科研費報告書収録論文(課題番号:09355012・基盤研究(A)(2)・H9～H11/研究代表者:中村, 慶久/垂直ハード磁気ディスク装置を用いる超大容量ストレージシステムの研究

Jiang, W. H.

Muraoka, H.

Nakamura, Y.

Sugita, Y.

IEEE Transactions on Magnetics

English

Institutional Repositories DataBase (IRDB)

Thermal relaxation in perpendicular double-layered media

application/pdf学術論文 (Article)科研費報告書収録論文(課題番号:09355012・基盤研究(A)(2)・H9～H11/研究代表者:中村, 慶久/垂直ハード磁気ディスク装置を用いる超大容量ストレージシステムの研究)307385 bytesjournal articl

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1645 IEEE TRANSACTIONS ON MAGNETICS, VOL. 34, NO. 4, JULY 1998 Thermal Relaxation in Perpendicular Double-Layered Media W. H. Jiang, H. Muraoka, Y. Sugita and Y. Nakamura RIEC, Tohoku Univ, 2-1-1, Katahira. Aoba-ku, Sendai 980-77, Japan Absfracf- Output decay by thermal relaxation in perpendicular double-layered media is studied by numerical calculation and experiments. In addition some influences due to a single pole head are pointed out. We suggest that if these influences could be avolded, thermal relaxation In perpendicular magnetic recording would be much smaller than reported so far. High recording denslty and low noise is possible with relatively small particles, even at large film thickness in perpendicular magnetic recording. Index t e e  Thermal relaxation, output decay, single- , pole head, perpendicular magnetic recording. I. IN-IRODUCIION Thermal relaxation is becoming significant because a small bit size and a thin film are necessary for achieving high recording density and signal-to-noise ratio in longitudinal recording [1],[2]. Perpendicular magnetic recording is thought to be suitable for ultrahigh-density magnetic recording due to its low demagnetization character [3], however, this has not been sufficiently studied. In this paper, the relaxation dependence on recording density is investigated by experiments, and is compared with other results that have been reported [4],[5]. A single-pole writing head and a MR flying head were used for the experiments. 11. SIMULATION A two dimensional FEM(finite element method) simulation [6] associated with a curling switching model was modified and used by introducing an exponential time decay. The grains of the media were assumed to be cylindrical with heights equal to the media thickness (here 50nm). The exchange coupling between the particles was approximated by Manuscript received October 17,1997 W.H. Jiang, +81-22-217-5494, fax: +81-22-217-5496, E-mail:kyouQ kiroku.riec.tohoku.ac.jp; H.Muraoka; ~-Mail:muraoka@riec.tohoku.a~jp; Y. Nakamura; E-Mail:ynakamura@kioku.riec.tohoku.ac.jp. This research was partly supported by Association of Super-Advanced Electronics Technologies, Storage Research Consortium, and Japanese Govemment under Grant-in-Aid for ScientEic Research. the mean-field method [7]. The coefficient of the mean-field was determined to be 0.14 by fitting the slope of a measured M-H loop. The saturation magnetization M,, anisotropy field Hk and coercivity H, were 430 emu / cm3, 4360 Oe and 1100 Oe, respectively, the same as the medium parameters in the experiments. The parameters for the simulation were the grain diameter, the average particle volume and the standard deviation of volume. These will be denoted as D, V and 0, respectively hereafter. First of all, the recording process was simulated at one density under a no-relaxation condition. Next, the demagnetization field, calculated according to the original magnetization, was subtracted from the coercivity to determine the reverse field of the grain. Here K,, (anisotropy constant) was calculated from the coercivity Hc, which is equal to the grain switching field, instead of from the anisotropy field Hk in the S-W model. Thus, the time constant t was determined as X (K, V / kT), where k is the Boltzmann constant, and T is the absolute temperature. Following a time step At, the magnetization was calculated by a self-consistent calculation according to the time decay defined as M(tJ = M(t,,) X exp(-(-t,,t~t) / t ). The head output was obtained by reciprocity. Fig. 1 shows the time decay at various recording densities and volume distributions. For a small 0 / V (=0.1), there was almost no output reduction at both high and low recording density. The severe output reduction at large 0 / V coIlfirms that the relaxation of grains whose volumes are smaller than the average dominates the time decay. Meanwhile, the simulation indicates that the relaxation becomes less at high recording density than low density. This confirmed the low demagnetization characteristic at high recording density for perpendicular magnetic recording 181. 111. EXPERIMENTS AND RESULTS A. Recording Density Dependence The track width and the main pole thickness of the single-pole writing head used in the experiment were 50 /L m and 300nm, respectively. The flying height of the MR head was 50nm. The medium whose specification is mentioned 0018-9464/98$10.00 0 1998 IEEE Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on January 4, 2010 at 01:42 from IEEE Xplore.  Restrictions apply. 0.7 \ I * . , , , , , . I  I . . 1 1 , . 1 1  , I * ....*? . . , . 1 00 10' 102 i 03 i 04 1 os Time(seconds) Fig. 1 Time decay at various recording densities and volume deviations. 