- Email: [email protected]
Past, present, and future of perpendicular magnetic recording
Journal of Magnetism and Magnetic Materials 235 (2001) 235–240
Invited paper
Past, present, and future of perpendicular magnetic recording Jack H. Judy* Department of Electrical and Computer Engineering, The Center for Micromagnetics and Information Technologies (MINT), University of Minnesota, Minneapolis, Minnesota 55455-0154, USA
Abstract The past, present, and future developments of perpendicular magnetic recording are discussed. r 2001 Published by Elsevier Science B.V. Keywords: Perpendicular magnetic recording; Longitudinal magnetic recording
1. Introduction At the Toronto Intermag Conference in April 2000, Fujitsu [1] using longitudinal magnetic recording (LMR) and Hitachi [2] using perpendicular magnetic recording (PMR) reported demonstrations of hard disk storage at 56 and 52.5 Gbits/ in2, respectively. However, development of manufacturable disk drives constitutes a major challenge for the magnetic recording industry. In fact, since longitudinal recording may have difficulty achieving acceptable thermal stability from 40 to 100 Gbits/in2, PMR is now being seriously considered for storage at 100–1000 Gbits/in2. To sustain even 100 Gbits/in2, either the recording media must possess an average grain and magnetic cluster domain size near 10 nm, possess high coercivity of 5–10 kOe to resist bit demagnetization, and simultaneously allow only 10% signal amplitude loss in 10 years. For longitudinal recording, the thickness of the media must also approach 10 nm which will induce significant time*Tel.: +1-612-625-4583; fax: +1-612-625-7381. E-mail address: [email protected] (J.H. Judy).
decay of signal and signal-to-noise ratio (SNR). However, a 20 nm thick PMR media will easily be able to provide the required thermal stability for 10 years, but the media transition noise and soft underlayer domain noise must be reduced significantly to increase media SNR. The purpose of this talk is to review past, present, and future developments of perpendicular magnetic recording and discuss the intrinsic advantages of PMR to overcome key challenges facing hard disk storage at areal densities beyond 100–1000 Gbits/in2.
2. Past developments of perpendicular magnetic recording The past of perpendicular magnetic recording started with the invention of magnetic recording by Valdemar Poulsen [3] who attempted in 1898 to record perpendicular to a carbon steel wire placed inside the gap of a electromagnet driven by currents generated in a microphone by his voice. The invention was a success even though the longitudinal fringing fields of the electromagnet recorded the wire instead of the perpendicular
0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 3 4 5 - 6
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fields. Ludwig Mayer [4] demonstrated perpendicular contact writing in 1958 using a permanently magnetized tip of a Vicalloy wire which was dragged on the surface of a high-coercivity MnBi film having perpendicular magnetic anisotropy. The recording was imaged using an electron mirror microscope. This form of PMR was pursued for magneto-optical beam-addressable memories using Curie-point writing which is a product today. A large-scale demonstration of the true vector nature and spatial distribution of the remanent magnetization of longitudinal oxide tape was given in 1963 by Tjaden and Leyten [5]. Sheets of longitudinally-oriented particulate tape were stacked to make a 5 cm thick media and a 5000 : 1 large-scale ring head with a 2 cm gap was used for recording. The remanent magnetization vector was found to follow circular paths and exhibit a large perpendicular component near the center of the recorded transitions. This experiment advanced the fundamental understanding of the physical behavior of thick longitudinal recording media by showing clearly that both longitudinal and perpendicular magnetization components in thick longitudinal media participate in the writing and reading processes and that neither should be neglected at short wavelengths comparable to the media thickness. Even though these three experiments showed the coexistence of perpendicular and longitudinal recording, only LMR had been developed since 1898 until Professor Shun-ichi Iwasaki invented the first practical implementation of perpendicular magnetic recording for high densities in 1977.
