Computer simulation of coercive force and thermal viscosity in perpendicular recording media

Abstract

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 -<I.( /Ill 115 111 I ! H i H i Fig. 3. M-H loops of a CoCrPl medium with Hi=lflk(k and IId4nM,=3.19 in a) perpendicular direction and h) in-plane direction. idong with the conventional case of coherent magnctization rolation. It is seen that the coercive force H , decrenses with decreasing the exchange rtilfness const~nI. I t is to be noled lhat this decrease of H, does nul U C C U~ withuul thcrmal nocluatiun ( H , , at high frcqsency or T=UK). I1 is also to be noted that the value of HdHL is 0.85 bccnuse of the aspect ratio of the grain. When A is smnller. the cffcctive switching voliimc hccomes smaller llian the grain voliime. -rhcdurc the lhcrmid fluctuation dkct becomes stronger and causes incoherent magnetization rotttion, leading Io lower coercive force. The number of subgains largcr t l m four does not affecl these results. Cunaequently the problem oi H, and lcmpurill change of / I , are hasically explained without introducing any W 0 z 8 8 8 models of nucleation \ i t a with b i n d l .mirotropy. imediiirri ilrc sImw81 in Fig. 3 (a (pcrpcndicularl and (b (in-plane) along wilh calcul.ilrd luups using i n fitted consistent with the experimental value o f CoCr based a 7J 2 P P 'Typical experimental M-H loops of B CaCrPt e w value of the A of fl.15rIO"er~cm. This value is alloy 161. The Ciltuhlion'i bared on the i...