Hidekazu Ikeno

Room temperature ferromagnetism in Mn-doped γ-Ga2O3 with spinel structure

Mn-doped Ga2O3 (7 cation % of Mn) thin film has been grown on c-cut sapphire substrate using pulsed-laser deposition technique. Electron diffraction analyses by transmission electron microscopy found that the Mn-doped film shows γ phase with spinel structure, which is different from undoped film showing β phase. No secondary phase can be detected. Combination of Mn-L2,3 near edge x-ray absorption experiments with first-principles many-electron calculations unambiguously implies that Mn atoms are located at tetrahedrally coordinated Ga sites with a valence of +2. The doped sample shows ferromagnetism up to 350K.

Theoretical ELNES using one-particle and multi-particle calculations

One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L(2,3) edges of Mg and Al. Many-particle interactions, including both electron-electron and electron-hole interactions, are indispensable for the L(2,3) edges of transition metals and the M(4,5) edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.

Local environment of Mn dopant in ZnO by near-edge x-ray absorption fine structure analysis

High-resolution near-edge x-ray absorption fine structure (NEXAFS) at MnK edge is employed to probe the local environment of Mn dopant in ZnO. First-principles supercell calculations are systematically made to obtain theoretical NEXAFS. Mn is found to substitute for Zn up to 5at.%Mn in polycrystalline samples sintered at 1623K in air. Presence of Mn3O4 is apparent for samples with higher Mn content. The NEXAFS does not change in the range of Mn concentration from 0.01 to 5at.%, indicating the absence of Mn precipitates. The results are confirmed by examining the polarization dependence of the NEXAFS for a 5at.%-doped ZnO thin film.

Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites

Bifunctional electrocatalysts for oxygen evolution/reduction reaction (OER/ORR) are desirable for the development of energy conversion technologies. It is discovered that the manganese quadruple perovskites CaMn7 O12 and LaMn7 O12 show bifunctional catalysis in the OER/ORR. A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO4 and octahedral MnO6 units to promote direct OO bond formations.

Mn L2,3-edge X-ray absorption spectroscopic studies on charge-discharge mechanism of Li2MnO3

The redox reaction of Mn in Li2MnO3 was studied by X-ray absorption spectroscopy and ab initio multiplet calculation. Associated with the de-intercalation of Li-ion, small but clear spectral changes were observed in Mn-L2,3 X-ray absorption near edge structure (XANES). The systematic ab initio multiplet calculations of Mn-L2,3 XANES revealed that the spectral changes in the experiment could not simply be ascribed to the change of the valency from Mn4+ to Mn5+ but can be explained well by the changes of local atomic structures around Mn4+ due to the Li de-intercalation. Our results suggest that the electronic state of oxygen should change during charging in Li2MnO3.

Variation of Zr-L2,3 XANES in tetravalent zirconium oxides

Zr-L2,3 XANESs of tetravalent zirconium oxides with different coordination numbers and local symmetries are systematically investigated by ab initio multiplet calculations using fully relativistic molecular spinors for model clusters. Experimental Zr-L2,3 XANESs are obtained for SrZrO3, m-ZrO2 (monoclinic) and t-ZrO2 (tetragonal). The theoretical spectra are in good agreement with the experimental data. The multiplet effects are found to play essential roles in determining the peak shape. The shapes of L3- and L2-edges are systematically different. The intensity ratios of the doublet peaks at both L3- and L2-edges are found to be sensitive to the coordination number of Zr. The ratio can therefore be used to estimate the coordination number of Zr in such oxides.

First principles multiplet calculations of the calcium L-2,L-3 x-ray absorption spectra of CaO and CaF2

First principles calculations are performed for the interpretation of the L₂,₃ x-ray absorption spectrum of calcium oxide and calcium fluoride. The first principles calculations are based on configuration interaction (CI) calculations using fully relativistic molecular spinors. The first principles results are compared to experimental data and also to calculations based on a semi-empirical crystal field multiplet model and also on a multichannel multiple scattering method. We show that the CI calculations show good agreement with experiment, both for bulk and for surface experiments. The remaining differences with experiment and between the theoretical models are discussed in detail.

Effect of local coordination of Mn on Mn-L2,3 edge electron energy loss spectrum

The effects of the local coordination environment of Mn ions in perovskite manganese oxides on the Mn-L2,3 edge electron energy loss (EEL) spectra was experimentally and theoretically investigated. The Mn-L2,3 edge EEL spectra were observed for various perovskite manganese oxides, including YMnO3, LaMnO3, BaMnO3, SrMnO3, and CaMnO3, in which the Mn ions have different valence states and local coordination. The experiment revealed that the Mn L3/L2 ratio is influenced not only by the valence state but also by the local environment of the Mn ions. Furthermore, compared to the Mn L3/L2 ratios of Mn3+ compounds, the Mn L3/L2 ratios of the Mn4+ compounds are found to be much more sensitive to local distortions. The ab-initio multiplet calculation of the Mn-L2,3 edge EEL spectra revealed that the effects of local coordination on the spectral features are dependent on the local electronic structures of the Mn ions. These findings indicate that the valence state as well as the local environments of the Mn ions can be unraveled by combining experimental and theoretical investigations of Mn-L2,3 edge EEL spectra.

Awaking of ferromagnetism in GaMnN through control of Mn valence

Room temperature ferromagnetism of GaMnN thin film is awaked by a mild hydrogenation treatment of a sample synthesized by molecular beam epitaxy. Local environment of Mn atoms is monitored by Mn-L2,3 near edge x-ray absorption fine structure technique. Doped Mn ions are present at substitutional sites of Ga both before and after the hydrogenation. No secondary phase can be detected. Major valency of Mn changes from 3+ to 2+ by the hydrogenation. The present result supports the model that the ferromagnetism occurs when Mn2+ and Mn3+ are coexistent and holes in the midgap Mn band mediate the magnetic coupling.

Magnetic structures of FeTiO3-Fe2O3 solid solution thin films studied by soft X-ray magnetic circular dichroism and ab initio multiplet calculations

The solid solutions between ilmenite (FeTiO3) and hematite (α-Fe2O3) have recently attracted considerable attention as a spintronic material due to their interesting magnetic and electrical properties. In this study, the electronic and magnetic structures of epitaxially grown 0.6FeTiO3·0.4Fe2O3 solid solution thin films were investigated by combining x-ray absorption near-edge structure (XANES), x-ray magnetic circular dichroism (XMCD) for two different crystallographic projections, and first-principles theoretical calculations. The Fe L-edge XANES and XMCD spectra reveal that Fe is in the mixed-valent Fe2+–Fe3+ states while Fe2+ ions are mainly responsible for the magnetization. Moreover, the experimental Fe L-edge XANES and XMCD spectra change depending on the incident x-ray directions, and the theoretical spectra explain such spectral features. We also find a large orbital magnetic moment, which can originate the magnetic anisotropy of this system. On the other hand, although the valence state of Ti was interpreted to be 4+ from the Ti L-edge XANES, XMCD signals indicate that some electrons are present in the Ti-3d orbital, which are coupled antiparallel to the magnetic moment of Fe2+ ions.