1. c-' a 0 +- U -+ - simulation (D = 70 nm, (J / V = 0.2) t 0 Head: SPT(write) Z Medium: 6 Go-Cr = 50 nm (double-layer) 0.8 1 00 10' 1 02 lo3 io4  i o 5  Time (seconds) Fig. 2 Dependence of time decay upon recording density. above was used. To avoid any influence from the writing head during the reading process, the single-pole head was removed from the medium immediately after writing, while the MR head kept reading. Fig. 2 shows the dependence of the time decay upon the recording density. Over the measured time range, almost no output reduction of the signal of 100 kFRPI was observed, in good agreement with the simulation as shown by the filled circles in the figure. Even at a very low recording density of 5 kFRpI, where the demagnetization field is not small in perpendicular magnetic recording, the reduction in output was less than 3%. The results in the experiment also confirm the advantage of low demagnetization field at high recording density for perpendicular magnetic recording. B. Influence of Single-Pole Writing Head 1) The anisotropy field effect in the head: Although the time decay in the simulation and the experiment were shown above to be not very large, this result is inconsistent with a previous experiment [4]. In order to confirm this, measurements with single-pole heads of Various Hk were carried out. Fig. 3 shows the dependence of time decay upon 8 0.9 Density = 110 kFRPl Head: Bulk SPT (readlwrile) 0:  Hk = 4.0 Oe z 0: Hk = 14.8 Oe Media: 6 Q-Cr E 50 nm (double-layer) 0.8 I I 102 1 o3 1 o4 Time (seconds) Fig. 3 Dependence of time decay upon main pole anisotropy field H,. I 1 Head: Thin-film SPT Time (seconds) Fig. 4 Time decay of output with thin-film return path head. the main pole anisotropy field (Hk). The output much larger when a low H, head was used. In contrast, the output reduction was reduced to less than 3% after two hours by increasing H, to 14.8 Oe. It has been suggested that the domain structure in a single pole head having a main pole with a low anisotropy field may not be stable during the reading process [9] and that this domain instability would demagnetize the medium. It is well-known that the domains are stable in high H, pole materials. 2) The head structure: A low inductance thin-film single-pole flying head was used to measure the output reduction in the perpendicular medium [lo]. The coil was designed to be near the main pole so that the recording sensitivity and overwrite cteristic are improved compared with the bulk single-pole head. Fig. 4 shows time decay using this thin-film head for writing and reading. At a recording density of lOOkFRF'1, about 3% output reduction was measured after 2 hours; if this logarithmic reduction is maintained, the data suggests that the output will be reduced to 95% of the original output after 10 years. The thin-film return path performs like a shield, shunting the extemal field Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on January 4, 2010 at 01:42 from IEEE Xplore.  Restrictions apply. 1647 1 5 0.9 B 0.8 0. 2 0 c N .-  E 5 0.7 z 0.6 -30 -20 -10 0 10 20 30 External field (Oe) Fig. 5 Iduence of external field for various single pole heads with a double- layered medium. affecting the main pole. 3) The external field influence: Fig. 5 shows the output reduction against external field. For a regular bulk single pole head the output reduced to 60% after 1500 seconds for H,=4.0 Oe and 80% for H,=14.8 Oe, at an applied field of about 2 Oe. On the other hand, the output reduced to 80% at an applied field of 20 Oe for a thin-film return path single- pole head. This field is much larger than reported before [4], which means that head structure is also important for the signal erasure with a head. That is, the extemal magnetic flux flow passes through the return path and reduces the influence of the external field on the main pole. N. CONCLUSION Thermal relaxation in perpendicular double-layered media was investigated by simulation and experiments. The severe reduction in output observed was accounted for by a low head anisotropy field and the effects of an external field. The head condition should be carefully examined for the measurements of the output decay in perpendicular double layer recording. Because of the small demagnetization field at high recording density in perpendicular recording, we can reduce thermal relaxation by using relatively small particles and still achieve high recording density and low noise because of the large film thickness. However, this requires a small volume and coercivity distribution and a consideration of the influence of domain instability in single-pole head and head structure. &TFiRENCES [l] P.-L. Lu and S. H. Charap, "Magnetic viscosity in high-density recording," J. AppL Phys., vo1.75, pp. 5768-5770, May 1994. [2] P.-L. Lu and S. H. Charap, "Thermal instability at 10 GbiVin' magnetic recording," IEEE Trans. Magn., ~01.30, N0.6, pp. 4230-4232, Nov 1994. [3] S .  Iwasaki and Y. Nakamura, "An analysis for the magnetization mode for high density magnetic recording," IEEE Trans. Magn., ~01.13, pp. 1272-1277, 1977. [4] W. Cain, A. Payne, M. Baldwin and R. Hempstead, "Challanges in the practical implementation of perpendicular magnetic recording," IEEE Trans. [5] N. Honda, K. Ouchi and S. Iwasaki, "1.5Gbit/in2perpendicular recording with inductive head," IEEE Trans. Magn., vo1.33, pp. 3097-3099, Sep 1997. [6] Y. Nakamura and I. Tagawa, "An analysis of perpendicular magnetic recording using a newly-developed 2D-FEM combined with a medium magnetization model," IEEE Trans. Mag., vo1.25, pp. 4159-4161, Sep 1989. [7] I. Tagawa, A. Takeo and Y. Nakamura, "Interparticle interaction field in magnetic recording media," J. Magn. Soc. Jpn, vo1.18, pp. 95-98, 1994 (in Japanese). 181 W. H. Jiang, H. Muraoka, Y. Tagawa and Y. Nakamura, "Bit density dependence of thermal relaxation in perpendicular magnetic recording," J. Mugn. Soc. Jpn., v01.21, pp. 293-295, 1997 (in Japanese). [9] K. Yamakawa, I. Watanabe, Y. Nakamura and S. Iwasaki, "Narrow track single pole head for perpendicular magnetic recording," Technical report of IEICE. MR84-28, pp.7-12, 1984 (in Japanese). [lo] H. Muraoka, K. Sato and Y.Nakamura et al. "Low inductance Thin-film single-pole flying head," to be presented Intermag'98. Map . ,  ~01.32, pp. 97-102, 1996. Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on January 4, 2010 at 01:42 from IEEE Xplore.  Restrictions apply. 

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Thermal relaxation in perpendicular double-layered media

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