Micromagnetic calculation of hysteresis as a function of temperature in pseudo-single domain magnetite

Using an unconstrained three-dimensional micromagnetic model, magnetic hysteresis was simulated in magnetite in the grain size range 0.08-0.3 mu m as a function of temperature between 100 K and the Curie temperature (851 K). For all temperatures in the cubic phase above the Verwey transition (T-v approximate to 120 K), 0.08 mu m grains switched by coherent rotation, and 0.3 mu m grains switched with an intermediate vortex phase. The mode of switching for 0.1 mu m grains was found to be dependent on both field direction and temperature. For all grain sizes in the monoclinic phase below T-v, switching was by coherent rotation, with the domain state rotating in the monoclinic magnetocrystalline anisotropy "easy" a-plane. Calculations for coercive force (H-c) as a function of temperature display similar trends to experimental data. Below T-v, the model correctly predicts the large increase in H-c. However, the absolute values of H-c are higher than the experimental data.