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Real-time observation of an electrochemical reaction at a solid–liquid interface was performed using fluorescence-yield wavelength-dispersive soft X-ray absorption spectroscopy (XAS). A dedicated e...

The Fe/MgO interface is an essential ingredient in spintronics as it shows giant tunneling magnetoresistance and strong perpendicular magnetic anisotropy (PMA). A recent study demonstrated that the insertion of an ultra-thin LiF layer between the Fe and MgO layers enhances PMA significantly. In this study, we perform x-ray magnetic circular dichroism measurements on Fe/LiF/MgO multilayers to reveal the origin of the PMA enhancement. We find that the LiF insertion increases the orbital-magnetic-moment anisotropy and thus the magnetic anisotropy energy. We attribute the origin of this orbital-magnetic-moment-anisotropy enhancement to the stronger electron localization and electron-electron correlation or the better interface quality with fewer defects.

Metal surface oxidation is a well-known phenomenon, but the oxidation states of metal surfaces have not been observed

Fe3O4 thin films on Si (100) substrate were prepared by chemical solution deposition (CSD) technique. In the present work, we have investigated the control of magnetization of Fe3O4 thin films by N2 ion implantation. The dosage of N2 ion implantation in Fe3O4 thin films varies from 0 to 3 × 1016 ions/cm2. The magnetization decreased as a function of dosage concentration that changed the electronic and magnetic properties of the thin films. Advanced characterization techniques, such as X-ray absorption spectroscopy (XAS) X-ray magnetic circular Dichroism (XMCD), were used for the first time to estimate the electronic and magnetic properties of the thin films in surface-sensitive total electron-yield mode. The temperature-dependent XMCD measurement suggests that with an increase in the dosage of N2 from 0 to 3 × 1016 ions/cm2, Fe3O4 transitioned from a high-magnetization phase to a low magnetization phase. The observation was further supported by vibrating sample magnetometer (VSM) measurements, which pointed toward the same magnetic-phase transition in the films.

Intermediate species formed on Ag(110) surfaces under the presence of ethylene and oxygen in the sub-Torr pressure range were studied by near-ambient-pressure X-ray photoelectron spectroscopy (XPS)...

In this work, we report the electronic and magnetic properties of epitaxial BiFeO3 (BFO) thin film deposited on SrTiO3 (001) substrate via the chemical solution deposition technique. These properties of pure BFO thin films were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques (XMCD). The large magnetization at Fe-edge could be due to annealing in nitrogen. Annealing in nitrogen atmosphere increases the grain size and porosity which lead to create more oxygen vacancy. The presence of oxygen vacancies which compensate the bismuth vacancies created due to bismuth volatilization. The origin of large orbital momentum in BFO thin film is also discussed. The O K-edge spectra display large spectral variation depending on the polarization vector E. The structural distortion at oxygen octahedral induces large anisotropy due to Fe 3d–O 2p hybridization. The hybridized Fe 3d orbitals contribute a much larger orbital momentum in BFO thin film.

Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Auphotolithography pattern on Si signifies the beam spot size of 10 $\mu$m (horizontal) x 12 $\mu$m (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal., Comment: 21 pages, 8 figures

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of

Oxidation of monometallic Pd and bimetallic Pd3Au alloy surfaces are observed by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at an elevated pressure (100 mTorr O2 ambient). I...

In situ monitoring of initial oxidation of GaAs surfaces was performed under (near-) realistic oxidizing environments, using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The surface chemical states drastically change with time. The oxidation process at the sub-nano-meter-scale exhibits a significantly small activation energy, which can be regarded as a quasi-barrier-less oxidation.

In this study, a method for reflection-mode soft x-ray absorption spectroscopy was developed to realize three-dimensional chemical-state imaging. Soft x rays from a pinhole were reflected by the sample, and the magnified image was observed with a two-dimensional detector. This technique was applied to a Co film with an Au-island-covered surface to obtain the surface chemical state images with a spatial resolution of several tens of micrometers. Furthermore, the soft x-ray reflection spectra within and outside the Au layer were extracted from the images by changing the photon energy. Distinct differences were observed at the Co absorption edge. By considering anomalous x-ray scattering around the Co L-edges in the simulation, the reflection spectrum near the absorption edge in the nm depth resolution was reproduced. In the region without the Au layer, the results were well reproduced, assuming that 4 nm CoO was formed at the surface. These results demonstrate the feasibility of three-dimensional imaging of the chemical states in multilayer films.

