believed to be able to extend the superparamagnetic limit in conventional longitudinal recording only up to B100 Gbit/in2 [1,2]. If the current 100% compound annual growth rate (CAGR) in areal density continues, a 100 Gbit/in2 laboratory demonstration should be achieved within 1 yr. Therefore, a new technology should be chosen for the progress to continue. Besides the other closest candidates, AFM longitudinal recording and patterned media, perpendicular recording was predicted to be extendible to at least 500 Gbit/in2 due to a thicker recording layer and/or the use of a soft underlayer (SUL) [2]. Due to a SUL, a perpendicular recording system is capable of producing a significantly larger *Corresponding author. Fax: +1-412-918-7010. E-mail address: [email protected] (S. Khizroev).
recording field than an equivalent longitudinal system [3,4]. In longitudinal recording, writing is produced by the fringing field emanating from the gap region of a ring head, as shown in Fig. 1a. At densities exceeding 100 Gbit/in2, a typical longitudinal system suffers from the fringing field being insufficiently strong (B2pMs of the head material) to saturate the medium [5]. In perpendicular recording, however, writing is produced by the field in ‘‘the effective gap’’ (‘‘the effective gap’’ is created between the trailing pole of the write head and the trailing pole of the magnetic image due to a SUL), as shown in Fig. 1b. As a result, the perpendicular field can be significantly larger than 2pMs of the head material (approximately by a factor of two). The larger the recording field a head can generate, the larger anisotropy medium can be utilized, and the larger anisotropy medium, in turn, implies the higher density the superparamagnetic limit can be extended to. However, the capability of generating a strong field alone is not sufficient to achieve ultra-high densities.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 5 5 - 1
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2.1. Geometrical factor
Fig. 1. Schematic diagrams of a (a) longitudinal head and (b) a perpendicular head using a SUL.
Another critical requirement is the capability of producing a sufficiently large recording field gradient [6]. The field gradient, in turn, is strongly sensitive to the choice of the SUL. In this work, a detailed experimental and theoretical study of a perpendicular recording system using a medium with a SUL is presented. Fundamental guidelines for choosing optimum head/media parameters to operate at densities above 100 Gbit/in2 are defined [7].
2. Theory 3D-boundary element modeling (BEM) (using commercial software, Amperes) was used to find an optimum media/head yoke geometry.1 Although, micromagnetics become critical at relatively high densities when the bit cell dimensions are comparable to the characteristic exchange length of the soft materials the head and the SUL are made of, a macroscopic model, which assumes a non-linear magnetic behavior of the soft materials, can still be used to give a good insight into the physics and performance of a recording system at high areal densities [8]. This assumption is justified by the results of the previously published experiments which indicated that despite the micromagnetically governed processes, inside the magnetic head with dimensions o100 nm the magnetization switching takes place according to a substantially linear and sufficiently high permeability magnetic characteristics [9]. 1
Amperes, Integrated Engineering, Winnipeg, Canada.
Both the pole tip and the SUL are modeled to be made of the same magnetic material, FeAlN, with a saturation moment, 4pMs ; of 20 kG and an anisotropy field, Hk ; of 10 Oe. The choice of the optimum head/media geometry is determined in a similar way as for a longitudinal ring head. The goal is to minimize the saturation write current and maximize the recording fields. Assuming perfect imaging by a SUL, a good estimate of parameters in a perpendicular magnetic system circuit can be obtained via a simple magnetic circuit expression, 1 I ¼ Hg Lg 1 þ ; ð1Þ ðRg =Rr Þ where I; Hg ; Lg and Rg =Rr are the write current, the deep gap field, the gap length (twice the distance between the air bearing surface (ABS) and the underlayer) and the reluctance ratio of the gap region and the rest of the magnetic path, respectively (see Fig. 1b) [10]. It is desired to keep the magnetic path (in this case, the path is the yoke length plus a region in the SUL) as short as possible for relatively good high frequency performance [11]. It can be seen that (to the approximation of Eq. 1) increasing the reluctance ratio of the gap region and the rest of the circuit and minimizing the effective gap length (in this case, the gap length is approximately twice the distance between the ABS and the SUL) is the approach to fulfill the goal. Although, this approximation is not sufficient to define a process according to which the magnetic flux propagates in the gap region itself, it is a good starting point to define the optimum system geometry. As mentioned above, the missing information on how the flux propagates in the gap region, which is necessary for determination of the recording field and the field gradient, was obtained using 3D-BEM. To take complete advantage of perpendicular recording (to have the recording field as close to 4pMs of the head material as possible), the ABS pole tip dimensions should be substantially larger than the ABS-to-SUL separation. Modeled vertical fields at saturation versus the distance down the track with a pole tip
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moment of 20 kG (same as for the SUL) with two values of the pole tip thickness, 0.5 and 0.1 mm, are shown in Fig. 2. The fields were calculated at a flying height of 10 nm for a 30-nm separation between the ABS and the SUL. These results show recording fields of B16 and 11 kOe, respectively. The values of the saturation current for these two cases are B75 and 85 mA, respectively. The exact values of the saturation currents obviously depend on the configuration of each specific magnetic flux system. Nevertheless, a reason to expect relatively small saturation currents can be understood with help of the magnetic image model: in comparison with an equivalent longitudinal system, there are
Fig. 2. Modeled vertical fields near the trailing edge for two values of the pole thickness, PT, of 0.5 and 0.1 mm, with the same trackwidth of 0.1 mm.
