SURFACE SPIN CANTING IN BAFE12O19 FINE PARTICLES

Several samples of BaFe12O19 were prepared by the 'liquid mix' technique. The dependence of both particle size and macroscopic magnetic parameters on the synthesis procedure was studied by means of X-ray powder diffraction and high-field magnetization measurements. It is shown that particle size increases and magnetic parameters approach the intrinsic ones as the synthesis temperature and time rise. Besides, the non-collinearity of the surface spins tends to reduce the saturation magnetization and the degree of reduction is strongly dependent on particle size through the surface-to-volume ratio. Some approximations have been considered to check this effect: we have assumed that the spin canting takes place in the outer region near the surface, while in the inner region the magnetic structure is that of a bulk and the magnetic moments remain collinear to the easy magnetization axis (shell model). The possibility of surface spin canting in the basal and side planes is also discussed. Some other hypotheses concerning the evolution of the spin canting angle inside the particle are tested. The results obtained suggest that the width of the surface region where spins are non-collinear depends on particle size and it may range from a few to some tens of Angstroms.