和達 大樹

We demonstrate ultrafast magnetization dynamics in a 5d transition metal using circularly-polarized x-ray free electron laser in the hard x-ray region. A decay time of light-induced demagnetization of L1${}_0$-FePt was determined to be $\tau_\textrm{Pt} = 0.6\ \textrm{ps}$ using time-resolved x-ray magnetic circular dichroism at the Pt L${}_3$ edge, whereas magneto-optical Kerr measurements indicated the decay time for total magnetization as $\tau_\textrm{total} = 0.1\ \textrm{ps}$. A transient magnetic state with the photo-modulated magnetic coupling between the 3d and 5d elements is firstly demonstrated.

Time-resolved magneto-optical Kerr effect (MOKE) measurement was demonstrated on a sample of the Au/Fe/Au heterostructure with the Fe layer of 0.35 nm thickness under Fe M-edge resonance condition. An ultrabrilliant free electron laser (FEL) in the soft X-ray range was facilitated for the detection of transient signals of resonant MOKE from the ultrathin Fe film. A variation in the Kerr rotation angle was successfully observed on the femtosecond timescale. This technique enables us to reveal the transient magnetization dynamics of such a-few-monolayer magnetic films, which promote the development of spintronic devices. (C) 2018 The Japan Society of Applied Physics

In order to realize superconductivity in cuprates with the T′-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T′-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T′-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd2-xCexCuO4 (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO2 plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd2CuO4 film is doped with electrons despite the absence of the Ce substitution.

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.

The electronic-structural modulations of Ir1-xPtxTe2 (0x0.12) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructures with wave vectors of Q=(1/50-1/5), (1/80-1/8), and (1/60-1/6) are observed on the same sample of IrTe2 at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around Q=(1/50-1/5) are observed for doped samples (0.02x0.05). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge-density wave (CDW) can coexist with superconductivity. The analysis of the energy-dependent REXS and RIXS line shape reveals the importance of the Te 5p state rather than the Ir 5d states in the formation of the spatial modulation of these systems. The phase diagram reexamined in this work suggests that the CDW incommensurability may correlate with the emergence of superconducting states such as CuxTiSe2 and LixTaS2.

The spin states of Co3+ ions in perovskite-type LaCoO3, governed by the complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can directly detect them. In this Letter, we revealed the tensile-strain dependence of spin states, i.e., the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO3 via resonant inelastic soft x-ray scattering. A tensile strain as small as 1.0% was found to realize different spin states from that in the bulk.

The spin states of Co3+ ions in perovskite-type LaCoO3, governed by the complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can directly detect them. In this Letter, we revealed the tensile-strain dependence of spin states, i.e., the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO3 via resonant inelastic soft x-ray scattering. A tensile strain as small as 1.0% was found to realize different spin states from that in the bulk.

Conventionally, the parallelism between the physical properties of Ce and Yb<br /> based magnets and heavy fermions has been considered to arise as a consequence<br /> of the electron-hole symmetry known for the band theory in condensed matter<br /> physics. Namely, the trivalent state of Ce has $4f^{1}$ configuration, while<br /> $4f^{13}$ configuration for Yb$^{3+}$ can be viewed as a state carrying one<br /> 4$f$ hole. However, in the light of this parallelism, the magnetism of Yb based<br /> intermetallic compounds has been a mystery over decades; the transition<br /> temperature of the Yb based compounds is normally very small, as low as $\sim$<br /> 1 K or even lower, while Ce counterparts may often have the transition<br /> temperature well exceeding 10 K. Here, we report our experimental discovery of<br /> the transition temperature reaching 20 K for the first time in an Yb based<br /> compound. Our single crystal study using transport, thermomagnetic and<br /> photoemission spectroscopy measurements reveals that the Mn substitution at the<br /> Al site in an intermediate valence state of $\alpha$-YbAlB$_{4}$ not only<br /> induces antiferromagnetic transition at a record high temperature of 20 K but<br /> also transform the heavy fermion liquid state in $\alpha$-YbAlB$_{4}$ into a<br /> highly resistive metallic state proximate to a Kondo insulator. The high<br /> temperature antiferromagnetism in a system close to a Kondo insulator is<br /> unusual and suggest a novel type of itinerant magnetism in the Yb based heavy<br /> fermion systems.

We report on directly determining the complex permittivity tensor using a method combining a developed light source from a segmented cross undulator of synchrotron radiation and the magneto-optical Kerr effect. The empirical permittivity, which carries the electronic and magnetic information of a material, has element specificity and has perfect confirmation using the quantum-mechanical calculation for itinerant electrons systems. These results help in understanding the interaction of light and matter, and they provide an interesting approach to seek the best materials as optical elements, for example, in extended-ultraviolet lithographic technologies or in state-of-the-art laser technologies.

The resonant magneto-optical Kerr effect of an Fe nanofilm at the L-edge was investigated by theoretically and experimentally using the polarization controlled undulator. Large values of the Kerr rotation angle (Be) were measured at the L-2 and L-3 absorption edges for both of s- and p-polarized incident lights. Furthermore, the sign changes of OK depending on the photon energy and the polarization of incident light were also observed. (C) 2016 Elsevier B.V. All rights reserved.

