Saturation magnetization of γ-Fe2O3 nanoparticles dispersed in a silica matrix

The relative variation of the saturation magnetization of non-interacting nanoparticles of gamma-Fe2O3 (0.68 vol%) that are isolated in an amorphous SiO2 matrix is of 69.7% when the temperature decreases from 300 to 77 K. This variation is much higher than the relative variation of the spontaneous magnetization corresponding to bulk ferrite, which is of only 9.5% in the same temperature range. We have attributed the observed anomaly to the increase of the mean magnetic diameter of the particles (attached to the particle's core where the spins are aligned due to the superexchange interaction) from 10.20 to 11.76 nm when the temperature decreases from 300 K to the temperature of liquid nitrogen. The increase of the magnetic diameter is caused by the presence of a paramagnetic layer on the particle surface, a layer that becomes ferrimagnetically ordered at low temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.