The ferrimagnet Mn4N forms a family of compounds useful in spintronics. In a compound comprising non-magnetic and magnetic elements, one basically expects the compound to become ferromagnetic when the proportion of the magnetic element increases. Conversely, one does not expect ferromagnetism when the proportion of the non-magnetic element increases. Surprisingly, Mn4N becomes ferromagnetic at room temperature when the Mn content is decreased by the addition of In atoms, a non-magnetic element. X-ray magnetic circular dichroism measurement reveals that the magnetic moment of Mn atoms at face-centered sites, Mn(II), reverses between x= 0.15 and 0.27 and aligns parallel to that of Mn atoms at corner sites, Mn(I), at x = 0.27 and 0.41. The sign of the anomalous Hall resistivity also changes between x = 0.15 and 0.27 in accordance with the reversal of the magnetic moment of the Mn(II) atoms. These results can be interpreted using first-principles calculations, showing that the magnetic moment of Mn(II) sites which are the nearest neighbors to the In atom align to that of Mn(I) sites.

Perpendicular magnetic anisotropy (PMA) in ferromagnet/oxide systems has attracted considerable attention owing to its significant potential in spintronic applications; however, an understanding of the causes of PMA enhancement in such systems remains inadequate. This study investigates the influence of epitaxial strain on PMA in the Fe/MgO system. The PMA energy increases monotonically as the in-plane lattice constant of Fe decreases, and increases further after annealing. We find that the interface component of the PMA energy is largely unaffected by the epitaxial strain and annealing process, and that changes in its bulk component play an important role. This study leads to a comprehensive understanding of PMA and voltage-controlled magnetic anisotropy effects in ultrathin magnetic films.

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

Thermal dehydrogenation of long n-alkane on Au(111) and Pt(111) surfaces is investigated by near the carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS) and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). On Au(111), dehydrogenation and formation of unsaturated bonds start at temperatures above 480 K, ending at 650 K. This is confirmed by the decrease in the 1 s → σ*CH/R resonance (σ*CH/R resonance) and the evolution of the π*CC resonance in the C K-NEXAFS spectra. The aliphatic or non-aromatic unsaturated bonds are generated at the early stage of the dehydrogenation on the Au(111), which is examined by the reaction with bromine. The dual peaks of the π*CC resonance at 284–286 eV suggest the formation of polycyclic aromatic hydrocarbons (PAHs). These PAHs exhibit a ribbon or rosary and planer shape by heating at 800 K, which is observed by the STM. In contrast, dehydrogenation begins at RT on Pt(111), and is almost completed at 480 K. π*CC resonance exhibits a main peak at 286 eV, which corresponds to a graphene skeleton, with a sub peak or a shoulder at 284.6 eV, which is attributed to a carbon atom at an edge or defect. The main peak intensity increases as the temperature is raised up to 820 K. STM images obtained after heating at 820 K show nanodots having a diameter of ca. 2–3 nm. The final products upon heating to 1000 K on Pt(111) are graphene nanoflakes with a size of tens of nanometers. The difference between the products is related to the difference between the reaction pathways on Au(111) and Pt(111) surfaces.

The active oxygen species for ethylene epoxidation on Ag surfaces has been a long-standing issue. We conducted in situ observations of Ag(111) surfaces under exposure to ethylene and oxygen gases w...

X-ray magnetic circular dichroism (XMCD) signals are usually absent in antiferromagnets. In this letter, we report the observation of spontaneous XMCD spectra originating from the inverse triangular spin structure, or the polarization of the cluster magnetic octupole, in the chiral antiferromagnet Mn$_{3}$Sn thin film. The result is consistent with the recent theoretical predictions that the inverse triangular spin structure can give rise to finite XMCD signals in the absence of net magnetization [J. Phys. Soc. Jpn. 89, 083703 (2020) and Phys. Rev. Lett. 126, 157402 (2021)].

A fluorescence-yield wavelength-dispersive x-ray absorption spectroscopy technique in the soft x-ray region, by which the x-ray absorption spectra are recorded without scanning the monochromator, has been developed. The wavelength-dispersed soft x rays, in which the wavelength (photon energy) continuously changes as a function of the position, illuminate the sample, and the emitted fluorescence soft x rays at each position are separately focused by an imaging optics onto each position at a soft x-ray detector. Ni L-edge x-ray absorption spectra for Ni and NiO thin films taken in the wavelength-dispersive mode are shown in order to demonstrate the validity of the technique. The development of the technique paves the way for a real-time observation of time-dependent processes, such as surface chemical reactions, with much higher gas pressure compared to the electron-yield mode, as well as under magnetic and electric fields.

Ferrimagnets in the vicinity of the magnetic compensation point are of particular interest for spin-torque-based spintronics applications, because they require lower switching currents and thus smaller switching energies. Room-temperature compensation is rather uncommon, but can be obtained by tuning the chemical concentrations of compounds such as ${\mathrm{Co}}_{0.79}{\mathrm{Tb}}_{0.21}$ or ${\mathrm{Co}}_{0.56}{\mathrm{Gd}}_{0.44}$, in order to find the single concentration at which one sublattice compensates the other. Here we study the ferrimagnetic ${\mathrm{Mn}}_{4\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}\mathrm{N}$ films, showing that they presumably possess at room temperature not only one but two different Co concentrations at which the magnetic compensation occurs. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we show that this startling behavior can be explained by the existence of two nonequivalent sites for the Co, when it is inserted in the antiperovskite structure of epitaxial ${\mathrm{Mn}}_{4\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}\mathrm{N}$ films.

