SOME EFFECTS OF GRAIN SHAPE AND VARYING EXTERNAL MAGNETIC-FIELDS ON THE MAGNETIC-STRUCTURE OF SMALL GRAINS OF MAGNETITE

The change in the magnetic spin structure of simple domain configurations has been investigated as a function of grain size, shape and varying external magnetic fields at elevated temperatures. The results indicate that simplification of domain configuration occurs as grains become elongated, and as grain sizes increase from small pseudo-single-domain (PSD) to large PSD. In these cases domains become nearly lamellar, as suggested by earlier Bitter pattern observations on large PSD grains of magnetite, although the spins on the crystal surface in the theoretical models would not produce the Bitter lines representative of its body domain structure. Simulation of domain rotation under the influence of an external uniform magnetic field shows that the rotation mode depends upon the grain's temperature. At higher temperatures coherent rotation is favoured, whereas at lower temperatures the non-uniform domain structure prohibits coherent rotation and forces a wave-like transition via a lamellar three-domain stage.