btrduction rhtinuing to increase the dak-transfer rate of h d d i s k drives is becoming much more difficult. These drives are expected to operate at over 100 MB/s in the near fiaurr, but dynamic coercivity, d i c h increases rapidly at such high hquencies [IJ], might prevent the recording head &om frequency region was measured by using the short-pulse-field method [ I ]. However, we pointed out h u g h numerical calculations that H, measured by a shot-pdscfield with slow rise &ne was larger
As the recording density of hard-disk drives increases, it becomes more important to suppress media noise, which is deeply related to magnetic cluster. In this study, the magnetic cluster, thermal activation volume and the media noise in perpendicular recording media have been analyzed by using micro-magnetic simulation, and compared with data for longitudinal recording media [I].A 64x64 array of hexagonal single-domain grains was used ' to simulate a perpendicular recording medium. The easy axis of each grain was perpendicular to the film plane with a normal distribution of 3.0 degrees. The value of the uniaxial anisotropy energy was assumed to have a normal distribution of f l 7 % . The time evolution of the magnetizations of the grains was calculated by solving the LLG-equation and the Langevin equation. Figure 1 shows magnetization distributions of demagnetized states by DC reverse field and 488kFC1 recorded states in media with different. inter-grain surface exchange coupling w. Magnetic cluster size Dclus,er for these states can be defined from autocorrelation function of magnetization distributions [I]. Dclus,e:s from the demagnetized states are almost the same as that of the recorded states. It should be noticed that thermal activation volume is much smaller than D,I.,,, [2, 31. Figure 2 shows Dcfusler as a function of w divided by 2rr M: Dm.(where D, i . is the grain size). It is seen that Dcluste;s for media with various M, and are on one line and increase linearly with increasing w i~n M: DAlso the slope of the line is significantly smaller than that for longitudinal media shown by the dashed line. The media noise has been calculated to consist of DC and transition noise, and increases linearly with Thermal activation volume [2, 31 deeply related with signal degradation due to thermal fluctuation will he discussed also.
Tohoku Iii\titute ofTechnolog): Tiiihaku-ku, Sendai, YX2-8577, Japan.It was reported that measured coercive forces /IC's, o f perpendicular recording media were significantly smaller than the ebtimated values fi-om perpendicular amisotropy field Hk or anisotropy cnccgy [I. 21. Nuclcation sifc imodcls lhnt includc weak anisotropy rcgions in a medium wcrc insufficicnl to cxplain tcmpoml change of H, by thcrmal filmation [3l. In previous calculations, coherent magnetization rotation was a-sumed in normal grains or regions. In this study. H , and the temporal change of H, have been analyzed by using micromagnetic $muliltion including incoherent inagnctiration rofation in il grain.A 32x32 array of hexagonal prism gwins with il diametcr D o f 10 nm and it height fnui of I X nm was used to simulate B medium. A single grain was divided inlo five sub-grains in the direction o f film thickness. which is much larger than D (Fig. I). In each sab-gmin. the magnetization rotates coherently atid interacts with the neighbors by the exchange coupling. The easy axis of each grain wilh perpendicular to Ihe film lplane wilh a d i h h u t i o n of three deglees. The vdue of the iiniiixial perpendicular iiniaotropy energy K,, wit5 aaaumed to have a normid distribution of f17%. The time evolution o f the magnetizations of the grains due to the thermal fluctuation was calculalcd by solving thc Langevin equation 141. To compare M-H loops in long time scales. magnetizations were calculated using the temperature acceleration method 14.5 I. b-grain ~i~~~~~ 2 s h w a M-H loops rccunling media with intra-grain exchange stiffness tmd5 Fig. I . C a l c u l a t i~n m u d e l o f~ medium. cOnStsnl of o,l, and xIo6 AE-12 b' -1,s -1.0 -
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