Structural and magnetic properties of Ba-ferrite fine particles grown by glass crystallization

Abstract

The dependence of the specific saturation magnetization on the particle size of pure and Co-Ti-substituted Ba-ferrites was interpreted in terms of a nonmagnetic surface layer parallel to thec-planes of 1.3 nm and 0.7 nm, respectively, in thickness. Electron microscope investigations of the particles demonstrate that this effect arises neither from a sufficient fraction of superparamagnetic particles in the ensemble nor from the Co-Ti substitution or from surface defects such as steps and stacking disorder of the building blocks. Extrapolating this surface effect to small sizes yields a critical size, with the particles losing their ferrimagnetic behaviour. This critical size agrees surprisingly well with the volume calculated for the transition of the single domain state to the superparamagnetic one.