Abstract
The saturation magnetization of ferrimagnetic nanoparticles of γ-Fe2O3 that are isolated in an amorphous SiO2 matrix, has a high increase when the temperature decreases from 300 to 77K. In this case, the relative variation (69.7 magnetization corresponding to bulk ferrite (9.5 is attributed to the narrowing of the paramagnetic layer that exists at the surface of nanoparticles; with the decrease of temperature, the mean magnetic diameter, and implicitly the magnetic moment of the particles increases, which in turn leads to an increase of the saturation magnetization. The existence of the paramagnetic layer is a result of the modification of the superexchange interaction between iron ions at the nanoparticle surface, due to the distortion of the crystalline network in the presence of the silica matrix. Based on the experimental results we propose a core-shell model that explains the abnormal increase of the saturation magnetization of γ-Fe2O3 nanoparticles dispersed in a silica matrix compared to bulk γ-Fe2O3.
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