3. Present developments of perpendicular magnetic recording In 1968, Iwasaki and Suzuki [6] reported making self-consistent iterative calculations for the first time by taking into account the vector demagnetization fields and the nonlinear field-dependence of magnetization components in stand-still LMR. Their calculations of the spatial distributions of both longitudinal and perpendicular remanent magnetization confirmed the Tjaden and Leyten [5] large-scale LMR experimental model. In 1975,
Iwasaki and Takemura [7] presented a dramatic demonstration of small-scale LMR using a single thick layer of standard longitudinal particulate tape media and a ring head. Bitter patterns of cross-sections of sinusoidal recordings provided confirmation of the vector nature and circular distribution of the remanent magnetization in true size LMR media. The pure circular mode of magnetization was shown to correspond to a null in the output as a function of write current in short wavelength sinusoidal longitudinal recording. By applying a strong magnetic field nearly normal to the media, they showed that the circular mode could be decoupled into a perpendicular mode. This demonstration constituted a major advancement in the understanding of magnetic recording because it resulted in the first proposal of using the PMR mode for data storage at very short wavelengths. Perpendicular magnetic recording initially received considerable attention when Iwasaki and Nakamura [8] first demonstrated the high-density recording potential of PMR at 50 Kfcpi by writing with an auxiliary-driven single-pole probe head on a CoCr single layer thin film having perpendicular magnetic anisotropy and reading with a ring head. This first demonstration of PMR marked the beginning of an extraordinary new era in the evolution of magnetic recording. Perpendicular recorded bits are intrinsically stabilized at high densities due to the reduction of the demagnetization fields by the fields of the adjacent bits whereas the LMR become severely demagnetized at high densities due to the demagnetization fields of the adjacent bits. In 1978, Iwasaki and Ouchi [9] reported that RF sputtered CoCr films exhibit a hexagonal-close-packed (hcp) perpendicular oriented columnar structure. A single pole type (SPT) head with a 3.5 mm thick main pole was used to write through a 1 mm thick CoCr film and a ferrite ring head with a gap of 1 mm was used to recover signal near 50 Kfci. In 1979, Iwasaki, Nakamura, and Ouchi [10] reported RF sputtering of a double-layer PMR media on a flexible plastic substrate consisting of 0.5 mm CoCr on a 0.5 mm soft NiFe underlayer. A 1 mm thick SPT head was used both as the write head and the read head near 100 Kfcpi. The NiFe underlayer
J.H. Judy / Journal of Magnetism and Magnetic Materials 235 (2001) 235–240
enhanced both the writing field amplitude and gradient and the readback output by a factor 10 by decreasing the demagnetizing fields at the pole tip of the head and by increasing the flux density for reading. In 1987, Yamamoto, Nakamura, and Iwasaki [11] reported contact writing and reading of an all ‘‘ones’’ pattern on a 0.1/0.5 mm CoCr/ NiFe double-layer media at an incredible high density of 680 Kfcpi using a single 0.4 mm thick SPT head for both writing and reading. However, the PMR has not yet been commercialized today because LMR has been improved significantly since the first demonstration of PMR and SPT heads have not been available. Magnetic force microscopy (MFM) studies of relationships between the physical microstructure and the magnetization microstructures of longitudinal and perpendicular thin film media indicate the existence of magnetic irregularities or magnetic cluster domains [12] which are usually many times larger than the physical grains and are located mainly between bit transitions and in the noisy dc-erased areas. Even if intergranular exchange interactions can be reduced to zero, the size of these magnetostatically-induced interaction cluster domains must be reduced significantly to achieve acceptably low noise levels [13]. A new approach to PMR media having unity squareness without correction for demagnetization fields utilizing multilayer superlattice perpendicular media has been pursued in recent years [14–17]. The nano-structure, grain size, texture, and magnetic properties of the interfaces of the multilayer superlattices are extremely sensitive to the surface roughness, grain size, and crystallinity of the interfaces, and the smoothness of the ultra-thin underlayers. The nano-structures of PMR superlattice media must have magnetically-isolated grains for high-SNR at high densities and thickness equal to several grain diameters for highthermal stability. These new PMR superlattice media being fabricated are modeled by micromagnetic simulations using Voronoi cells and are interface-induced high-magnetic anisotropy hetero-epitaxial multilayer superlattices of CoB/Pt, CoB/Pd, CoB/Pt/Pd, CoBCrTaPt/Pt/Pd on supersmooth hard disk glass or alternative substrates. Double-layer PMR media is being deposited on
237
optimized ultra-thin amorphous underlayers such as InSn oxide (ITO), CoZrTa, Ta, or SiN, deposited on magnetically soft high-saturation flux-density NiFe, CoFe, FeN, CoZr, or FeTaN/ IrMn underlayers. PMR superlattice media must have saturation flux-densities of 200–900 G, 10–4 nm grain diameter, 10–24 nm thickness, 5–15 kOe remanent coercivity, loop side slope of unity, unity magnetization squareness, and have an extremely smooth (0.2–0.1 nm) surface. Electronic structure theory, computer calculations of effects of grain boundaries, separations, and defects on the magnetization, exchange interactions, and magnetic anisotropy are being used as inputs to micromagnetic simulations of perpendicular hysteresis loops, magnetic cluster domains, magnetization reversal, dynamic coercivity, thermal stability, and magnetic recording. Measurements of grain size distribution, surface roughness, texture, and boundary defects such as stacking faults and pinning sites should be made using high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Magnetic cluster domains, recorded bits, and media noise will have to be studied using magnetic force microscopy (MFM) and Lorenz TEM in order to understand the key interrelationships between magnetic and physical structure. There are potentially three different PMR writing modes: (1) single-layer media using a thin film ring write head (TFH), (2) double-layer media having a soft magnetic keeper underlayer using a single pole head (SPH) or a pseudo-pole head (PPH) constructed from a standard TF write head by focused-ion-beam etching, and (3) triple-layer media having a soft magnetic keeper overlayer and underlayer using a single pole head (SPH) or a pseudo-pole head (PPH) constructed from a standard TF write head by focused-ion-beam etching. In the 1st mode, the PMR media must be highly-oriented with unity perpendicular squareness. In the 2nd and 3rd modes, the media must also have unity perpendicular squareness, but the soft underlayer and overlayer enhance the magnitude of the writing field and reduce the demagnetizing fields. However, the main advantages of single and double-layer PMR media are
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that PMR media can support extremely high densities and can be significantly thicker than LMR media for the same density so that the resistance of PMR to thermally decay is increased. An SPH or PPH with a keeper underlayer provides a 2 larger write field and higher head field gradient so that PMR media with coercivity higher than LMR can be used to give sharper transitions and sharper track edges suitable for achieving densities of 100–1000 Gbits/in2.
