CRYSTALLITE SIZE EFFECT ON SATURATION MAGNETIZATION OF FINE FERRIMAGNETIC PARTICLES

Fine equiaxed gamma-CoxFe2-xO3 (x = 0, 0.06) particles with diameters of 200-1000 Angstrom were prepared by chemical precipitation. The average crystallite sizes of the particles were determined from X-ray line broadening measurements. The saturation magnetization of the particles were measured at 295 and 79 K, respectively. An empirical linear dependence of the specific saturation magnetization sigma(s) on the specific surface area S-a of the fine crystallites was obtained in the form of sigma(s)(S) = sigma(s)(infinity)(1-AS(a)), where the slope A is different for gamma-Fe2O3 and gamma-Co0.06Fe1.94O3 particles. Under the supposition that the fine crystallite consists of two parts, i.e. a surface layer, whose magnetic moment can not be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and an inner part, whose magnetic moment can be aligned along the direction of the applied field, the above empirical formula can be interpreted well.