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.
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