FR-08 : Tuning the structural and magnetic properties of iron oxide nanoparticles

Magnetic iron oxide nanoparticles (NPs) have gained significant attention in biomedical applications such as targeted drug delivery and hyperthermia therapy. This is due to their magnetic properties and non-toxic nature. Combining magnetic iron oxide NPs with plasmonic gold NPs, novel multifunctional nanomaterials can be produced. Depending on their oxidation states, iron may form various crystal structures, thus showing different magnetic properties. However, their magnetic properties in nanoscale may differ from the bulk behavior due to the finite size effect. Moreover, the synthesis of single-phase oxide NPs is challenging, often resulting in the presence of an oxidized layer at the surface. We observe a shift in the center of the hysteresis loops of iron oxide NPs. This is due to the exchange interaction between the magnetite core and a shell with disordered/oxidized surface spins. By comparing hysteresis loops cooled at different magnetic fields, a hardening effect is observed, i.e. the squareness and hardness of hysteresis loops is significantly enhanced with increasing magnetic cooling field. This indicates that an anisotropy axis is induced due to the exchange bias effect. To synthesize nanomaterials with tunable magnetic properties, it is important to understand how the crystallographic structure or the phase composition inside the NPs influences their magnetic properties. In this work, we performed systematic investigations on single iron oxide NPs as well as Au@FeOx multifunctional NPs with various sizes and shapes. The crystallographic structure and the magnetic properties of the samples were tuned by oxidation or reduction via different annealing procedures. From the X-ray absorptions spectroscopy results, one obtained the ratio of different iron oxide phases existing inside the particles. Anomalous small-angle X-ray scattering provides us with information about the size and thickness of core/shell structures. Combining these X-ray techniques with magnetometry, one can better understand the relationship between structural and magnetic properties. Furthermore, we studied the influence of a thin Au shell or a small Au NP attaching to the surface of the iron oxide NPs on their magnetic properties.