MONTE-CARLO SIMULATION OF 2-DIMENSIONAL DOMAIN-STRUCTURES IN MAGNETITE

A Monte Carlo method was applied for micromagnetic studies of two-dime nsional domain structures of a 1-mu m magnetite cube in zero field. By using this method we could incorporate the effect of thermal agitatio n into micromagnetic modeling. Starting from an initial single-domain structure (a saturated state), we obtained a closure domain structure with three body and four closure domains. This structure is quite diff erent from the checkerboard-like structure obtained by a conjugate gra dient method, and it gives a much lower energy and saturation remanenc e ratio. An initial lamellar two-domain structure evolved into a vorte x structure, which was also reached from an initial quasi-vortex struc ture. Although such a vortex structure has a lower energy than the clo sure domain structure, it was not attainable from an initial single-do main structure at room temperature. The Monte Carlo method is effectiv e in finding a path to escape from unstable local energy minima and re ach a stable local energy minimum, although not necessarily a global m inimum, at a given temperature. The structure corresponding to such a stable local minimum should represent a realistic domain structure, co mparable to what would be attained in nature with the aid of thermal f luctuations of spins.