Evolution of Morphology and Magnetic Properties in Silica/Maghemite Nanocomposites

The morphological evolution of SiO2/I?-Fe2O3 nanocomposites was systematically synthesized in a one-step flame spray pyrolysis. Under these conditions, a gradual transformation from discrete I?-Fe2O3 nanoparticles to thin SiO2 coatings, segregated single I?-Fe2O3 core, and multiple I?-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 I?-Fe2O3 crystallite structure (transforming from P4132 to Fdm space group) and its cationic vacancy ordering. Decrease in the latter was further reflected through the intrinsic magnetic properties of the I?-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.