GRAIN-SIZE DEPENDENCE OF 2-DIMENSIONAL MICROMAGNETIC STRUCTURES FOR PSEUDO-SINGLE-DOMAIN MAGNETITE (0.2-2.5 MU-M)

Micromagnetic modelling with a Monte Carlo method was performed to obt ain 2-D micromagnetic structures of pseudo-single-domain (PSD) magneti te (0.2-2.5 mu m). Cubic model grains were subdivided into a 2-D array with a maximum of 100 x 100 cells. In order to find an appropriate ce ll size, the variation of the self-consistency parameter (S) with cell size was examined. We regard solutions with S>0.95, which corresponds to cell sizes less than or equal to 0.03 mu m, as representing realis tic micromagnetic structures. For saturation remanence states, we foun d vortex structures for small PSD grains (less than or equal to 0.25 m u m) and closure domain structures for larger PSD grains (0.6-2.5 mu m ). Between 0.25 and 0.6 mu m, structures remained chaotic even after a very large number of Monte Carlo steps. In grains with a closure doma in structure, domain walls were usually subdivided into several segmen ts with opposite polarities of magnetization rotation.