Anomalous viscosity and thermal remagnetization in Monte Carlo simulations of planar random spin-models

A ferromagnetic, planar spin-model with random anisotropies in two dimensions is used to simulate thermally activated processes in exchange-coupled, two-phase permanent magnets by Monte Carlo dynamics. Simulations of hysteresis and slow relaxation processes at finite temperature are reported. The model displays exchange-spring behavior, anomalous viscosity along recoil loops, and thermal remagnetization in qualitative accord with experimental findings on spring magnets. The anomalous relaxation of the model can be explained by shrinking of subcritical interaction domains along the recoils. It is argued that disorder is important for the observability of anomalous viscosity in materials, which undergo magnetic reversal by nucleation and growth of interaction domains.