Title: Magnetization processes in non single domain magnetite particles

Abstract: Quasi-static hysteresis loops of spherical and spheroidal magnetite nanoparticles with semi-axes ratio a/b = 1.5 and 2.0 with different types of combined magnetic anisotropy are calculated using numerical simulation. For particles of each type the critical diameters Dcr are determined so, that above Dcr the magnetization curling becomes the easiest mode of particle magnetization reversal. Hysteresis loops are calculated both for single-domain nanoparticles in the diameter range Dcr < D < Dc, and for vortex particles with diameters D > Dc, where Dc is the single-domain diameter. The results obtained are compared with the usual hysteresis loops of particles with diameters D < Dcr. Hysteresis loops of dilute oriented assemblies are studied for various angles of the external magnetic field relative to the particle symmetry axis. For the corresponding randomly oriented assemblies the hysteresis loops are obtained by averaging over this angle. It is shown that the magnetization reversal of nanoparticles studied occurs through the nucleation of the curling mode, which can be accompanied by the formation of various vortices in finite intervals of the external magnetic field. The remanent magnetization and coercive force of oriented and non-oriented dilute assemblies of magnetite nanoparticles with different aspect ratios are determined as the functions of the transverse particle diameter.