Influences of cobalt substitution and size effects on magnetic properties of coprecipitated Co-Fe ferrite nanoparticles

Co-substituted ferrite nanoparticles with narrow size distribution have been prepared by coprecipitation method. X-ray diffraction (XRD) showed that the samples have cubic spinel structure of which the lattice constant slightly decreases upon cobalt substitution. The mean crystallite size of the samples was in the range 9.5-11 nm as deduced from the XRD line broadening. Energy dispersive X-ray spectroscopy (EDX) verified the presence of cobalt in the substituted samples. The morphology and size distribution of the nanoparticles were studied using transmission electron microscopy (TEM). Magnetic properties were determined using a vibrating sample magnetometer (VSM). The samples are characterized by a superparamagnetic transition at blocking temperatures T_B below room temperature. The coercivity H_c at low temperatures follows a simple model of thermal activation of particle's moment over the anisotropy barrier in the temperature range below T_B which is in accordance with Kneller's law for ferromagnetic materials. From the blocking temperature and from the thermal decay of the coercivity, the effective anisotropy constant values were determined to be in order of 10⁶ erg/cm³. The Curie temperature T_C and saturation magnetization M_s at nanoscale are lower than those of the bulk and decrease with the increase of cobalt content.