MICROMAGNETIC MODELING OF 2-DIMENSIONAL DOMAIN-STRUCTURES IN MAGNETITE

Micromagnetic studies of two-dimensional domain structures in various local energy minimum states were carried out for 1- and 5-mum cubes of magnetite. Domain structures obtained for the cubes are relatively si mple compared to ones from previous two- and three-dimensional calcula tions for smaller magnetite grains (Newell et al., 1993a; Williams and Dunlop, 1989, 1990). With an initially assumed one-dimensional lamell ar structure, the final two-dimensional structure reveals the formatio n of closure domains at the grain surfaces. The magnetization directio ns in closure domains are found to be determined largely by the magnet ostatic energy, rather than the magnetocrystalline anisotropy as norma lly expected. The number of body domains that gives the lowest-energy state for the 1-mum magnetite cube is 2, while the number for the 5-mu m cube is 4. These are much smaller equilibrium numbers of domains tha n the 6 and 13 domains obtained from a one-dimensional model by Moon a nd Merrill (1985) and the 4 and 10 domains predicted from a quasi-two- dimensional analysis by Ye and Merrill (1991) for 1- and 5-mum cubes, respectively. The numbers of domains predicted are comparable to the n umbers observed in magnetite grains of similar sizes.