Evolution of Morphology and Magnetic Properties in Silica/Maghemite Nanocomposites

The morphological evolution of SiO2/γ-Fe2O3 nanocomposites was systematically synthesized in a one-step flame spray pyrolysis. Under these conditions, a gradual transformation from discrete γ-Fe2O3 nanoparticles to thin SiO2 coatings, segregated single γ-Fe2O3 core, and multiple γ-Fe2O3 cores within a SiO2 matrix was obtained as a function of SiO2 loading. The presence of SiO2 up to 13% has a pronounced effect on the γ-Fe2O3 crystallite structure (transforming from P4132 to Fd3̅m space group) and its cationic vacancy ordering. Decrease in the latter was further reflected through the intrinsic magnetic properties of the γ-Fe2O3 cores (i.e., decreasing specific saturation magnetization and increasing coercivity and exchange bias). Deviation from the magnon-type thermal dependence, T3/2 Bloch law, was observed for nanocomposites with SiO2 content above 13%. The Ms vs T curves could be fitted with the sum of an exponential component and a Bloch law component, where the magnitude of the exponential component increased with increasing SiO2 content above 13% SiO2. The thermal dependence of the saturation magnetization for these samples could not be adequately explained by a finite size effect or via freezing of canted spins.