4. Future development of perpendicular magnetic recording At PMRC’97 in Akita, Dr. David Thompson of IBM challenged the PMR community to achieve by 12/31/02 a type 2 demo of writing and reading random data with a real HDD channel at 100 Gbits/in2. In my opinion, such a demonstration will confirm the intrinsic advantages of PMR with higher signal-to-noise and greater thermal stability to allow PMR to ultimately replace LMR beyond 100–1000 Gbits/in2. The challenges facing perpendicular magnetic recording are to simultaneously achieve high thermal stability with high signal amplitude and high media signal-to-noise for low bit-error-rate. Meeting these challenges will require extremely uniform average magnetic cluster diameters of 10–5 nm, double-layer 50– 25 nm thick media with a high-saturation 10– 20 kG low-noise magnetic underlayer or overlayer with permeability of 100–50, average surface roughness of 0.2–0.1 nm, remanent magnetization of 200–900 emu/cc, unity squareness without any demagnetization field correction, remanent coercivities of 5–15 kOe which can be written with subnanosecond rise times using a microstrip singleturn perpendicular single pole head and read with a 10–20% GMR spin-valve read head. The basic theory of thermally activated timedecay of remanent magnetization or the ultimate superparamagnetic limit is based on the assumption that for a single magnetic energy barrier, the exponential relaxation time constant t is given by the Arrhenius-N!eel law as t ¼ ð1=f0 ÞexpðDE=kTÞ;
ð1Þ
where DE is the uniaxial energy barrier, T is the absolute temperature, k is the Boltzmann’s constant, and f0 is the frequency of approach to the energy barrier which is taken as 109 s1. For most magnetic materials, this exponential decay is not observed and the time-decay of remanent magnetization MR follows a logarithmic time dependence for a wide uniform distribution of energy barriers as follows: MR ðt; Hint Þ ¼ MR ðt0 ; Hint Þ S lnðt=t0 Þ
ð2Þ
where the initial time is t0 and S is the magnetic viscosity or decay rate which is taken to be in units of %/decade (s), and Hint is the magnetic interaction field acting to reverse the MR : For a system of magnetic particles having a uniaxial anisotropy energy density Ku and magnetic volume V; the effective energy barrier to kT ratio can be shown to be given by DE=kT ¼ ðKV=kTÞeff ¼ ðKV=kTÞ0 ½1 ðHint =HK Þ3=2 ;
ð3Þ
where HK ¼ 2 Ku =MS is the uniaxial anisotropy field, and Ms is the saturation magnetization. Since the term ðKV=kTÞ0 is the energy barrier without magnetic interactions, the interaction field Hint tends to reduce the energy and increase the magnetic viscosity or time-decay rate. A ratio of about 42.543 corresponds to an Mr or signal loss of only 10% in 10 years at 651C. In order to better understand the subtle dependence of the thermal-activated time decay brickwall facing ultra-high-density recording (UHDR) densities beyond 100 Gbits/in2, the constraints and trends can be predicted by an UHDR areal storage roadmap. The results of Tarnopolsky [18] are used to generate the constraint to first achieve an on-track bit-error rate (BER) for an assumed media signal-to-noise-ratio (SNR)0-peak/ rms for a given bit length (B) and a user density U ¼ PW50=B is ðSNRÞpeak ¼ ð2=pÞUðB=sjitter Þ2 ½ðpU=4Þ=sinhðpU=4Þ2
ð4Þ
by calculating the timing position jitter (sjitter ) which is directly related to the cross-track correlation length (s) and the transition length parameter
J.H. Judy / Journal of Magnetism and Magnetic Materials 235 (2001) 235–240
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Table 1 UHDR roadmap for perpendicular magnetic recording from 100–1000 Gbpi2 Areal density (Gbpi2) Bit density (Kfci) Rack density (Ktpi)) BI aspect ratio=BAR BIT length=B (nm) Read track width=W (nm) FLY height=d (nm) Pulse Width=PW50 (nm) log on-track bit ero rate Signal-to-noise SNR (rms/rms) Number grains per bit=10^SNR/10 Average grain diameter=D (nm) Magnetization squareness=S Media thickness=d (nm) Anisotropy constant Ku (106 erg/cc) Media saturation=Ms (emu/cc) Remenant coercivity=Hc (Oe)