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effectively twice as many current sources due to the image in the SUL, as shown in Fig. 1b. The next approximation step is to take into consideration the finite thickness, magnetic moment and permeability of the SUL, which, in turn, makes the imaging non-perfect [12]. The vertical fields at saturation versus the distance down the track for two values of the underlayer thickness, 0.1 and 0.3 mm, are shown in Figs. 3a and b, respectively. The values of the corresponding saturation currents are B90 and 75 mA, respectively. The maximum fields at saturation are B16.3 and 17.7 kOe for thickness values of 0.1 and 0.3 mm, respectively. As mentioned above, besides the capability of generating a sufficiently strong field, to produce sufficiently sharp transitions necessary for achieving ultrahigh density recording, the recording head should be also capable of sufficiently large trailing field gradient. At this point, it is worth giving the most meaningful definition to the ‘‘trailing gradient.’’ Obviously, the most meaningful definition of the ‘‘trailing gradient’’ is strongly sensitive to every specific perpendicular magnetic system, which includes both a head and a medium. For example, if a system includes a perpendicular medium with a negligibly small distribution of the anisotropy field, the ‘‘trailing gradient’’ can be defined in a relatively local sense around the anisotropy field, Hk ; i.e. as the tangent of the slope at the point corresponding to Hk ; as shown in Fig. 4a. On the
Fig. 3. The vertical field versus the distance down the track for a system with (a) 0.1 and (b) 0.3 mm thick FeAlN as a SUL at different write current values.
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Fig. 4. Graphs helping understand how the ‘‘trailing gradient’’ is defined in (a) the local sense and (b) the integral sense.
Fig. 5. (a) Trailing gradient at saturation and (b) saturation current versus the underlayer thickness.
other hand, if a perpendicular medium with a relatively large distribution of Hk is utilized, the ‘‘trailing gradient’’ should be defined in a more integral (or global) sense, including the whole region of the Hk distribution, Hk distr ; as shown in Fig. 4b. Because the intention of this paper is to explore the writing process in perpendicular recording and, therefore, a medium is not a subject of this investigation, the authors used an arbitrary definition of the ‘‘trailing gradient’’ as of the maximum field gradient in the local sense at a point on the trailing slope, for which the field gradient reaches maximum. The maximum ‘‘trailing gradient’’ of the recording field at saturation and the saturation current versus the underlayer thickness are shown in Figs. 5a and b, respectively. Although, the saturation current significantly changes as the thickness is reduced to 0.1 mm, the maximum field remains
approximately the same. The other essential effect to observe is the fact that the trailing gradient for the case of a 0.1 mm thick SUL, B175 Oe/nm, is significantly less than the trailing gradient for the case of a 0.3 mm thick SUL, B550 Oe/nm. (It is worth mentioning that at 100 Gbit/in2, assuming a 4:1 BAR, the trailing gradient is required to be larger than B300 Oe/nm to provide good recordings onto a medium with an anisotropy field of 10 kOe.) The fact that the value of the saturation current practically does not depend on the thickness value for thickness larger than 0.1 mm indicates that saturation occurs first in the pole tip rather than in the SUL. If the saturation current were sensitive to the SUL thickness, it would have indicated that the SUL starts to saturate before the pole tip does, thus distorting the magnetic imaging. As can be seen, the latter is the case for a SUL thickness oB0.1 mm, thus
