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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.

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.

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.

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.

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.

A twisted magnetic structure in Ni ultrathin films attached to antiferromagnetic FeMn is revealed by a combination of the depth-resolved x-ray magnetic circular dichroism (XMCD) and the polarized neutron reflectivity (PNR) techniques. The depth-resolved XMCD at remanent magnetization shows that the perpendicular magnetization component in the Ni film decreases around the interface to FeMn when the film exhibits perpendicular magnetization, whereas the in-plane component is kept constant through the whole film in the case of in-plane magnetization. Moreover, the PNR data shows that when a weak in-plane magnetic field is applied to a film, which exhibits perpendicular magnetization at the remanent state, an in-plane magnetization component is induced in Ni around the interface to FeMn. These results are reasonably interpreted by assuming that the magnetic moment in the Ni layer is twisted from the perpendicular to the in-plane directions towards the interface to FeMn. Such a magnetic structure is supposed to be induced by a magnetic anisotropy interaction at the interface between ferromagnetic Ni and antiferromagnetic FeMn, which makes the Ni magnetic moment around the interface rotate towards the in-plane direction.

The ferromagnetic oxide ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ has been studied by magnetic circular x-ray dichroism (MCXD) in core-level absorption. In the Co ${2p}_{3/2,1/2}$ absorption, the orbital and spin magnetic moments of the Co $3d$ are deduced with the use of magneto-optical sum rules and compared with magnetization measurements. In the O $1s$ absorption, large negative MCXD structures are observed, showing that the O $2p$ orbitals are heavily mixed with the Co $3d$ orbitals and that the orbital magnetic moment of O $2p$ is parallel to the orbital magnetic moment of Co $3d$ and the total magnetic moment. The relative magnitudes of the spin and orbital magnetic moments are most consistently interpreted based on the intermediate-spin states of Co $3d$ within the ionic model, although the absolute values of the spin and orbital magnetic moments are found to be smaller, probably indicating that the itinerant electron description of the Co $3d$ states is more appropriate in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$

The atomic and electronic structures of the methoxy $({\mathrm{CH}}_{3}\mathrm{O})$ adsorbate formed on the Cu(111) and Ni(111) surfaces have been studied by NEXAFS (near-edge x-ray-absorption fine structure) and SEXAFS (surface extended x-ray-absorption fine structure) with a newly constructed soft x-ray beamline in the Photon Factory. The transition from the O $1s$ core to the $2e$ level of the methoxy species was found to be noticeably stronger on Ni(111) than on Cu(111), indicating that the methoxy adsorbate on Cu(111) is more anionic. The C-O bond lengths of the species on Cu(111) and Ni(111) were estimated using the ${\ensuremath{\sigma}}^{*}(\mathrm{C}\ensuremath{-}\mathrm{O})$ resonance to be $1.46\ifmmode\pm\else\textpm\fi{}0.05$ and $1.43\ifmmode\pm\else\textpm\fi{}0.05\AA{},$ respectively. The polarization-dependent NEXAFS measurements showed that the C-O axis of the methoxy adsorbate is nearly perpendicular to the surface on both Cu(111) and Ni(111). From the SEXAFS analysis on Cu(111), it was revealed that the oxygen atom of the methoxy species locates on a threefold hollow site and the nearest-neighbor O-Cu distance was determined to be $2.00\ifmmode\pm\else\textpm\fi{}0.03\AA{}.$

Structure and magnetic anisotropy of alternately layered FeNi thin films grown on Ni ($x=3$--21 ML)/Cu(001) are investigated by means of reflection high-energy electron diffraction (RHEED) and x-ray magnetic circular dichroism (XMCD). From the RHEED analysis, we find that $n$ ML FeNi films grow with a similar in-plane lattice constant, $a$, to that of Cu up to the total film thickness, $n+x$, of $\ensuremath{\sim}$13 ML, from which the decrease in $a$ occurs. For the Ni-sandwiched Fe layer, we find from the XMCD analysis that the perpendicular component of the orbital magnetic moment increases and the in-plane component decreases as the in-plane lattice constant gets smaller. This suggests that perpendicular magnetic anisotropy is enhanced when the compressive strain in the in-plane direction is applied to the Fe layer. On the other hand, in the case of the Fe-sandwiched Ni layer, the orbital magnetic moment in both directions shows a small dependence on $a$. The change in magnetic anisotropy of Ni as a function of the structural strain is within the error margins.

