Structural and Magnetic Properties of Fe₂O₃ Nanoparticles Dispersed over a Silica Matrix

Abstract

The structure and the magnetic properties of a series of Fe2O3−SiO2 nanocomposites (9−33 wt % Fe2O3), prepared by a sol−gel method and submitted to thermal treatments in the temperature range 300−900 °C, were investigated through XRD, TEM, EPR, and magnetic susceptibility measurements. Superparamagnetic iron(III) oxide nanoparticles with a narrow size distribution, dispersed over the amorphous silica matrix, are present in all the samples. They are mostly amorphous, antiferromagnetic in the samples treated at low temperatures. At T > 700 °C, a lot of γ-Fe2O3 crystalline ferrimagnetic nanoparticles (4−6 nm) are formed, while a further increase of the temperature results in the γ- to α-Fe2O3 transformation. The variation of iron oxide content affects the abundance of γ-Fe2O3 formation, which reaches the maximum percent values in the more dilute samples. In the more concentrated samples, while the amount of maghemite is still growing, antiferromagnetic α-Fe2O3 begins to form. As a consequence, the saturation magnetization lowers in the samples with higher Fe2O3 content. Also, interparticle interactions, evidenced by fitting susceptibility values versus temperature and by EPR observations, contribute to such a decrease.