Materials that possess nontrivial topology and magnetism is known to exhibit exotic quantum phenomena such as the quantum anomalous Hall effect. Here, we fabricate a novel magnetic topological heterostructure Mn4Bi2Te7/Bi2Te3 where multiple magnetic layers are inserted into the topmost quintuple layer of the original topological insulator Bi2Te3. A massive Dirac cone (DC) with a gap of 40–75 meV at 16 K is observed. By tracing the temperature evolution, this gap is shown to gradually decrease with increasing temperature and a blunt transition from a massive to a massless DC occurs around 200–250 K. Structural analysis shows that the samples also contain MnBi2Te4/Bi2Te3. Magnetic measurements show that there are two distinct Mn components in the system that corresponds to the two heterostructures; MnBi2Te4/Bi2Te3 is paramagnetic at 6 K while Mn4Bi2Te7/Bi2Te3 is ferromagnetic with a negative hysteresis (critical temperature ~20 K). This novel heterostructure is potentially important for future device applications., T.H. was supported by JSPS KAKENHI Grant Number 18H03877, the Murata Science Foundation (No. H30-084), the Asahi Glass Foundation, Tokyo Tech. Challenging Research Award, and the Iketani Science and Technology Foundation (No. 0321083-A). M.K. was supported by the Spintronics Research Network of Japan (Spin-RNJ). The ARPES measurements were performed under the UVSOR proposal Nos. 29-837, 30-571, 30-860 and the SARPES measurements were performed under the HiSOR proposal No. 16BG001. The XMCD measurements were performed at JAEA beamline BL-23SU in SPring-8 (Proposal No. 2018B3843) and also at PF-KEK (PF PAC No. 17P006). The work at SPring-8 was performed under the Shared Use Program of JAEA Facilities (Proposal No. 2018B-E21) with the approval of Nanotechnology Platform project supported by the Ministry of Education, Culture, Sports, Science and Technology (Proposal No. A-18-AE-0039). The support by Tomsk State University competitiveness improvement program (No. 8.1.01.2018), the Saint Petersburg State University (Project ID 51126254), the Russian Science Foundation (Grant No. 18-12-00169) and the Government research assignment for ISPMS SB RAS, project No. III.23.2.9 is gratefully acknowledged. M.M.O. acknowledges the support by Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00).

NO reduction by CO on Rh(111) was investigated by near-ambient pressure X-ray photoelectron spectroscopy, mass spectrometry, and kinetic analysis. Under exposure to NO + CO mixed gases and with heating the surface from room temperature to 450 °C, NO dissociation and NO reduction reaction start simultaneously independent of gas pressure ratio of NO/CO, which indicates that NO dissociation triggers this reaction. From kinetic analyses based on observed adsorbate coverages under reaction conditions, the following two points are suggested: (i) NOhollow is a reactive species for N2 and N2O formation via N + NO reaction. (ii) At low temperatures, the N + NO reaction is dominant for N2 production, whereas above around 400 °C, the N + N reaction becomes dominant, which leads to an increase in N2 selectivity at the higher temperatures. Compared with the NO + CO reaction on Ir(111) surfaces, which exhibits a high N2 selectivity, the adsorption site of reactive NO and the availability of vacant surface sites could b...

To investigate the relationship between the charge redistribution and ferromagnetism at the interface between perovskite transition-metal oxides $\mathrm{LaNi}{\mathrm{O}}_{3}$ (LNO) and $\mathrm{LaMn}{\mathrm{O}}_{3}$ (LMO), we performed x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements on the sandwiched layer in the trilayer structures. In the LNO/LMO heterostructures, one electron (hole) per unit-cell area is transferred from Mn (Ni) to Ni (Mn) ions across the interface unless there is a spatial limitation in the layer accommodating the transferred charges. A detailed analysis of the $\mathrm{Ni}\ensuremath{-}{L}_{2,3}$ and $\mathrm{Mn}\ensuremath{-}{L}_{2,3}$ XMCD spectra reveals that the magnetization of Ni ions is induced only in $\mathrm{N}{\mathrm{i}}^{2+}$ ions at the interface, while that of Mn ions is induced in both $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ ions. Combining these results with the previous XAS results on bilayer structures revealing the asymmetric charge redistribution of the LNO/LMO interface [Kitamura et al., Appl. Phys. Lett. 108, 111603 (2016)] shows that Ni magnetization is induced only in the $\mathrm{N}{\mathrm{i}}^{2+}$ ions of the 1 ML LNO adjacent to the interface, while the magnetization of Mn ions is increased in the 3--4 ML LMO of the interfacial region. These results indicate a close relationship between the charge redistribution due to the interfacial charge transfer and the ferromagnetism of the LNO/LMO interface. Furthermore, the XMCD spectra clearly demonstrate that the vectors of induced magnetization of both ions are aligned ferromagnetically, suggesting that the delicate balance between the exchange interactions occurring inside each layer and across the interface may induce the canted antiferromagnetism of $\mathrm{N}{\mathrm{i}}^{2+}$ ions, resulting in weak magnetization in the 1 ML LNO adjacent to the interface.