100 635 160 4 40 140 8 100 8 18 63 9 1 18 2 400 8500
(a) according to Bertram [20] by the following simple formula s2jitter ¼ ðp4 =48Þs a2 =w;
ð5Þ
where (w) is the read track width. For perpendicular recording media with Hc ¼ Hk and S ¼ 1; the average grain diameter D is assumed to be equal to the average magnetic cluster diameter and equal to the dross-track correlation length s; and the transition length parameter a ¼ 3D=4; the SNR is then determined to take the simple form given by Bertram [19] as ðSNRÞpeak ¼ 0:314 w PW50 B=s a2
ð6Þ
However, a simpler method is to use the Poisson statistics of additive noise for the magnetic grains in the media to find the (SNR)rms/rms as given by ðSNRÞrms=rms ¼ 10 log 10 ðNÞ;
ð7Þ
where N is the number of magnetic grains per bit. This simpler approach has been followed to determine the average grain diameters assuming hexagonal close packing of the grains as shown in Table 1. Once the grain diameter has been determined and the media thickness has been chosen to insure sufficient thermal stability and resolution, the uniaxial crystalline anisotropy can be determined. Taking into account the demagne-
250 1000 250 4 255 89 7 64 7 17 50 7 1 20 3 550 11000
500 1400 354 4 18 63 6 45 6 16 40 5 1 22 4 700 12500
1000 2000 500 4 13 45 5 32 5 15 32 4 1 24 6 875 1430
tizing fields of three adjacent recorded bits and using the optimum coercivity as a constant between 1 and 3 times the saturation magnetization 4pMs to maximize the height of the energy barrier assuming Hc ¼ Hk ; the saturation magnetization Ms and remanent coercivity were determined as given in Table 1. This resulting ultra-high density roadmap for perpendicular magnetic recording given in Table 1 shows that PMR is capable of achieving 100 Gbits/ in2 easily with on-track BER of 108 and 10 nm grain size with Ku=1.7 106 ergs/cc and a remanent coercivity of 8500 Oe. Furthermore, it appears that 1000 Gbits/in2 may be possible with D=4 nm Ku=6 106 and a coercivity of 14300 Oe even though the on-track BER is only 105. Either the SNR of the media will have to be increased or the required SNR will have to be provided by improving signal processing techniques. If SNR=18 is assumed for 1000 Gbits/in2, D=3 nm and Ku=12.5 106 and the remanent coercivity required would be about 20 kOe. This may be possible? In conclusion, the future of perpendicular magnetic recording will be extremely challenging [20–24]. The goals are to reduce grain size and dispersion significantly, enhance magnetic isolation so that the cluster size is a single grain, reduce transition and DC noise, optimize design a
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single-turn single-pole head for high-speed writing, and achieve durable contact recording.
References [1] H. Takano, et al., Realization of 52.5 Gb/in2 perpendicular recording, AD-06. Intermag, 2000. [2] E.N. Abarra, et al., Thermal stability of longitudinal media for 56 Gb/in2 Recording, AA-01, Intermag, 2000. [3] V. Poulsen, Method of recording and reproducing sounds or signals, U.S. Patent: 661-619-1900, 1898. [4] D.L.A. Tjaden, J. Leyten, A 5000 : 1 Scale model of the magnetic recording process, Philips Res. Repts. 25 (1963–1964) 319. [5] L. Mayer, General Mills, Minneapolis, Minnesota, 1958. [6] S. Iwasaki, T. Suzuki, Dynamical interpretation of magnetic recording process, IEEE Trans. Magn. MAG-4 (1968). [7] S. Iwasaki, K. Takemura, IEEE Trans. Magn. MAG-11 (1975) 1173. [8] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. MAG-13 (1977) 1272. [9] S. Iwasaki, K. Ouchi, IEEE Trans. Magn. MAG-14 (1978) 849.
