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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