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indicating that the SUL starts to saturate before the pole tip does for a SUL thickness oB0.1 mm. This is in agreement with the result that the trailing gradient at saturation is the smallest in the thinnest underlayer case. In this case, the magnetic field tends to fringe through the edges of the pole tip, because the SUL gets saturated right under the pole tip center, thus reducing the trailing field gradient. The thickness cross-over can be approximately predicted as the thickness, TSUL ; at which the amount of the magnetic flux coming from a pole tip is equal to the amount of the magnetic flux which can ‘‘freely’’ go into the underlayer. Assuming that both the pole tip and the SUL are made of the same soft material, to the first order approximation, the cross-over condition is satisfied when the pole tip cross-section area, Apt ; is approximately equal to the circumference of the pole tip cross-section, Spt ; multiplied by the underlayer thickness, TSUL ; i.e., Apt BSpt TSUL :
ð2Þ
To some degree, the effect of using too thin SUL is similar to that due to a non-optimum material used as a SUL and will be described below from the materials perspective (see Fig. 6). This effect can, also, be looked at from a very different perspective. With the SUL saturated, the perpendicular head turns into a regular longitudinal ring head with an extremely large gap length. In turn, a ring head with an extremely large gap length drastically suffers in the field gradient, not to mention the intrinsic inability of a ring head in combination with a medium without a SUL to produce a sufficiently large recording field [6].
Fig. 6. Diagrams showing flux propagation in the case of the SUL moment (a) less and (b) larger than the pole tip moment. The saturated region is highlighted in black.
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2.1.1. Material factor Another critical parameter for designing the optimum recording system is the combination of materials used for the pole tip and the SUL. From the magnetic image model, described above, the purpose of the SUL is to provide sufficiently good magnetic imaging so that sufficiently large recording field and field gradient can be generated. However, theoretically, the magnetic imaging can be substantially distorted by having the SUL not sufficiently thick (see Section 2). Image distortion can also occur if the SUL material has a significantly lower moment than the pole tip, regardless of the SUL thickness. When the write current is below any saturation point, i.e. both materials are relatively soft and, therefore, there is no significant field, H; in either material, according to the Maxwell’s equation, div B ¼ 0; the vertical moments should be approximately the same at the interface. This implies that as soon as the vertical moment reaches the moment of the lower moment material (underlayer in this case) the SUL gets saturated right under the pole tip, thus enhancing the flux fringing from the pole tip edge regions, which, in turn, deteriorates the trailing field gradient. To help understand this phenomenon, schematic diagrams showing the magnetic flux propagation in the two cases, with the SUL moment lower and larger than the head moment, are shown in Figs. 6a and b, respectively. To demonstrate the importance of the right choice of the SUL material, three materials were explored, permalloy with a Bs of 10 kG and a Hk of 5 Oe, FeAlN with a Bs of 20 kG and a Hk of 10 Oe, and an FeCo compound with a Bs of 24 kG and a Hk of 100 Oe. The head was always assumed to be made of FeAlN, and the SUL material was varied. The vertical fields versus the distance down the track at different write current values with a 0.5 thick SUL made of each of the three materials are shown in Figs. 7a, b and c, respectively. One can see that the curves look similar for FeAlN and FeCo, despite the fact that they have different values of the saturation moment and the anisotropy field. Both these materials have at least one similar property: contrary to permalloy, the magnetic moment of each of the two materials is
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Fig. 8. (a) The trailing gradient and (b) the maximum vertical field versus the write current for permalloy and FeAlN.