[10] S. Iwasaki, Y. Nakamura, K. Ouchi, IEEE Trans. Magn. MAG-15 (1979) 1456. [11] S. Yamamoto, Y. Nakamura, S. Iwasaki, IEEE Trans. Magn. MAG-23 (1987) 2070. [12] M. Futamoto, et al., J. Mag. Soc. Japan 21 (S2) (1997) 141. [13] K. Ouchi, N. Honda, IEEE Trans. Magn. MAG-36 (2000) 4. [14] Y. Tyan, et al., Jpn. J. Appl. Phys. 18 (1997) 418. [15] T.K. Hartwar, C.F. Brucker, J. Appl. Phys. 79 (1996) 15. [16] G.A. Bertero, R. Sinclair, Appl. Phys. Lett. 64 (1994) 13. [17] G. Chen, Recording properties of Co/Pd multilayers using a magnetoresistive sensor, J. Appl. Phys (1999). [18] G. Tarnopolski, et al., J. Appl. Phys. 81 (1997) 4837. [19] H.N. Bertram, M. Williams, IEEE Trans. Magn. MAG-36 (2000) 4. [20] D. Weller, A. Moser, IEEE Trans. Magn. MAG-35 (1999) 1. [21] D. Weller, et al., IEEE Trans. Magn. MAG-36 (2000) 10. [22] A. Takeo, et al., A new recording design to suppress recording demagnetization for PMR, BB-03, Intermag, 2000. [23] N. Honda, et al., Low noise PMR media for deep submicron track width recording, DP-13, Intermag, 2000. [24] K. Ise, et al., High writing-sensitivity single-pole head with a cusp field coil, CB-08, Intermag, 2000.
Invited paper
Past, present, and future of perpendicular magnetic recording Jack H. Judy* Department of Electrical and Computer Engineering, The Center for Micromagnetics and Information Technologies (MINT), University of Minnesota, Minneapolis, Minnesota 55455-0154, USA
Abstract The past, present, and future developments of perpendicular magnetic recording are discussed. r 2001 Published by Elsevier Science B.V. Keywords: Perpendicular magnetic recording; Longitudinal magnetic recording
1. Introduction At the Toronto Intermag Conference in April 2000, Fujitsu [1] using longitudinal magnetic recording (LMR) and Hitachi [2] using perpendicular magnetic recording (PMR) reported demonstrations of hard disk storage at 56 and 52.5 Gbits/ in2, respectively. However, development of manufacturable disk drives constitutes a major challenge for the magnetic recording industry. In fact, since longitudinal recording may have difficulty achieving acceptable thermal stability from 40 to 100 Gbits/in2, PMR is now being seriously considered for storage at 100–1000 Gbits/in2. To sustain even 100 Gbits/in2, either the recording media must possess an average grain and magnetic cluster domain size near 10 nm, possess high coercivity of 5–10 kOe to resist bit demagnetization, and simultaneously allow only 10% signal amplitude loss in 10 years. For longitudinal recording, the thickness of the media must also approach 10 nm which will induce significant time*Tel.: +1-612-625-4583; fax: +1-612-625-7381. E-mail address: [email protected] (J.H. Judy).
decay of signal and signal-to-noise ratio (SNR). However, a 20 nm thick PMR media will easily be able to provide the required thermal stability for 10 years, but the media transition noise and soft underlayer domain noise must be reduced significantly to increase media SNR. The purpose of this talk is to review past, present, and future developments of perpendicular magnetic recording and discuss the intrinsic advantages of PMR to overcome key challenges facing hard disk storage at areal densities beyond 100–1000 Gbits/in2.
2. Past developments of perpendicular magnetic recording The past of perpendicular magnetic recording started with the invention of magnetic recording by Valdemar Poulsen [3] who attempted in 1898 to record perpendicular to a carbon steel wire placed inside the gap of a electromagnet driven by currents generated in a microphone by his voice. The invention was a success even though the longitudinal fringing fields of the electromagnet recorded the wire instead of the perpendicular
0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 3 4 5 - 6
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fields. Ludwig Mayer [4] demonstrated perpendicular contact writing in 1958 using a permanently magnetized tip of a Vicalloy wire which was dragged on the surface of a high-coercivity MnBi film having perpendicular magnetic anisotropy. The recording was imaged using an electron mirror microscope. This form of PMR was pursued for magneto-optical beam-addressable memories using Curie-point writing which is a product today. A large-scale demonstration of the true vector nature and spatial distribution of the remanent magnetization of longitudinal oxide tape was given in 1963 by Tjaden and Leyten [5]. Sheets of longitudinally-oriented particulate tape were stacked to make a 5 cm thick media and a 5000 : 1 large-scale ring head with a 2 cm gap was used for recording. The remanent magnetization vector was found to follow circular paths and exhibit a large perpendicular component near the center of the recorded transitions. This experiment advanced the fundamental understanding of the physical behavior of thick longitudinal recording media by showing clearly that both longitudinal and perpendicular magnetization components in thick longitudinal media participate in the writing and reading processes and that neither should be neglected at short wavelengths comparable to the media thickness. Even though these three experiments showed the coexistence of perpendicular and longitudinal recording, only LMR had been developed since 1898 until Professor Shun-ichi Iwasaki invented the first practical implementation of perpendicular magnetic recording for high densities in 1977.
