Microemulsion-Assisted Synthesis of Tunable Superparamagnetic Composites

Tunable superparamagnetic materials have been obtained by the thermal annealing of iron oxide/silica composites synthesized using microemulsions with a high content of the iron oxide precursor. The crystallochemical characteristics of the powders obtained after controlled hydrolysis within the reverse micelle nanocavities and their thermal evolution up to the formation of the γ-Fe 2 O 3 crystalline phase were studied. The thermally annealed composites clearly exhibited superparamagnetic behavior with the value of the saturation magnetization easily tuned by changing the iron oxide content and the annealing temperature. Information on the influence of particle size and interparticle interactions on the magnetic properties of the composites was inferred from zero field and field cooling measurements and also from the decay of the reduced remanence with temperature. It was observed that the blocking temperature increased with content and particle size of γ-Fe 2 O 3 . We have also determined a strong dependence of the effective anisotropy with the particle size associated with the existence of surface anisotropy.