Micromagnetic modeling of two‐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-μm cubes of magnetite. Domain structures obtained for the cubes are relatively simple compared to ones from previous two-and three-dimensional calculations for smaller magnetite grains (Newell et al., 1993a; Williams and Dunlop, 1989, 1990). With an initially assumed one-dimensional lamellar structure, the final two-dimensional structure reveals the formation of closure domains at the grain surfaces. The magnetization directions in closure domains are found to be determined largely by the magnetostatic energy, rather than the magnetocrystalline anisotropy as normally expected. The number of body domains that gives the lowest-energy state for the 1-μm magnetite cube is 2, while the number for the 5-μm cube is 4. These are much smaller equilibrium numbers of domains than the 6 and 13 domains obtained from a one-dimensional model by Moon and Merrill (1985) and the 4 and 10 domains predicted from a quasi-two-dimensional analysis by Ye and Merrill (1991) for 1-and 5-μm cubes, respectively. The numbers of domains predicted are comparable to the numbers observed in magnetite grains of similar sizes.

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