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Mössbauer Spectroscopic Study of Fe2O3 Nanoparticles Dispersed over a Silica Matrix

A series of Fe2O3 -SiO2 nanocomposites (9 -33 wt% of Fe2O3) has been prepared by a sol-gel method and submitted to thermal treatments at 300 -900 °C. The samples were characterized by X-ray diffraction and Mössbauer Spectroscopy measurements. Superparamagnetic behavior is exhibited by all the samples, indicating that the size of iron oxide grains is in the nanometer range (4-6 nm). Increase of iron content and temperature treatment give rise to a small particle growth, while the spread of sizes around the average value increases with the iron concentration. The Mössbauer spectra, at all the explored compositions, show a very steep increase of the peak width by treatment temperature at 900 °C, indicating the formation of the ferrimagnetic γ-Fe2O3 phase from the antiferromagnetic amorphous Fe2O3 phase, which dominates in the samples treated at lower temperatures. The samples at 28.5% and 33.2%, treated at 900 °C, also show a component of γ-Fe2O3 in the blocked state at room temperature. Moreover, the growth of the particles favours the formation of other oxide phases (α and ε phase) along with the γ phase.