We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

Graphene growth on a Pt(111) substrate exposed to benzene was studied at substrate temperatures between 800 K and 1100 K. The effects of the substrate temperature during benzene exposure and post-annealing were examined. It was revealed that the substrate temperature during benzene exposure determined the graphene size, which was measured using scanning tunneling microscopy (STM). Nanographenes (NGs) of ~10 nm in size. were obtained by exposing the substrate to benzene at 800 K and post-annealing at 1100 K. The presence of a C–Pt bond, which may fix the carbon atoms on the Pt(111) surface and suppress any further growth of graphene, was confirmed by the carbon K-edge near-edge X-ray absorption fine structure spectroscopy (C K NEXAFS) and X-ray photoelectron spectroscopy (XPS) for the NG. Angle resolved photoemission spectroscopy (ARPES) performed on the NG has a reduced density of states at the Fermi level.

The influence of the preparation method and adsorbed amount of n-tetratetracontane (n-C44H90) on its orientation in a monolayer on the Au(111) surface is studied by near carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS), scanning tunneling microscopy (STM) under ultrahigh vacuum, and infrared reflection–absorption spectroscopy (IRAS) at the electrochemical interface in sulfuric acid solution. The n-C44H90 molecules form self-assembled lamellar structures with the chain axis parallel to the surface, as observed by STM. For small amounts adsorbed, the carbon plane is parallel to the surface (flat-on orientation). An increase in the adsorbed amount by ∼10–20% induces compression of the lamellar structure either along the lamellar axis or alkyl chain axis. The compressed molecular arrangement is observed by STM, and induced conformation and orientation changes are confirmed by in situ IRAS and C K-NEXAFS.

The nitric oxide (NO) reduction by carbon monoxide (CO) on Ir(111) surfaces under near ambient pressure conditions was studied by a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and mass spectrometry (MS), particularly paying attention to the dominant reaction pathway to formation of molecular nitrogen (N2). Under a relatively low CO pressure condition (50 mTorr NO + 10 mTorr CO), two reaction pathways to form N2 are clearly observed at different ignition temperatures (280 and 400 °C) and attributed to a reaction of NO adsorbed at atop site (NOatop) with atomic nitrogen (Nad) and associative desorption of Nad, respectively. Since the adsorption of NOatop is inhibited by CO adsorbed at atop site (COatop), the ignition of the NOatop + Nad reaction strongly depends on the coverage of COatop; the ignition temperature shifts to higher temperature as increasing CO pressure. In contrast, for the Nad + Nad reaction the ignition temperature keeps almost constant (∼400 °C). The onl...

Catalytic CO oxidation over Pd(111) and Pd70Au30(111) surfaces was investigated by in situ spectroscopic observations to understand the alloying effect. The reaction behaviour on Pd70Au30(111) is greatly different from that on Pd(111). Pd monomer and dimer ensembles can act as active centers, whereas triangular-shaped trimers and larger ensembles are inactive.

A room-temperature skyrmion-hosting compound ${\mathrm{Co}}_{8}{\mathrm{Zn}}_{8}{\mathrm{Mn}}_{4}$ has been examined by means of soft x-ray absorption spectroscopy, resonant small-angle scattering, and extended reference holography. An element-selective study was performed by exciting the $2p$-to-$3d$ transitions near Co and Mn ${L}_{2,3}$ absorption edges. By utilizing the coherence of soft x-ray beams the element-specific real-space distribution of local magnetization at different temperatures has been reconstructed using iterative phase retrieval and holography with extended reference. It was shown that the magnetic moments of Co and Mn are ferromagnetically coupled and exhibit similar magnetic patterns. Both imaging methods provide a real-space resolution of 30 nm and allowed us to record a magnetic texture in the temperature range between $T\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}20$ K and $T\phantom{\rule{0.16em}{0ex}}=120\phantom{\rule{0.16em}{0ex}}$ K, demonstrating the elongation of the skyrmions along the principal crystallographic axes at low temperatures. Micromagnetic simulations have shown that such deformation is driven by a decreasing ratio of symmetric exchange interaction to antisymmetric Dzyaloshinskii-Moriya interaction in the system and effect of the cubic anisotropy.