Evolution of morphology and magnetic properties in silica/maghemite nanocomposites

The morphological evolution of SiO₂/γ-Fe₂O ₃ 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 SiO ₂ up to 13% has a pronounced effect on the j γ-Fe ₂O₃ crystallite structure (transforming from P4132 to Fd3m 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. © 2009 American Chemical Society.