Magnetic Study of SiO₂/γ-Fe₂O₃ Nanocomposites Prepared by the SOL-GEL Method

Abstract
Nanocomposites of γ-Fe₂O₃ in a silica matrix were prepared by the sol-gel method using tetramethylorthosilicate (TMOS) as a precursor of silica and introducing iron as Fe(NO₃)₃ with Fe/Si ratios of 2, 5, 10 and 20%. The obtained gels were calcinated at temperatures between 600°C and 1000°C. Magnetic measurements performed with a vibrating magnetometer showed that superparamagnetic γ-Fe₂O₃ nanoparticles began to form at 700°C as confirmed by X-rays and Mössbauer spectroscopy. Superparamagnetic magnetization curves were fitted by a Langevin function considering a log-normal particle size distribution. This allowed to determine the saturation magnetization and the particle size distribution of the samples. The latter results were in good agreement with observations made by Transmission Electronic Microscope. For all studied concentrations, the γ-Fe₂O₃ particles size increased with the calcination temperature up to a maximal average diameter of 40 Å. This maximum was reached at 900°C for the highest concentrations and l000°C for the lowest. Calcinations at higher temperatures led to the degradation of the γ-Fe₂O₃ phase with creation of defects in the particles and formation of α-Fe₂O₃. This phenomenon was accompanied by a decrease in the saturation magnetization and the presence of an hysteresis in the magnetization curve.