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_c. It was found that the critical thickness δ_c exists, beyond which the incoherent magnetization rotation occurs. δ_c is approximately proportional to 2.5^*(A/K_u)¹2/, where A is the intra-grain exchange stiffness constant and K_u is the uniaxial perpendicular anisotropy energy. In the incoherent magnetization rotation, the decreasing rate of H_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.