Ga${}^{+}$ irradiation-induced changes in magnetic anisotropy and crystalline structure of a Pt/Co/Pt ultrathin film are investigated by means of x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) techniques. The XMCD analysis shows a large orbital moment difference between out-of-plane and in-plane directions, which corresponds to perpendicular magnetic anisotropy (PMA), at moderate Ga${}^{+}$ fluences, while further increased fluences reduce the orbital moment difference resulting in in-plane magnetization. From the EXAFS analysis, enhancement of PMA is directly related to an in-plane lattice expansion caused by ion irradiation and Co-Pt intermixing, which results in a large lattice distortion. Thus, the origin of the ion irradiation-induced changes in magnetic anisotropy is successfully explained.

The depth-resolved x-ray magnetic circular dichroism (XMCD) technique was applied to observe the interfacial and inner magnetic moments in CoFe/MgO and Co${}_{2}$MnSi(CMS)/MgO. The magnetic moments of the real interface region (two monolayers from the interface) and the inner layers were separately analyzed using the XMCD sum rules. The observed interfacial moments of Co at the CMS/MgO interface show a remarkable reduction from that in the inner layers compared with the CoFe/MgO structure, suggesting small exchange stiffness in the CMS at the MgO interface. The weak exchange stiffness of Co is a possible reason for the large temperature dependence of tunnel magnetoresistance in CMS-based magnetic tunnel junctions.

The electronic structure of nanographene in pristine and fluorinated activated carbon fibers (ACFs) have been investigated with near-edge x-ray absorption fine structure (NEXAFS) and compared with magnetic properties we reported on previously. In pristine ACFs in which magnetic properties are governed by nonbonding edge states of the $\ensuremath{\pi}$ electron, a prepeak assigned to the edge state was observed below the conduction electron ${\ensuremath{\pi}}^{*}$ peak close to the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak, which was assigned to the $\ensuremath{\sigma}$-dangling bond state, was observed between the prepeak of the edge state and the ${\ensuremath{\pi}}^{*}$ peak in the NEXAFS profile. The intensities of the extra peak correlate closely with the spin concentration created upon fluorination. The combination of the NEXAFS and magnetic measurement results confirms the coexistence of the magnetic edge states of $\ensuremath{\pi}$ electrons and dangling bond states of $\ensuremath{\sigma}$ electrons on fluorinated nanographene sheets.

We present direct evidence of carrier localization in (Zn,Al)O by using Auger resonant Raman scattering, which makes use of the competition between the core-hole decay time and the motion of the photoexcited electrons. From the branching ratio of the competing two channels, i.e., normal Auger-like and spectator Auger processes, we found that the average carrier transfer time from a donor site is in the range of 6--140 fs depending on structural disorder. The transfer time shows a rapid increase at a certain energy in the conduction band. This result suggests that the disorder-induced localization limits the electronic conductivity in the heavily $n$-doped ZnO.