3. Present developments of perpendicular magnetic recording In 1968, Iwasaki and Suzuki [6] reported making self-consistent iterative calculations for the first time by taking into account the vector demagnetization fields and the nonlinear field-dependence of magnetization components in stand-still LMR. Their calculations of the spatial distributions of both longitudinal and perpendicular remanent magnetization confirmed the Tjaden and Leyten [5] large-scale LMR experimental model. In 1975,
Iwasaki and Takemura [7] presented a dramatic demonstration of small-scale LMR using a single thick layer of standard longitudinal particulate tape media and a ring head. Bitter patterns of cross-sections of sinusoidal recordings provided confirmation of the vector nature and circular distribution of the remanent magnetization in true size LMR media. The pure circular mode of magnetization was shown to correspond to a null in the output as a function of write current in short wavelength sinusoidal longitudinal recording. By applying a strong magnetic field nearly normal to the media, they showed that the circular mode could be decoupled into a perpendicular mode. This demonstration constituted a major advancement in the understanding of magnetic recording because it resulted in the first proposal of using the PMR mode for data storage at very short wavelengths. Perpendicular magnetic recording initially received considerable attention when Iwasaki and Nakamura [8] first demonstrated the high-density recording potential of PMR at 50 Kfcpi by writing with an auxiliary-driven single-pole probe head on a CoCr single layer thin film having perpendicular magnetic anisotropy and reading with a ring head. This first demonstration of PMR marked the beginning of an extraordinary new era in the evolution of magnetic recording. Perpendicular recorded bits are intrinsically stabilized at high densities due to the reduction of the demagnetization fields by the fields of the adjacent bits whereas the LMR become severely demagnetized at high densities due to the demagnetization fields of the adjacent bits. In 1978, Iwasaki and Ouchi [9] reported that RF sputtered CoCr films exhibit a hexagonal-close-packed (hcp) perpendicular oriented columnar structure. A single pole type (SPT) head with a 3.5 mm thick main pole was used to write through a 1 mm thick CoCr film and a ferrite ring head with a gap of 1 mm was used to recover signal near 50 Kfci. In 1979, Iwasaki, Nakamura, and Ouchi [10] reported RF sputtering of a double-layer PMR media on a flexible plastic substrate consisting of 0.5 mm CoCr on a 0.5 mm soft NiFe underlayer. A 1 mm thick SPT head was used both as the write head and the read head near 100 Kfcpi. The NiFe underlayer
J.H. Judy / Journal of Magnetism and Magnetic Materials 235 (2001) 235–240
enhanced both the writing field amplitude and gradient and the readback output by a factor 10 by decreasing the demagnetizing fields at the pole tip of the head and by increasing the flux density for reading. In 1987, Yamamoto, Nakamura, and Iwasaki [11] reported contact writing and reading of an all ‘‘ones’’ pattern on a 0.1/0.5 mm CoCr/ NiFe double-layer media at an incredible high density of 680 Kfcpi using a single 0.4 mm thick SPT head for both writing and reading. However, the PMR has not yet been commercialized today because LMR has been improved significantly since the first demonstration of PMR and SPT heads have not been available. Magnetic force microscopy (MFM) studies of relationships between the physical microstructure and the magnetization microstructures of longitudinal and perpendicular thin film media indicate the existence of magnetic irregularities or magnetic cluster domains [12] which are usually many times larger than the physical grains and are located mainly between bit transitions and in the noisy dc-erased areas. Even if intergranular exchange interactions can be reduced to zero, the size of these magnetostatically-induced interaction cluster domains must be reduced significantly to achieve acceptably low noise levels [13]. A new approach to PMR media having unity squareness without correction for demagnetization fields utilizing multilayer superlattice perpendicular media has been pursued in recent years [14–17]. The nano-structure, grain size, texture, and magnetic properties of the interfaces of the multilayer superlattices are extremely sensitive to the surface roughness, grain size, and crystallinity of the interfaces, and the smoothness of the ultra-thin underlayers. The nano-structures of PMR superlattice media must have magnetically-isolated grains for high-SNR at high densities and thickness equal to several grain diameters for highthermal stability. These new PMR superlattice media being fabricated are modeled by micromagnetic simulations using Voronoi cells and are interface-induced high-magnetic anisotropy hetero-epitaxial multilayer superlattices of CoB/Pt, CoB/Pd, CoB/Pt/Pd, CoBCrTaPt/Pt/Pd on supersmooth hard disk glass or alternative substrates. Double-layer PMR media is being deposited on