Fig. 7. The vertical field at different write current versus the distance down the track for (a) permalloy, (b) FeAlN, and (c) FeCo as a 0.5 mm thick underlayer.
equal or larger than the moment of the head material, FeAlN. The ‘‘trailing gradient’’ and the maximum field versus the write current are shown for permalloy and FeAlN in Figs. 8a and b, respectively. The magnetic field dependencies for the two materials look similar. If permalloy is used as the SUL, however, the ‘‘trailing gradient’’ starts to significantly degrade as the recording field increases. It can be noticed that for permalloy at the write current value when the maximum field is achieved, the ‘‘trailing gradient’’ significantly
drops. On the other hand, for FeAlN, the maximum of the ‘‘trailing gradient’’ is achieved at the same write current value as the maximum field is achieved. This is consistent with the scenario described above, indicating that the lower moment SUL gets saturated under the pole tip, thus reducing the ‘‘trailing gradient’’. It is worth noticing that FeAlN and FeCo show no difference, despite the fact that they have different values for permeability, 2000 and 240, respectively. To explore in more detail the effect of permeability on the recording process, the recording fields at a write current value of 25 mA for FeAlN as the SUL with a 0.3 mm thickness for 3 values of permeability, 2000, 240, and 20, were calculated, as shown in Fig. 9. The write current value was chosen to be small to avoid the
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Fig. 9. The vertical field versus the distance down the track at a 25 mA write current for 3 cases: SUL permeability of 2000, 240 and 20. The maximum recording field versus permeability is shown in the inset.
possibility of saturation, so that only the effect of permeability could be observed. Naturally, identical results are expected for the other two cases of the SUL materials, permalloy and FeCo and, therefore, are not shown to avoid the redundancy. A graph of the recording field at 25 mA versus permeability for a 0.3 mm thick SUL is shown in the inset of Fig. 9. One can see that only when permeability is oB100, does it start to affect efficiency of the system. This result is consistent with the previous conclusion derived for the playback process in perpendicular recording [4]. It should be noticed that the knowledge of the lower boundary value for the SUL permeability is critical for different aspects of the future perpendicular recording implementation, for example, such as the flexibility in controlling the domain wall noise in the SUL and biasing the SUL for the playback process [12].
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tion. The other top layers represent a write coil and the single pole head. The inductance of this structure was measured as a function of the dimensions of the layers and the set of layer materials. The head material used in this test structure was Ni/Fe (45/55) with a 4pMs of 16 kG and a Hk of 5 Oe. The SUL was made of one of the following three materials: permalloy (4pMs ¼ 10 kG and Hk ¼ 5 Oe), FeAlN (4pMs ¼ 20 kG and Hk ¼ 10 Oe), or FeCo (4pMs ¼ 24 kG and Hk ¼ 100 Oe). The inductance versus the write current for the test structures with the three materials used as a 0.3 mm thick underlayer is shown in Fig. 11. One can see that the dependencies are very similar for the FeAlN and FeCo structures. However, for permalloy, the inductance roll-off region starts at a different write current value and is significantly broader than for the other two cases. This is in agreement with the previous conclusion that in this case, the SUL
Fig. 10. A diagram showing a test structure imitating a perpendicular recording system with a SUL.
3. Experiment 3.1. Test structure A diagram of a test structure developed for a study of a perpendicular recording system with a SUL is shown in Fig. 10. A soft material imitating the SUL is deposited as the bottom layer. The next insulating layer imitates the ABS-to-SUL separa-
Fig. 11. Inductance versus the write current for the test structures with the three materials used as a 0.3 mm thick underlayer, permalloy, FeAlN, and FeCo.
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saturates before the pole tip does, while in the other two cases the pole tip saturates first. 3.2. Spin-stand measurements A focused ion beam (FIB) trimmed single pole head made of Ni/Fe 45/55 material with a Bs of B16 kG was used for the spin-stand measurements. A FIB image of a trimmed head with a 60nm wide pole tip is shown in Fig. 12 [5]. An MFM image of two adjacent tracks with a 65-nm trackpitch recorded using the described head onto a 25 nm thick CoCr based perpendicular medium with an anisotropy field, Hk ; of B7 kOe in combination with a 300 nm thick FeAlN SUL is shown in Fig. 13. Relatively well-defined ultranarrow tracks and sharp transitions indicate recording onto a medium with a sufficiently small distribution of the anisotropy field, as explained in Section 2. The roll-off curves at saturation, obtained using a recording head of the type described above with
a trackwidth value of 100 nm and a perpendicular medium with a 25 nm thick hard layer and biased FeAlN SULs of different thicknesses, 50 and 300 nm, are shown in Fig. 14. In these experiments, a conventional longitudinal GMR shielded head with a 70-nm separation between the soft magnetic shields at a flying height of B25 nm was utilized. Although, it is obvious that the roll-off curve is specific to each head/media combination not only from the dimensions but also from the materials perspective, the drastic difference between the two cases of the SUL thickness clearly indicates the importance of the proper choice of the SUL. The deteriorated with the thickness reduction roll-off curve is in agreement with the theoretical conclusions (see Section 2) which indicate the ‘‘trailing gradient’’ degradation with the SUL thickness reduction. At the same time, it should be well remembered that the particular roll-curves are made not only of the writing capability of the perpendicular system, but also of complex dependencies on the density of properties of the perpendicular medium and the perpendicular playback process. For example, the demagnetization and stray fields in a perpendicular medium is relatively strongly sensitive to the recording density: unlike in a longitudinal medium, in a perpendicular medium the dominant contribution into the demagnetization and stray fields is not
Fig. 12. A FIB image of a FIB trimmed perpendicular head with a 60 nm trackwidth and a 1.5 mm gap length.
