High-resolution micromagnetic models of fine grains of magnetite

High resolution three dimensional models are presented of cubic grains of magnetite in the grain size range 0.03 mu m to 4.0 mu m, achieved using a resolution of up to 262,144 elements to represent a single grain. Previous models have been limited in the maximum grain size that could be investigated because of the lack of resolution and increase in complexity of domain structure with grain size. This paper confirms the simple flower and vortex states found previously, but the classical two-domain (with closure domains) Kittel structure is shown not to be a local energy minimum state for magnetite. The concept of a single-domain to two-domain transition is now seen as a gradual unwinding of a flower state, and pseudo-single domain grains are shown to be vortex and multiple-vortex states. For grains greater than about 2 mu m in size the center of vortices act a nucleation centers for the formation of domain walls. At the largest grains size modeled (4.0 mu m), the expected features of multidomain grains are seen, with well-defined domain walls, and body domains with their magnetization aligned along the easy magnetocrystalline anisotropy axis.