Structural and magnetic susceptibility studies Of SiO2 : Fe2O3 nano-composites prepared by sol-gel technique

Silica gel doped with different concentrations of Fe2O3 ranging from 5 up to 40 wt% (SiO2: 5-40 wt% Fe2O3) was prepared by sol-gel method, using tetra-ethoxysilane as precursor material. The prepared samples were submitted to thermal treatments in the temperature range from room temperature (RT) up to 1150 degrees C. Structural and magnetic characteristics were investigated through XRD, TEM, and magnetic susceptibility measurements. At 900 degrees C many gamma-Fe2O3 crystalline nano-particles formed, while a further increase of the temperature resulted in the gamma to alpha and/or epsilon Fe2O3 transformation. By increasing the Fe2O3 content, the appearance of the alpha and/or epsilon phase is accompanied by a further growth of the gamma phase. It seems, therefore, that the progressive gamma-to-alpha and/or epsilon transformation is accompanied by a contemporary growth of the gamma phase at the expense of the residual amorphous phase. The samples show low-magnetic susceptibility chi T value typical to anti-ferro magnetic substances at lower sintering temperature ranging from 200 up to 700 degrees C, while transition to ferrimagnetic behavior is observed at higher sintering temperature ranging from 700 up to 1150 degrees C. It is clearly seen that between 700 and 900 degrees C, a broad transition occurs from anti-ferromagnetic amorphous phase to a ferrimagnetic one, corresponding to gamma-Fe2O3. At constant sintering temperature 1150 degrees C the samples show lower magnetic susceptibility chi T value at higher concentrations of Fe2O3 embedded in silica gel matrix. The observed behavior can be explained by the presence of gamma-Fe2O3, whose contribution to the magnetic saturation at higher Fe2O3 content is much lower than that of alpha-Fe2O3 (and/or epsilon-Fe2O3) at this higher temperature. (c) 2004 Elsevier B.V. All rights reserved.