Yasuhiro Kobayashi

We investigate the microscopic origins of magnetic anisotropy in the ferromagnetic binary alloy Fe3Si film while applying reversible strain on a ferroelectric Pb(Mg,Nb)O3–PbTiO3 substrate. X-ray magnetic circular dichroism with the introduction of reversible strain clearly demonstrates the change in the magnetic easy axis direction, even with an anisotropy energy of 1.6 × 103 J/m3. The strain-dependent Mössbauer spectroscopy detects changes in the quadrupole shift, which are related to variations in the electric field gradient from the valence electron distribution. The density-functional-theory calculation also supports the electron quadrupole contribution to magnetic anisotropy. These results indicate that the strain-dependent changes in the magnetic easy axis in Fe3Si arise from the formation of the charge quadrupole at the strained Fe sites. This characteristic is notably different from that of the ferromagnetic Heusler alloy Co2FeSi, where one of the Fe sites in Fe3Si are replaced by Co. This study provides a significant approach to understanding and designing interfacial multiferroic materials.

High-valent Fe ^IV =O species are common intermediates in biological and artificial catalysts. Heme and nonheme S=1 Fe ^IV =O sites have been synthesized and studied for decades but little quantitative experimental comparison of their electronic structures has been available, due to the lack of direct methods focused on the iron. This study allows a rigorous determination of the electronic structure of a nonheme Fe ^IV =O center and its comparison to an Fe ^IV =O heme site using 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy (XAS). Further, variable temperature magnetic circular dichroism (VT-MCD) of the ligand field transitions, combined with nuclear resonance vibrational spectroscopy of the two S=1 Fe ^IV =O systems show that the equatorial ligand field decreases from a nonheme to a heme Fe ^IV =O site. Alternatively, RIXS and Fe L-edge XAS combined with MCD show that the Fe d π orbitals are unperturbed in the Fe ^IV =O heme relative to the nonheme site because the strong axial Fe-O bond uncouples the Fe d π orbitals from the porphyrin π -system. As a consequence, the thermodynamics and kinetics of the H-atom abstraction reactions are actually very similar for heme compound II and nonheme Fe ^IV =O active sites.

The mechanical properties of TRIP steels depend on heterogeneities of chemical composition and grain size in the retained γ structure, although these heterogeneities have not been characterized in detail. Therefore, in this study, we quantitatively investigate the inhomogeneous carbon concentration and grain size distribution, and its effects on the thermal stability of the retained γ in Fe-2Mn-1.5Si-0.4C (mass%) TRIP steel using FE-EPMA, EBSD, Mössbauer spectroscopy, and in-situ neutron diffraction during bainitic transformation at 673 K. In-situ neutron diffraction experiments detects high-carbon γ evolving during bainite transformation, in addition to the original γ, and the time variation of the volume fraction of high-carbon γ agrees well with the fraction of γ retained at room temperature. Williamson-Hall analysis based on peak width suggests that heterogeneity of carbon content exists even within the high-carbon γ. Compositional analysis using FE-EPMA and three-dimensional atom probe directly revealed that fine filmy γ was highly enriched with carbon compared to larger blocky γ, and the carbon content in blocky γ decreases with increasing blocky γ size. DICTRA simulation qualitatively reproduces the size dependency of carbon enrichment into γ. It was also found that γ tends to be retained at higher carbon content and smaller γ grain size since the smaller grain size directly improves thermal stability and the smaller γ size further contributes to the thermal stability via enhanced carbon enrichment.

Die Optimierung von Prozessen und Materialien für die Valorisierung von CO₂ zu Wasserstoffträgern oder Plattformchemikalien ist ein wichtiger Schritt zur Eindämmung der globalen Erwärmung und zur nachhaltigen Verwendung von erneuerbaren Energien. Wir berichten hier über die Hydrierung von CO₂ in Wasser an ZnO‐geträgerten CuAu‐Nanolegierungen, basierend auf ≤7 mol % Au. Die Cu_xAu_y/ZnO‐Katalysatoren wurden mit Hilfe von ¹⁹⁷Au‐Mössbauer, In situ‐Röntgenabsorption (Au L_III‐ und Cu K‐Kanten) und Umgebungsdruck‐Röntgenphotoelektronen (APXPS) spektroskopische Methoden zusammen mit Röntgenbeugung und hochauflösender Elektronenmikroskopie charakterisiert. Bei 200 °C stieg der CO₂‐Umsatz bei einer Erhöhung der Cu₉₃Au₇‐Beladung von 1 auf 10 Gew.‐% signifikant um das 34‐fache (von 0.1 auf 3.4 %), wobei die Methanolselektivität bei 100 % blieb. Eine begrenzte CO‐Selektivität (4–6 %) wurde bei einer Temperaturerhöhung bis auf 240 °C beobachtet, die jedoch mit einem ≈3‐fachen Anstieg der CO₂‐Umwandlung einherging. Auf der Grundlage von APXPS, während der CO₂‐Hydrierung in einem H₂O‐reichen Gemisch segregiert Cu bevorzugt an der Oberfläche in einem hauptsächlich metallischen Zustand, während leicht negative‐geladenes Au tiefer in den unterirdischen Bereich eintaucht. Diese Ergebnisse und detaillierte Strukturanalysen sind Gegenstand des vorliegenden Beitrags.

Optimizing processes and materials for the valorization of CO₂ to hydrogen carriers or platform chemicals is a key step for mitigating global warming and for the sustainable use of renewables. We report here on the hydrogenation of CO₂ in water on ZnO‐supported CuAu nanoalloys, based on ≤7 mol % Au. Cu_xAu_y/ZnO catalysts were characterized using ¹⁹⁷Au Mössbauer, in situ X‐ray absorption (Au L_III‐ and Cu K‐edges), and ambient pressure X‐ray photoelectron (APXP) spectroscopic methods together with X‐ray diffraction and high‐resolution electron microscopy. At 200 °C, the conversion of CO₂ showed a significant increase by 34 times (from 0.1 to 3.4 %) upon increasing Cu₉₃Au₇ loading from 1 to 10 wt %, while maintaining methanol selectivity at 100 %. Limited CO selectivity (4–6 %) was observed upon increasing temperature up to 240 °C but associated with a ≈3‐fold increase in CO₂ conversion. Based on APXPS during CO₂ hydrogenation in an H₂O‐rich mixture, Cu segregates preferentially to the surface in a mainly metallic state, while slightly charged Au submerges deeper into the subsurface region. These results and detailed structural analyses are topics of the present contribution.

Understanding the strange metallic behavior that develops at the brink of localization in quantum materials requires probing the underlying electronic charge dynamics. Using synchrotron radiation–based Mössbauer spectroscopy, we studied the charge fluctuations of the strange metal phase of β-YbAlB ₄ as a function of temperature and pressure. We found that the usual single absorption peak in the Fermi-liquid regime splits into two peaks upon entering the critical regime. We interpret this spectrum as a single nuclear transition, modulated by nearby electronic valence fluctuations whose long time scales are further enhanced by the formation of charged polarons. These critical charge fluctuations may prove to be a distinct signature of strange metals.