AN INVESTIGATION OF PARTICLE-SIZE EFFECTS IN ULTRAFINE BARIUM FERRITE

Ultrafine particles of barium ferrite in the size range 5-100 nm have been synthesized by thermal decomposition of a citrate precursor. The precursor decomposed at 425-degrees-C is amorphous, but crystalline ba rium ferrite starts forming at temperatures of 550-degrees-C and above . Barium ferrite which shows a monophase X-ray diffraction pattern and well-resolved Mossbauer spectra is obtained at 700-degrees-C. The Mos sbauer spectra at both liquid helium and room temperature of samples a nnealed at 700, 750 and 800-degrees-C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be sati sfactorily resolved into five components corresponding to the five sub lattice sites in barium ferrite. The Mossbauer parameters: magnetic hy perfine field, quadrupole splitting and isomer shift, all show particl e size dependence. The magnetic hyperfine field and quadrupole splitti ng are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing tempera ture for all the five sublattice sites. Isomer shifts also differ from bulk values but the variation with particle size is also dependent on the sublattice sites.