The magnetization process and coercivity in random anisotropy systems

A numerical simulation of a two-dimensional XY model has been carried out to study the thermal behaviour of the magnetization process and the variation of the coercive field as a function of the random anisotropy amplitude. The existence of two different magnetic regimes is evidenced: a low-anisotropy regime, which is characterized at zero temperature by a power law increase of the coercive field as the anisotropy amplitude increases, and a regime for anisotropy values higher than D/zJ=0.5, for which the system behaves as an assembly of quasi-independent clusters of two or three atoms. In this regime, the coercive field increases linearly with the anisotropy strength. The numerical estimates of the power law exponent are in good agreement with the heuristic predictions of Imry and Ma (1975). The domain wall motion and the spin configuration for the two anisotropy regimes have also been studied, pointing out the differences in the pinning process between both regimes. The thermal dependence of the coercive field has been obtained and fitted to an exponential law.