The magnetic states of Mn and Co atoms in Co-rich ${\text{Co}}_{2}\text{MnGe}$ Heusler alloy thin films facing an MgO barrier were studied by means of soft x-ray magnetic circular dichroism (XMCD). In particular, the ${\text{Co}}_{2}\text{MnGe}$ film-thickness dependence of the Mn and Co magnetic moments was investigated. With a decrease in the ${\text{Co}}_{2}\text{MnGe}$ film thickness to 1--2 monolayers (MLs), the spin magnetic moment of Mn decreased and the $\text{Mn}\text{ }{L}_{2,3}$-edge x-ray absorption spectra (XAS) showed a ${\text{Mn}}^{2+}$-like multiplet structure in MnO, in contrast to samples thicker than 4 ML, indicating that the Mn atoms of the 1 and 2 ML samples were oxidized. The Co spin magnetic moment increased slightly with decreasing thickness. A ${\text{Co}}^{2+}$-like multiplet structure in CoO was not observed in all the $\text{Co}\text{ }{L}_{2,3}$-edge XAS and XMCD, indicating that, even in the ultrathin samples, the Co atoms were not oxidized, and were more strongly spin polarized than those in the thicker samples. Co spin magnetic moments of $1.40--1.77\text{ }{\ensuremath{\mu}}_{\text{B}}$ larger than the theoretical value for ideal stoichiometric ${\text{Co}}_{2}\text{MnGe}$ $(\ensuremath{\sim}1\text{ }{\ensuremath{\mu}}_{\text{B}})$ and the Co-rich film composition imply the presence of Co antisites that would lower the spin polarization.

J. Miyawaki,1,2 D. Matsumura,1,3 H. Abe,1,4 T. Ohtsuki,1 E. Sakai,1 K. Amemiya,5,6,* and T. Ohta7 1Department of Chemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2Soft X-ray Spectroscopy Laboratory, RIKEN SPring-8 Center, Sayo, Sayo, Hyogo 679-5148, Japan 3Kansai Photon Science Institute, Japan Atomic Energy Agency, Sayo, Sayo, Hyogo 679-5148, Japan 4Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, Kanagawa 223-8522, Japan 5Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan 6Japan Science and Technology Agency, CREST, Kawaguchi, Saitama 332-0012, Japan 7SR Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan Received 14 April 2009; revised manuscript received 14 July 2009; published 29 July 2009

The spin-reorientation transition (SRT) of Co/Pd(111) induced by Fe deposition was investigated using the x-ray magnetic circular dichroism method. In-plane magnetized 4--6 monolayer (ML) Co films undergo the SRT twice. First, a small amount of Fe deposition causes a transition to perpendicular magnetization; second, further Fe deposition (1--2 ML in total) causes a return to in-plane magnetization. A perpendicularly magnetized 3 ML Co film also exhibits a transition to in-plane magnetization by 1.5--2 ML Fe deposition. A precise magnetic anisotropy phase diagram was obtained using a combination of wedge-shaped Co samples with stepwise Fe deposition. Large perpendicular magnetic anisotropy was observed in a single layer Fe on Co/Pd(111).

The behavior of satellite features appearing in the Ni $L$-edge x-ray absorption spectrum has been investigated for Cu-capped Ni films on Cu(100) by extracting the interface component of the absorption spectrum. By comparing the results with the surface component, it was revealed that the satellite feature at $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the ${L}_{\mathrm{III}}$ peak is suppressed only at the surface. This behavior was well reproduced by a one-electron multiple-scattering calculation. On the other hand, an enhancement in the $3\text{\ensuremath{-}}\mathrm{eV}$ satellite was observed at the surface, associated with an increase in the $d$-hole number. This is interpreted as an increase in the $3{d}^{8}$ configuration in the ground state.

The spin-reorientation transition of $\mathrm{Ni}∕\mathrm{Cu}(100)$ and $\mathrm{CO}∕\mathrm{Ni}∕\mathrm{Cu}(100)$ films was investigated with the depth-resolved x-ray magnetic circular dichroism (XMCD) technique. The XMCD spectra from the surface and inner layers were separately extracted. As for the bare Ni films, the in-plane orbital magnetic moment in the surface layer is significantly enhanced. In contrast, the inner layers exhibit larger perpendicular orbital magnetic moment than the in-plane one. Upon CO adsorption, the surface magnetization is drastically reduced, while the inner layers are unaffected. These results directly explain the spin-reorientation transition in $\mathrm{Ni}∕\mathrm{Cu}(100)$ and $\mathrm{CO}∕\mathrm{Ni}∕\mathrm{Cu}(100)$.

