The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.

Abstract.We have employed first-principles calculations to study the magnetic properties of the binary system Co-O. Two types of calculations were carried out: i. Co-O clusters of 13, 14 and 19 atoms and ii. Co/CoO bilayer. The Co/CoO bilayer forms a ferromagnetic-antiferromagnetic interface. The geometrical structures of the clusters and of the Co/CoO bilayer were optimized via ab initio collinear and non-collinear calculations. The spin-orbit interaction was included, too. We found that the addition of the O atom in the Co cluster leads to an increase of the μ Co values of O's adjacent Co atoms. At the bilayer, the Co atoms in the interface have enhanced magnetic moment compared to the corresponding values of the bulk Co in agreement with the experiment.