Tuning the structural and magnetic properties of iron oxide nanoparticles

Due to their biocompatibility and magnetic properties, iron oxide nanoparticles (NPs) areespecially interesting for applications such as targeted drug delivery and hyperthermia therapy[1–3]. According to its oxidation states, iron may form various crystal structures and thus showdifferent magnetic properties. Divalent FeO is a bulk antiferromagnet with a rock salt crystalstructure at room temperature. When Fe2+ is oxidized towards the trivalent state, as found e.g. inFe3O4 and Fe2O3, one encounters a spinel structure and ferrimagnetic (FiM) behavior. The FiM toparamagnetic phase transition for bulk magnetite and maghemite occurs at TC = 858K and 948K,respectively. However, they may show different magnetic properties in nanoscale due to thefinite size effect. Synthesis of single-phase oxide NPs is challenging. An oxidized layer is oftenfound at the surface of the NPs. This leads to an exchange bias effect.We observe a shift in the hysteresis loops of various sizes of iron oxide NPs (5-20nm). This isdue to an exchange interaction between the magnetite core and a shell with disordered surfacespins. By comparing hysteresis loops cooled at different magnetic fields, a hardening effect isobserved, i.e. the squareness and hardness of hysteresis loops is significantly enhanced withincreasing magnetic cooling field. This indicates that an anisotropy axis is induced due to theexchange bias effect.In order to understand the origin of the exchange bias effect, we studied their crystallographicstructure using X-ray powder diffraction, total scattering experiments with pair distributionfunction analysis. The ratio of different phases of iron oxide (wustite, magnetite and maghemite)was obtained using X-ray absorptions spectroscopy. The morphology of the particles wascharacterized using scanning electron microscopy and small angle scattering. The relationshipbetween the composition of the NPs and the exchange bias effect is studied. Furthermore, themagnetic properties of the samples can be tuned by oxidation or reduction via differentannealing procedures. These results provide important information for the manipulation of theexchange bias in oxide NPs