Numerical modeling of the domain structure in magnetite grains of submicron sizes

The domain structure in submicron grains of magnetite is mainly determined by their size (d) and, to a lesser degree, by their geometry. Only a unifor m single-domain structure exists at d less than or equal to 50 nm. In cubic magnetite particles, at least two modes coexist at d = 55.5-110 nm: a pseu do-single-domain "flower" mode and a "curling" mode. The upper limit of the flower mode existence is 110 nm. The curling mode prevails (with its confi guration being somewhat complicated) throughout the interval d = 55.5-500 n m considered in this paper and has the lowest energy and the highest stabil ity. Accordingly, the flower mode is metastable nearly throughout the regio n of its existence d = 50-110 nm. In magnetite particles having the shape o f an oblong ellipsoid of revolution, the single-domain structure is transfo rmed directly into the curling mode, ignoring the flower mode. The critical size of this transition for a sphere is 53.5 nm. At a ratio of the ellipso id axes of 3.17, the grain is in a stable single-domain state regardless of its size. A critical size of the single-domain structure of 53.5 nm, obtai ned in our calculations For spherical particles, is in excellent agreement with the analytical estimate of Eisenstein and Aharoni [1976] (53.7 nm).