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 barium 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 Mossbauer spectra at both liquid helium and room temperature of samples annealed at 700, 750 and 800-degrees-C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be satisfactorily resolved into five components corresponding to the five sublattice sites in barium ferrite. The Mossbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift, all show particle size dependence. The magnetic hyperfine field and quadrupole splitting are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing temperature 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.