Mössbauer studies of ultrafine particles of NiO and α-Fe2O3

Ultrafine particles of antiferromagnetic NiO and ferromagnetic -Fe₂O₃, supported on a high area silica material, were studied with the Mössbauer effect. The superferromagnetic transition was observed as a function of particle size and temperature. From the temperature variation of the Mössbauer spectra, magnetic anisotropy constants and particle size distributions were determined. Small particles of -Fe₂O₃ show increasing quadrupole splitting as the particle size is decreased. This effect is attributed to a large quadrupole splitting for the surface shell which has a different electric field gradient than the particle core. Analysis of the spectra using a simple surface shell model gives an estimate of the thickness and the electric field gradient of this shell. In contrast to bulk NiO, small particles of NiO show predominantly trivalent Fe hyperfine spectra over a wide temperature range. The stability of the trivalent iron may be due to a slight oxygen atom excess in the NiO lattice resulting in trivalent charge stabilization.