Fig. 13. A MFM image of two tracks with a 65 nm trackpitch.
Fig. 14. Roll-off curves for the two different cases of the underlayer thickness, 50 and 300 nm.
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from the ‘‘magnetic charges’’ in the magnetization transitions but rather from the ‘‘magnetic charges’’ at the top and effective bottom (defined by the SUL properties) surfaces of the recording medium [12,13]. As to the playback process, previously it was shown that the magnetic imaging by the SUL in perpendicular recording gives rise to a fundamentally new property, which does not exist in longitudinal recording: the roll-curve is relatively sensitive to the quality of the SUL [12,14,15]. Therefore, it should be remembered that each of the roll-curves we see in Fig. 14 is the result of the superposition of the contribution due to the writing quality of the perpendicular system with the contributions due to the complex dependencies on the density of the perpendicular medium and the playback process. The saturation current versus the SUL thickness for permalloy and FeAlN as the SUL material is shown in Fig. 15. It can be seen that for FeAlN, the saturation current remains approximately constant at 4 mA down to approximately a 0.1 mm thickness. This is in agreement with the previous modeling conclusion that down to a 0.1 mm thickness the pole tip saturates before the SUL does regardless of the SUL thickness. When the thickness is further reduced, the saturation current starts to relatively rapidly increase. This is a direct indication that now the SUL saturates before the pole tip does. As expected from the theoretical model, in this case, we expect only the geometrical factor to influence the saturation current value. Otherwise, in the case of permalloy,
Fig. 15. Saturation current versus the underlayer thickness with permalloy and FeAlN as a SUL.
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it can be seen that the transition from the constant saturation current region at relatively high thickness values to the relatively large saturation currents at low thickness values is significantly broader, thus indicating a different nature of saturation. According to the modeling, in this case, due to a lower moment, saturation occurs first in the SUL. As a result, both the geometrical and material factors influence the saturation process.
4. Conclusions Theoretical arguments based on 3D-BEM indicate that for achieving areal densities above 100 Gbit/in2 using perpendicular recording with a SUL, it is critical to have the SUL with the magnetic moment equal or larger than the moment of the recording pole tip. Besides, having a relatively high moment, the SUL should be thick enough to provide a sufficiently large gradient. For example, assuming a recording head made of FeAlN with a 20 kG moment, a 300 nm thick SUL made of the same material, FeAlN, or even a higher moment material, such as FeCo with a 24 kG moment, provides a trailing field gradient of more than 550 Oe/nm (sufficient for more than 100 Gbit/in2 density). For comparison, if the same SUL were made of permalloy with a 10 kG moment, the trailing gradient would have been only B200 Oe/nm. FeCo did not show any difference from FeAlN despite the fact that relative permeability of FeCo, B240, was significantly smaller than the relative permeability of FeAlN. These conclusions were experimentally confirmed with inductance measurements of a test structure imitating a perpendicular system with a SUL and by spin-stand measurements. Both theoretically and experimentally, FeCo did not show any difference from FeAlN despite the fact that relative permeability of FeCo, B240, was significantly smaller than the relative permeability of FeAlN, indicating that a relatively high anisotropy material can be used as the SUL if required, for example, for the playback process.
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Acknowledgements The authors thank K. Howard, R. Gustafson, J. Wolfson, D. Karns, J. McCarthy, and C. Bowman for helpful discussions and maintenance of the experimental equipment.
[8]
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