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.

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The crystal and magnetic structures of BiMnO 3 were studied at high pressures up to 10 GPa by means of neutron diffraction in the temperature range 2-300 K. Three structural modifications, two monoclinic and one orthorhombic were found to exist in the pressure range studied and their structural parameters were determined. A suppression of the initial ferromagnetic state and formation of a new antiferromagnetic state with a propagation vector ͑1/2 1/2 1/2͒ was observed at P ϳ 1 GPa, accompanied with the monoclinic-monoclinic structural transformation. Possible mechanisms of the pressure-induced magnetic transition and origin of magnetoelectric phenomena in BiMnO 3 are discussed.

Simple diatomic molecules exhibit a variety of exciting physical phenomena under high pressures, including structural transitions, pressure induced metallization, and superconductivity. Oxygen is of particular interest because it carries a magnetic moment. For the first time we studied the magnetic structure in solid oxygen under very high pressure by a direct method, namely, neutron diffraction. A new type of magnetic order with ferromagnetic stacking of the antiferromagnetic O2 planes was discovered in delta-O2 at P=6.2 GPa. We show that all structural transformations at pressures <7 GPa are driven by spin interactions; therefore, high-pressure oxygen should be considered as a unique "spin-controlled crystal."

We have studied the magnetic and crystal structures of the hexagonal Laves phases RMn 2 H x (R ϭEr,Tm,Lu; xϭ4.2,4.6) by powder neutron diffraction. Hydrogen occupies interstitial sites of the metal lattice and forms ordered superstructures. Hydrogen stabilizes localized magnetic moments on the Mn sites by expanding the metal lattice. We have found a very strong coupling between the magnetic and structural properties of the hydrides. Very small modifications in the hydrogen sublattice result in drastic changes in the magnetic properties. The samples RMn 2 H 4.6 show a long-range antiferromagnetic ordering ͑propagation vector kϭ1/3 1/3 0͒, whereas the samples RMn 2 H 4.2 exhibit short-range magnetic correlations. We discuss our results in the framework of a model assuming that the magnetic ordering is driven by hydrogen superstructure which changes the local symmetries of the magnetic ions and releases the topological frustration in the Mn sublattice.

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