237
optimized ultra-thin amorphous underlayers such as InSn oxide (ITO), CoZrTa, Ta, or SiN, deposited on magnetically soft high-saturation flux-density NiFe, CoFe, FeN, CoZr, or FeTaN/ IrMn underlayers. PMR superlattice media must have saturation flux-densities of 200–900 G, 10–4 nm grain diameter, 10–24 nm thickness, 5–15 kOe remanent coercivity, loop side slope of unity, unity magnetization squareness, and have an extremely smooth (0.2–0.1 nm) surface. Electronic structure theory, computer calculations of effects of grain boundaries, separations, and defects on the magnetization, exchange interactions, and magnetic anisotropy are being used as inputs to micromagnetic simulations of perpendicular hysteresis loops, magnetic cluster domains, magnetization reversal, dynamic coercivity, thermal stability, and magnetic recording. Measurements of grain size distribution, surface roughness, texture, and boundary defects such as stacking faults and pinning sites should be made using high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Magnetic cluster domains, recorded bits, and media noise will have to be studied using magnetic force microscopy (MFM) and Lorenz TEM in order to understand the key interrelationships between magnetic and physical structure. There are potentially three different PMR writing modes: (1) single-layer media using a thin film ring write head (TFH), (2) double-layer media having a soft magnetic keeper underlayer using a single pole head (SPH) or a pseudo-pole head (PPH) constructed from a standard TF write head by focused-ion-beam etching, and (3) triple-layer media having a soft magnetic keeper overlayer and underlayer using a single pole head (SPH) or a pseudo-pole head (PPH) constructed from a standard TF write head by focused-ion-beam etching. In the 1st mode, the PMR media must be highly-oriented with unity perpendicular squareness. In the 2nd and 3rd modes, the media must also have unity perpendicular squareness, but the soft underlayer and overlayer enhance the magnitude of the writing field and reduce the demagnetizing fields. However, the main advantages of single and double-layer PMR media are
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J.H. Judy / Journal of Magnetism and Magnetic Materials 235 (2001) 235–240
that PMR media can support extremely high densities and can be significantly thicker than LMR media for the same density so that the resistance of PMR to thermally decay is increased. An SPH or PPH with a keeper underlayer provides a 2 larger write field and higher head field gradient so that PMR media with coercivity higher than LMR can be used to give sharper transitions and sharper track edges suitable for achieving densities of 100–1000 Gbits/in2.
4. Future development of perpendicular magnetic recording At PMRC’97 in Akita, Dr. David Thompson of IBM challenged the PMR community to achieve by 12/31/02 a type 2 demo of writing and reading random data with a real HDD channel at 100 Gbits/in2. In my opinion, such a demonstration will confirm the intrinsic advantages of PMR with higher signal-to-noise and greater thermal stability to allow PMR to ultimately replace LMR beyond 100–1000 Gbits/in2. The challenges facing perpendicular magnetic recording are to simultaneously achieve high thermal stability with high signal amplitude and high media signal-to-noise for low bit-error-rate. Meeting these challenges will require extremely uniform average magnetic cluster diameters of 10–5 nm, double-layer 50– 25 nm thick media with a high-saturation 10– 20 kG low-noise magnetic underlayer or overlayer with permeability of 100–50, average surface roughness of 0.2–0.1 nm, remanent magnetization of 200–900 emu/cc, unity squareness without any demagnetization field correction, remanent coercivities of 5–15 kOe which can be written with subnanosecond rise times using a microstrip singleturn perpendicular single pole head and read with a 10–20% GMR spin-valve read head. The basic theory of thermally activated timedecay of remanent magnetization or the ultimate superparamagnetic limit is based on the assumption that for a single magnetic energy barrier, the exponential relaxation time constant t is given by the Arrhenius-N!eel law as t ¼ ð1=f0 ÞexpðDE=kTÞ;
ð1Þ
where DE is the uniaxial energy barrier, T is the absolute temperature, k is the Boltzmann’s constant, and f0 is the frequency of approach to the energy barrier which is taken as 109 s1. For most magnetic materials, this exponential decay is not observed and the time-decay of remanent magnetization MR follows a logarithmic time dependence for a wide uniform distribution of energy barriers as follows: MR ðt; Hint Þ ¼ MR ðt0 ; Hint Þ S lnðt=t0 Þ
