Effect of calculation conditions on the numerical simulation of magnetic materials with random magnetic anisotropy

A numerical simulation of magnetic materials with random magnetic anisotropy was performed. The magnetization of an assembly of magnetically-interacting grains with randomly-oriented uniaxial anisotropy was calculated using the Landau-Lifshitz-Gilbert equation. For simplicity in the simulation, the magnetizations in a grain were assumed to be aligned in the same direction; this assumption is known as the single spin model. The interaction at the interface of the grains was taken into account by including an interaction energy between the unit vectors that represent the magnetization directions of the grains. Calculations were carried out for an N × N × N system, where the grain sizes D ranged from 5 to 40 nm and N ranged from 10 to 80. The relation between the coercive forces H _C and the grain size is represented by H _C ∼ D _k. For the case of N = 10, k = 5.7, which corresponds to the primitive theory of the random anisotropy model (RAM) where k = 6. As N increased, k decreased slightly from 5.7 to 4.2. The gradient of the log-log plot of the coercive force versus the grain size (d logH _C/d logD) was deduced by smoothing the data and was found to have a peak value of approximately 6. This result suggests that the RAM is supported by the simulation within the range of grain sizes where the peak was observed.