We report evidence for a preserved magnetic state in FeO up to 143 GPa at room temperature using high-resolution x-ray emission spectroscopy. This observation is based on the spectral line shape of the Fe Kb emission line. Up to the highest pressure, FeO remains a magnetic insulator. Combining our results with previous Mössbauer data, we present a new magnetic phase diagram of FeO. Features like a closed-loop P-T antiferromagnetic domain confirm that high-pressure investigations can reveal new physical properties and unexpected phenomena.

We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe Kβ emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphide-like phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure.The study of the electronic structure of highly correlated transition metal compounds has been an important subject in condensed-matter physics over the last several decades. The theoretical phase diagram proposed by Zaanen, Sawatzky, and Allen [1] is one of the key steps leading to a better understanding of the materials. In addition to the on-site d-d Coulomb interaction (U ) employed in the original Mott-Hubbard theory, the ligand-valence band width (W ), the ligand-tometal charge-transfer energy (∆), and the ligand-metal hybridization interaction (T ) are explicitly included as parameters in the model Hamiltonian. This classification scheme has been very successful in describing the diverse properties and some seemingly contradicting behavior of a large number of these compounds. However, these highenergy-scale charge fluctuations are primarily characteristic of the elements involved, and thus cannot be freely adjusted for systematic study of their effects, although they can be varied somewhat by external temperature and magnetic field. On the other hand, pressure can introduce much larger perturbations of these parameters than can either temperature or magnetic field. Hence, it is of great interest to study the high-pressure behavior of these systems, and specifically, to correlate observed transformations with changes in electronic structure.

The iron structural and magnetic transition between the magnetic ͑bcc-␣) and the nonmagnetic ͑hcp-⑀) phases has been studied monitoring the pressure dependence of the Fe-K␤ fluorescence line excited with monochromatic synchrotron radiation. The relative intensity of the two multiplets shows an S-shaped pressure curve with flex point at the known transition pressure of 130 kbar. The S width of Ϸ30 kbar also coincides with the one determined in structural determinations, and in magnetism studies using Mössbauer techniques. This shows how the x-ray emission method can be used to probe the local magnetic properties of atoms under extreme thermodynamic conditions. ͓S0163-1829͑99͒03745-5͔

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Transition metal oxide heterostructures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity, and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide heterostructures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard X-ray photoelectron spectroscopy (HAXPES) and high resolution X-ray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a nondestructive way to qualitatively probe locally the oxygen defects in transition metal oxide heterostructures. We studied two trilayer structures as model systems: the La 0.8 Sr 0.2 CoO 3−δ /(La 0.5 Sr 0.5 ) 2 CoO 4−δ / La 0.8 Sr 0.2 CoO 3−δ (LSC 113 /LSC 214 ) and the La 0.8 Sr 0.2 CoO 3−δ /La 2 NiO 4+δ /La 0.8 Sr 0.2 CoO 3−δ (LSC 113 /LNO 214 ) on SrTiO 3 (001) single crystal substrates. We found that the oxygen defect chemistry of these transition metal oxides was strongly impacted by the presence of interfaces and the properties of the adjacent phases. Under reducing conditions, the LSC 113 in the LSC 113 /LNO 214 trilayer had less oxygen vacancies than the LSC 113 in the LSC 113 /LSC 214 trilayer and the LSC 113 single phase film. On the other hand, LSC 214 and LNO 214 were more reduced in the two trilayer structures when in contact with the LSC 113 layer compared to their single phase counterparts. The results point out a potential way to modify the local oxygen defect states at oxide heterointerfaces.