Mossbauer spectroscopic study of Fe2O3 nanoparticles dispersed over a silica matrix

A series of Fe2O3-SiO2 nanocomposites (9 - 33 wt% of Fe2O3) has been prepar ed by a sol-gel method and submitted to thermal treatments at 300 - 900 deg rees C. The samples were characterized by X-ray diffraction and Mossbauer S pectroscopy measurements. Superparamagnetic behavior is exhibited by all th e samples, indicating that the size of iron oxide grains is in the nanomete r 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 aver age value increases with the iron concentration. The Mossbauer spectra, at all the explored compositions, show a very steep increase of the peak width by treatment temperature at 900 degrees C, indicating the formation of the ferrimagnetic gamma-Fe2O3 phase from the antiferromagnetic amorphous Fe2O3 phase, which dominates in the samples treated at lower temperatures. The s amples at 28.5% and 33.2%, treated at 900 degrees C, also show a component of gamma-Fe2O3 in the blocked state at room temperature. Moreover, the grow th of the particles favours the formation of other oxide phases (alpha and epsilon phase) along with the gamma phase.