Effect of citric acid on the morpho-structural and magnetic properties of ultrasmall iron oxide nanoparticles

The structural and magnetic properties of iron oxide nanoparticles obtained by the precipitation of iron ions in the presence of different amounts of citric acid were studied. The citric acid acts as a capping agent and provides control of the particle size to be obtained. The size dependence of the magnetic properties was investigated by comparing the results for ~2, 4, 6, and 8 nm particles. With decreasing the particle size, a reduction of the saturation magnetization (from ~84 to 33 Am2/kg) and an increase of coercivity (from ~32 to ~100 mT) were observed, due to the increasing role of the disordered surface. The behaviour of the zero-field cooled/field cooled magnetization curves indicates a superspin-glass freezing for 2 nm and 4 nm particle samples. Memory experiments below the freezing temperature on the 2 nm particle sample provide further evidence of a glassy magnetic state. The observation of exchange bias, due to core/shell interface exchange coupling, for 2 nm and 4 nm particle samples provides evidence of a surface spin-glass type freezing, which contributes to the observed glassy properties