Structural and magnetic susceptibility studies of SiO 2: Fe 2O 3 nano-composites prepared by sol–gel technique

Abstract

Silica gel doped with different concentrations of Fe 2O 3 ranging from 5 up to 40 wt% (SiO 2: 5–40 wt% Fe 2O 3) 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 °C. Structural and magnetic characteristics were investigated through XRD, TEM, and magnetic susceptibility measurements. At 900 °C many γ-Fe 2O 3 crystalline nano-particles formed, while a further increase of the temperature resulted in the γ to α and/or ε: Fe 2O 3 transformation. By increasing the Fe 2O 3 content, the appearance of the α and/or ε phase is accompanied by a further growth of the γ phase. It seems, therefore, that the progressive γ-to- α and/or ε transformation is accompanied by a contemporary growth of the γ phase at the expense of the residual amorphous phase. The samples show low-magnetic susceptibility χT value typical to anti-ferromagnetic substances at lower sintering temperature ranging from 200 up to 700 °C, while transition to ferrimagnetic behavior is observed at higher sintering temperature ranging from 700 up to 1150 °C. It is clearly seen that between 700 and 900 °C, a broad transition occurs from anti-ferromagnetic amorphous phase to a ferrimagnetic one, corresponding to γ-Fe 2O 3. At constant sintering temperature 1150 °C the samples show lower magnetic susceptibility χT value at higher concentrations of Fe 2O 3 embedded in silica gel matrix. The observed behavior can be explained by the presence of γ-Fe 2O 3 , whose contribution to the magnetic saturation at higher Fe 2O 3 content is much lower than that of α-Fe 2O 3 (and/or ε-Fe 2O 3) at this higher temperature.