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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Simulation Study of Bit Patterned Media With Weakly Inclined Anisotropy Naoki Honda1 , Kiyoshi Yamakawa2 , and Kazuhiro Ouchi2 , Fellow, IEEE Electronics and Intelligent System, Tohoku Institute of Technology, Sendai, Miyagi 982-8577, Japan Research Institute of Advanced Technology, Akita Prefectural R & D Center, Akita 010-1623, Japan Recording properties of bit patterned media with weakly inclined anisotropy were studied by simulation. When a shielded planar head was used, a recording system with an areal density of 2.6 Tbit/in2 was expected for 30-degree inclined anisotropy media. Relatively large tolerance of around 4% in the anisotropy field and 7 degrees in orientation dispersion as well as write margins in down- and cross-track directions would be expected. Recording densities would be extended to 4 Tbit/in2 with additional exchange coupling between the dots. It is expected that bit patterned media with weakly inclined anisotropy is one of promising media for ultra high density recording. Index Terms—Anisotropy dispersion, bit patterned media, recording simulation, weakly inclined anisotropy.
I. INTRODUCTION IT patterned media is one of promising candidates which extend recording densities of perpendicular magnetic . However, when the packing recording beyond 1 density of the magnetic dots is increased, magnetostatic interaction between the dots reduces the write shift margins in the down and cross track directions [1]. We have proposed bit patterned media with inclined anisotropy, which could reduce the effect of the magnetostatic interaction on the switching field and indicated high areal density recording beyond 2 [2]. Other approaches to increase the achievable recording density are to use relatively small dot size to avoid large magnetostatic interaction between the dots. However, these approaches require some method of reducing switching fields of the dots for write. Even use of ECC or super ECC dots with an exceptionally high magnetic anisotropy field still requires extensive effort to reduce magnetic spacing between the head and medium as small as 2 nm to acquire enough head field strength [3]. On the contrary, our approach of using an inclined anisotropy exhibited possibility of high areal density recording with moderate magnetic properties of the media of 5 and imposed no extreme condition for the head [4]. Although high density recording was expected by the approach, a large inclination angle of 60-degree from perpendicular direction was assumed for the anisotropy axis. It was anticipated that media with such a large inclination angle may not be easily fabricated. This paper studied simulation of high density recording of bit patterned media with weakly inclined anisotropy which is expected to be easier to fabricate. The effect of dispersions in the anisotropy field and the orientation angle on the recording properties were also studied.
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II. EFFECT OF ANISOTROPY ANGLE First, effect of the magnetic anisotropy angle of the dot on the remanence curve of the dot array was investigated by simulation. Fig. 1 shows normalized remanence curves of the dot ar-
Manuscript received October 30, 2009; accepted December 14, 2009. Current version published May 19, 2010. Corresponding author: N. Honda (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2039857
Fig. 1. Simulated perpendicular remanence curves for dot arrays with various anisotropy angles.
rays with various anisotropy angles, 30, 60 and 90 degrees from the film plane. The remanence curves were obtained using perpendicular applied field with a field step of 100 Oe for dot arrays arranged in 32 32 dots with a dot size of and the dot spacing of 5 nm, corresponding to 3.4 . Edge effect was eliminated by assuming attached areas in the calculation. An analytic soft magnetic underlayer (SUL) was assumed with a spacing of 1 nm between the dots and SUL. Av, was 22 kOe with a 2% dispersion, erage anisotropy field, saturation magnetization, , of 1000 , and the orientation dispersion of 1 degree for all the dot arrays. The curve for 90-degree array exhibited large shear due to magnetostatic , interaction between the dots. The switching field width, , which is defined as the difference between saturation field, , of the dot array, is as large and the reverse beginning field, as about 8 kOe, while the width is as small as around 2 kOe for 30-degree array. The 60-degree array showed intermediate value in the width. Therefore, the effect of the inclined angle on the switching field width is quite different from that of the tilted media where minimum SFD is expected at around 45 degrees [5].
0018-9464/$26.00 © 2010 IEEE
HONDA et al.: SIMULATION STUDY OF BIT PATTERNED MEDIA WITH WEAKLY INCLINED ANISOTROPY
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Fig. 4. Normalized perpendicular remanence curves for media with different anisotropy angle. Fig. 2. Areal density dependence of switching field width for dot arrays with various anisotropy angles.
Fig. 3. Simulation model of bit patterned medium with inclined anisotropy. Areal density of the dot array is 2.6 Tdot=in .
