The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ~1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L(2,3) absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

Element-and orbital-selective x-ray absorption and magnetic circular dichroism measurements are carried out to probe the electronic structure and magnetism of Ir 5d electronic states in double perovskite Sr2MIrO6 (M=Mg, Ca, Sc, Ti, Ni, Fe, Zn, In) and La2NiIrO6 compounds. All the studied systems present a significant influence of spin-orbit interactions in the electronic ground state. In addition, we find that the Ir 5d local magnetic moment shows different character depending on the oxidation state despite the net magnetization being similar for all the compounds. Ir carries an orbital contribution comparable to the spin contribution for Ir 4+ (5d 5 ) and Ir 5+ (5d 4 ) oxides, whereas the orbital contribution is quenched for Ir 6+ (5d 3 ) samples. Incorporation of a magnetic 3d atom allows getting insight into the magnetic coupling between 5d and 3d transition metals. Together with previous susceptibility and neutron diffraction measurements the results indicate that Ir carries a significant local magnetic moment even in samples without a 3d metal. The size of the (small) net magnetization of these compounds is a result of predominant antiferromagnetic interactions between local moments coupled with structural details of each perovskite structure.

When studying nominal magnetite nanoparticles, it is mandatory to obtain a precise structural characterization to get an accurate relationship with their physiochemical properties. The great deal of information accumulated to date on the characterization of nominal magnetite and maghemite NPs does not clarify if the synthesized materials are single o multiphase systems involving bulk-like stoichiometric oxides (Fe 3 O 4 , γ-Fe 2 O 3 , α-Fe 2 O 3 , ...), or single or multiphase entities formed by nonstoichiometric oxides. In this work we propose a new approach to determine the structure of Fe oxide NPs by using the Fe K-edge X-ray absorption near edge spectroscopy. We report here an X-ray absorption near edge spectroscopy study at the Fe K-and L 2,3 -edges, on nominal magnetite nanoparticles synthesized by different methods. In addition, X-ray magnetic circular dichroism was recorded at the Fe L 2,3 -edges, in selected samples. We have found that the experimental spectra are not well reproduced by any linear combination of the absorption spectra of Fe 3 O 4 and γ-Fe 2 O 3 bulk references, even taking into account other oxides as goethite or ferrihydrite. The analysis of the Fe K-edge XANES spectra shows that it is the size, and not the synthesis method, which determines the structure of the NPs. Our experimental results indicate that, irrespective of the synthesis method, the nominal magnetite NPs are, actually, a single phase non stoichiometric Fe 3−δ O 4 oxide. At the origin of this phase are the cation vacancies, which lead to the modification of the structural arrangements at the Fe sites with respect to those found in bulk-like iron oxides.