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

The micromagnetic simulations for perpendicular recording media with incoherent magnetization rotation was performed by dividing a grain into subgrains in the direction of film thickness. When intra-grain exchange coupling was smaller, incoherent magnetization rotation occurred, leading to a stronger thermal fluctuation effect and lower coercive force H/sub c/. It was found that the critical thickness /spl delta//sub c/ exists, beyond which the incoherent magnetization rotation occurs. /spl delta//sub c/ is approximately proportional to 2.5/sup */(A/K/sub u/)/sup 1/2/, where A is the intra-grain exchange stiffness constant and K/sub u/ is the uniaxial perpendicular anisotropy energy. In the incoherent magnetization rotation, the decreasing rate of H/sub c/ by logarithmic time (in thermal viscosity slopes) was larger than that for coherent rotation. The calculations were in good agreement with the experimental coercive forces and thermal viscosity slopes.