Fig. 2 shows areal dot density dependence of the switching field width, , for the three dot arrays of Fig. 1. Only the dot spacing was changed. The 60-degree array exhibited the intercompared with that of 30 and 90-degree arrays at mediate , but became similar high dot densities greater than 4 to that of 30-degree array for smaller dot densities less than 3 . Therefore, dot arrays with weakly inclined (30-degree from the film normal direction) anisotropy are expected to exhibit high recording performances at moderate areal densities. Films with such weakly inclined anisotropy would be much easier to fabricate compared with that of 30-degree anisotropy. III. RECORDING SIMULATION AT 2.6 A. Simulation Model with Magnetic dot was modeled as , and the dots were arranged in 8 tracks 128 dots with 12.5 and 20 nm in down and cross track periods, respectively, as shown in Fig. 3. The inclination of the anisotropy axis was set at 60 degrees from the reverse down track direction.
Fig. 5. Perpendicular field profiles of a shielded planar head in down- and cross-track directions.
The inclination in the cross track direction exhibited almost the same remanence property, but the distribution is widened for , was 22 smaller angles. The average anisotropy field, inkOe which assured the thermal stability with cluding demagnetizing field at room temperature. A non magnetic 1 nm inter-layer was assumed on an analytic SUL. Normalized remanence curves are shown in Fig. 4 for the bit patterned media with the anisotropy angle of 60 and 90-degree. A 90-degree medium with a reduced anisotropy field of 15 kOe is also shown in the figure, which was used for recording as a reference medium. Fig. 5 shows perpendicular field profiles in down- and crosstrack directions for a shielded planar head [6]. Maximum filed of in around 15 kOe was obtained with a pole size of spite of an assumed head-medium spacing of 6 nm. Recording simulation was performed using this head field.
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Fig. 6. Down track position dependence of on track write error rate for media with various H dispersions and anisotropy angles.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 8. Anisotropy field and orientation dispersion dependence of on track write error rate.
Fig. 7 shows cross track position dependence of on and adjacent track write error rate for media with weakly inclined anisotropy. Large cross-track shift margin of 10 nm was indidispersion is 2%. The margin was reduced cated when the to 2 nm for the medium with the dispersion of 4%, but the errors were mainly attributed to the on track errors. A large tolerance of around 7 degrees in the orientation dispersion was also obtained for the media as shown in Fig. 8. Further simulation suggested possibility of high areal denwhen additional exchange sity recording beyond 4 coupling [1] is introduced between the dots. It is expected that bit patterned media with weakly inclined anisotropy is one of promising media for ultra high density recording. ACKNOWLEDGMENT Fig. 7. Cross track position dependence of on and adjacent track write error rate for media with different H dispersions.
This work was supported in part by the Green IT Project of NEDO and in part by the Cultivation Research Project of Innovation Satellite Iwate, JST.
B. Recording Simulation Recording performances at an areal density of 2.6 was investigated for the media with weakly inclined anisotropy. Fig. 6 shows down track position dependence of on track write error rate for various media. Detectable error rate is limited to 1/128 due to the dot numbers. Reverse timing of the head field with 2031 kFCI was shifted in the down track direction and the write error rate was measured. The results indicated relatively large write shift margin of 6.5 nm in down-track direction when dispersion of the dot was as small as 2%, as seen in the figure. Although the shift margin was reduced to 2.5 nm with dispersion of 4%, it was still around 1/5 of the an increased dot pitch. However, no shift margin was obtained when the dispersion is as large as 6% or the 90-degree anisotropy angle dispersion of 2%, which are also was used even with a small shown in the figure.
REFERENCES [1] N. Honda, K. Yamakawa, and K. Ouchi, “Recording simulation of patterned media toward 2 Tb=in ,” IEEE Trans. Magn., vol. 43, no. 6, pp. 2142–2144, 2007. [2] N. Honda, K. Yamakawa, and K. Ouchi, “Simulation study of high-density bit-patterned media with inclined anisotropy,” IEEE Trans. Magn., vol. 44, no. 11, pp. 3438–3441, 2008. [3] A. Y. Dobin, H. J. Richter, J. Xue, and D. Weller, “10 Tb=in recording simulations on patterned domain wall assisted media,” presented at the Intermag 2008, Madrid, 2008, DA-02. [4] N. Honda, K. Yamakawa, and K. Ouchi, “Simulation study of bit patterned media with inclined anisotropy at 5 Tbit/in ,” presented at the Intermag 2009, Sacramento, 2009, CP-12. [5] K. Z. Gao and H. N. Bertram, “Magnetic recording configuration for densities beyond 1 Tb=in and data rates beyond 1 Gb/s,” IEEE Trans. Magn., vol. 38, no. 6, pp. 3675–3683, Nov. 2002. [6] K. Ise, S. Takahashi, K. Yamakawa, and N. Honda, “New shielded single-pole head with planar structure,” IEEE Trans. Magn., vol. 42, no. 10, pp. 2422–2424, 2006.