The surface and interface magnetic structures of Fe thin films (4-11 ML) grown on a 6 ML Ni/Cu(100) film have been directly investigated with the depth-resolved x-ray magnetic circular dichroism technique. An oscillatory behavior in the Fe surface magnetization as a function of Fe thickness is found. The spectra were analyzed with a simple magnetic model, in which only surface two layers and interface (bottom) single layer are magnetically active. The analysis clearly shows existence of an oscillatory or rotational interlayer magnetic coupling between the surface and interface of the Fe film via the inner layers with the interface magnetization essentially unchanged. Temperature dependent experiments showed that the surface magnetization vanishes with increasing temperature, while the interface one stays unchanged.

Spin reorientation transitions of Co and Ni thin films induced by surface chemisorption have been investigated for CO-adsorbed Co/Pd(111) and CO- and H-adsorbed Ni/Cu(001) by means of Co L I I I , I I - and Ni L I I I , I I -edge x-ray magnetic circular dichroism (XMCD). For four to six monolayer (ML) Co/Pd(111), a spin reorientation transition from surface parallel to perpendicular magnetization has been observed at 200 K after CO dosage. The region of perpendicular magnetic anisotropy (PMA) becomes about 3 ML thicker at this temperature. For Ni/Cu(001), similar transitions have been confirmed when the layers are covered with CO or H, and the PMA range is also expanded, in this case by about 2 ML. The observed stabilization of perpendicular magnetic anisotropy due to chemisorption is ascribed to quenching of the surface parallel magnetic orbital moment. The effects of the fashion in which adsorption occurs on the easy axis of thin-film magnetization are also examined by comparing the differences in behavior between CO- and H-adsorbed Ni films.

We observed O K-edge x-ray magnetic circular dichroism (XMCD) of atomic O adsorbed on a fcc Co thin film grown epitaxially on Cu(100). The XMCD of $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{O}$ showed a negative sign at the $\mathrm{O}1\stackrel{\ensuremath{\rightarrow}}{s}2p$ transition. The negative sign indicates parallel alignment of the O orbital moment with the substrate magnetization. This finding is in contrast to the CO/Co/Cu(100) case where a positive XMCD was observed at the $\mathrm{O}1\stackrel{\ensuremath{\rightarrow}}{s}\mathrm{CO}2\ensuremath{\pi}*$ transition. A possible explanation for this discrepancy is proposed referring to recent theoretical prediction [Pick et al., Phys. Rev. B 59, 4195 (1999)].

The electronic and magnetic structures of a Mn ultrathin film grown on a 3-ML (monolayer) Co film have been investigated during stepwise oxidation by means of O K-, Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-},$ and Co ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ x-ray-absorption spectroscopy and Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-},$ and Co ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ x-ray magnetic circular dichroism (XMCD). Without ${\mathrm{O}}_{2},$ strong interaction between the Mn and Co $3d$ orbitals was suggested and Mn-Co ferromagnetic coupling was confirmed. We observed significant suppression of the d hole number and the spin and orbital moments of Co after Mn deposition compared to those before Mn deposition. These findings imply that the Mn d electrons are transferred to the minority-spin levels of Co. At 0.5-L (Langmuir) ${\mathrm{O}}_{2}$ exposure, the spin and orbital moments of Co do not change noticeably, while the Mn ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ XMCD almost completely vanishes. After 5.5-L ${\mathrm{O}}_{2}$ exposure, an antiparallel spin alignment between Mn and Co was observed. The estimated orbital moments of Mn is reduced from 0.06 (before oxidation) to $l0.005{\ensuremath{\mu}}_{B}$ (after oxidation). It is concluded that unoxidized Mn is in the ${d}^{5}{+d}^{6}$ state while oxidized Mn is in the ${d}^{5}$ high-spin state. Such variance of the electron configuration of Mn can explain the unusual magnetic properties. Antiferromagnetic coupling between Co and oxidized Mn may originate from the ${d}^{5}$ high-spin configuration of Mn rather than from the superexchange interaction between Mn and Co via the O atom.