ð2Þ
where the initial time is t0 and S is the magnetic viscosity or decay rate which is taken to be in units of %/decade (s), and Hint is the magnetic interaction field acting to reverse the MR : For a system of magnetic particles having a uniaxial anisotropy energy density Ku and magnetic volume V; the effective energy barrier to kT ratio can be shown to be given by DE=kT ¼ ðKV=kTÞeff ¼ ðKV=kTÞ0 ½1 ðHint =HK Þ3=2 ;
ð3Þ
where HK ¼ 2 Ku =MS is the uniaxial anisotropy field, and Ms is the saturation magnetization. Since the term ðKV=kTÞ0 is the energy barrier without magnetic interactions, the interaction field Hint tends to reduce the energy and increase the magnetic viscosity or time-decay rate. A ratio of about 42.543 corresponds to an Mr or signal loss of only 10% in 10 years at 651C. In order to better understand the subtle dependence of the thermal-activated time decay brickwall facing ultra-high-density recording (UHDR) densities beyond 100 Gbits/in2, the constraints and trends can be predicted by an UHDR areal storage roadmap. The results of Tarnopolsky [18] are used to generate the constraint to first achieve an on-track bit-error rate (BER) for an assumed media signal-to-noise-ratio (SNR)0-peak/ rms for a given bit length (B) and a user density U ¼ PW50=B is ðSNRÞpeak ¼ ð2=pÞUðB=sjitter Þ2 ½ðpU=4Þ=sinhðpU=4Þ2
ð4Þ
by calculating the timing position jitter (sjitter ) which is directly related to the cross-track correlation length (s) and the transition length parameter
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Table 1 UHDR roadmap for perpendicular magnetic recording from 100–1000 Gbpi2 Areal density (Gbpi2) Bit density (Kfci) Rack density (Ktpi)) BI aspect ratio=BAR BIT length=B (nm) Read track width=W (nm) FLY height=d (nm) Pulse Width=PW50 (nm) log on-track bit ero rate Signal-to-noise SNR (rms/rms) Number grains per bit=10^SNR/10 Average grain diameter=D (nm) Magnetization squareness=S Media thickness=d (nm) Anisotropy constant Ku (106 erg/cc) Media saturation=Ms (emu/cc) Remenant coercivity=Hc (Oe)
100 635 160 4 40 140 8 100 8 18 63 9 1 18 2 400 8500
(a) according to Bertram [20] by the following simple formula s2jitter ¼ ðp4 =48Þs a2 =w;
ð5Þ
where (w) is the read track width. For perpendicular recording media with Hc ¼ Hk and S ¼ 1; the average grain diameter D is assumed to be equal to the average magnetic cluster diameter and equal to the dross-track correlation length s; and the transition length parameter a ¼ 3D=4; the SNR is then determined to take the simple form given by Bertram [19] as ðSNRÞpeak ¼ 0:314 w PW50 B=s a2
ð6Þ
However, a simpler method is to use the Poisson statistics of additive noise for the magnetic grains in the media to find the (SNR)rms/rms as given by ðSNRÞrms=rms ¼ 10 log 10 ðNÞ;
ð7Þ
where N is the number of magnetic grains per bit. This simpler approach has been followed to determine the average grain diameters assuming hexagonal close packing of the grains as shown in Table 1. Once the grain diameter has been determined and the media thickness has been chosen to insure sufficient thermal stability and resolution, the uniaxial crystalline anisotropy can be determined. Taking into account the demagne-
250 1000 250 4 255 89 7 64 7 17 50 7 1 20 3 550 11000
500 1400 354 4 18 63 6 45 6 16 40 5 1 22 4 700 12500
1000 2000 500 4 13 45 5 32 5 15 32 4 1 24 6 875 1430
tizing fields of three adjacent recorded bits and using the optimum coercivity as a constant between 1 and 3 times the saturation magnetization 4pMs to maximize the height of the energy barrier assuming Hc ¼ Hk ; the saturation magnetization Ms and remanent coercivity were determined as given in Table 1. This resulting ultra-high density roadmap for perpendicular magnetic recording given in Table 1 shows that PMR is capable of achieving 100 Gbits/ in2 easily with on-track BER of 108 and 10 nm grain size with Ku=1.7 106 ergs/cc and a remanent coercivity of 8500 Oe. Furthermore, it appears that 1000 Gbits/in2 may be possible with D=4 nm Ku=6 106 and a coercivity of 14300 Oe even though the on-track BER is only 105. Either the SNR of the media will have to be increased or the required SNR will have to be provided by improving signal processing techniques. If SNR=18 is assumed for 1000 Gbits/in2, D=3 nm and Ku=12.5 106 and the remanent coercivity required would be about 20 kOe. This may be possible? In conclusion, the future of perpendicular magnetic recording will be extremely challenging [20–24]. The goals are to reduce grain size and dispersion significantly, enhance magnetic isolation so that the cluster size is a single grain, reduce transition and DC noise, optimize design a
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single-turn single-pole head for high-speed writing, and achieve durable contact recording.
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