The Fe L-edge resonant magneto-optical Kerr effect of a buried Fe nanofilm was investigated by rotating-analyzer ellipsometry and the results were compared with those from three theoretical simulations. The reversal of the Kerr rotation angle theta(K) between the L-3 and L-2 edges, observed in the experiment, was consistent with classical electromagnetic simulation using empirical optical constants. The wspectral theta(K) feature was reproduced by the first-principles calculation of the KKR-Green's function method on the itinerant electronic system. The demonstration indicates that spectra of the L-edge resonant MOKE can be understood in terms of both the macroscopic and microscopic pictures.

The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.

We investigate the thickness effects on charge and magnetic orderings in Fe perovskite oxide La1/3Sr2/3FeO3/SrTiO3 thin films by hard x-ray and resonant soft x-ray scattering (RSXS) with changing thin film thickness systematically. We found that the correlation lengths of the magnetic ordering along the in-plane and out-of-plane directions are comparable and proportional to the thickness, and shows stronger thickness dependence than those of charge orderg. %the thickness dependence of correlation length of charge ordering is smaller than that of magnetic orderings. The magnetic ordered states disappear when the correlation length of magnetic ordering decreases to that of charge ordering through the intrinsic thickness effects. Surface sensitive grazing-incident RSXS revealed that the orderings exist even in the surface region, which indicates that the observed orderings is not affected by surface effect like oxygen vacancies. Critical thickness is in 5-15 nm, which corresponds to 4-11 antiferromagnetic ordering period. This critical value seems to be common to other ferromagnetic oxide thin films.

Experiments of time-resolved x-ray magnetic circular dichroism (Tr-XMCD) and resonant x-ray scattering at a beamline BL07LSU in SPring-8 with a time-resolution of under 50 ps are presented. A micro-channel plate is utilized for the Tr-XMCD measurements at nearly normal incidence both in the partial electron and total fluorescence yield (PEY and TFY) modes at the L2,3 absorption edges of the 3d transition-metals in the soft x-ray region. The ultrafast photo-induced demagnetization within 50 ps is observed on the dynamics of a magnetic material of FePt thin film, having a distinct threshold of the photon density. The spectrum in the PEY mode is less-distorted both at the L2,3 edges compared with that in the TFY mode and has the potential to apply the sum rule analysis for XMCD spectra in pump-probed experiments.

Resonant soft X-ray scattering is a new experimental technique to study charge-, orbital-, and spin- ordered states in transition-metal oxides. I present recent research using this technique, which includes future prospects.

We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (similar to 150 ps) to fast (&lt; 70 ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat-assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing one to spatially encode magnetic information.

We evaluate the utilities of fluorescence-yield (FY) modes in soft X-ray absorption spectroscopy (XAS) of several cathode materials for Li-ion batteries. In the case of total-FY (TFY) XAS for LiNi0.5Mn1.5O4, the line shape of the Mn L-3-edge XAS was largely distorted by the self-absorption and saturation effects, while the distortions were less pronounced at the Ni L-3 edge. The distortions were suppressed for the inverse-partial-FY (IPFY) spectra. We found that, in the cathode materials, the IPFY XAS is highly effective for the Cr, Mn, and Fe L edges and the TFY and PFY modes are useful enough for the Ni L edge which is far from the O K edge. (C) 2016 Author(s).

<p>トポロジカル絶縁体に磁性元素をドープすることで時間反転対称性が破れ、量子異常ホール効果が発現されることが近年発見された。今回VドープのBiSbTeに対して放射光を用いてX線磁気円二色性を測定し、キュリー温度以下でVの円二色性を観測した。トポロジカル系を構成する元素(Bi、Sb、Te)の円二色性とVの円二色性との関係を含めて、トポロジカル絶縁体中の局在スピン系の強磁性の性質について議論する。</p>

<p>垂直磁化膜として知られるFe(Pt,Pd)について電子密度を狙いとして硬X線光電子分光(SPring-8 BL47XU)をおこない、磁気モーメントを狙いとして軟X線磁気円二色性測定(Photon Factory BL16B)をおこなった。バンド計算から得られる電子密度と比較して議論したい。</p>

<p>Pr_2_Ir_2_O_7_はガンマ点でIr 5d電子のバンドがちょうどフェルミ面に接するNodal金属である。今回、我々はPr_2_Ir_2_O_7_の円偏光スピン分解光電子分光測定を行い、その結果、フェルミ面付近のバンドが111方向に大きなスピン分極を持っていることを確認した。</p>

<p>HiSORのBL-9Bにおいて鉄系超伝導体FeSe_1-x_Te_x_について円偏光スピン分解光電子分光を測定した結果を報告する。ホールポケットを形成するFe3dバンドにスピン分極が観測され、スピン軌道相互作用の効果が示唆された。測定結果に基づいて、FeSe_1-x_Te_x_の多バンド電子状態と超伝導の起源について議論する。</p>

<p>Ruddlesden-Popper型イリジウム酸化物Sr_2_IrO_4_,Sr_3_Ir_2_O_7_,SrIrO_3_について，軟X線角度分解光電子分光を行い，3次元波数空間での電子構造を明らかにした．その結果，IrO_2_面構造の次元性の増加によって，j_eff_=1/2バンドがフェルミ準位を横切り，金属化する様子を捉えた．講演では，実験的に決定したSrIrO_3_のフェルミ面形状についても議論する．</p>

<p>HiSORのBL9Bにおいて三角格子系IrTe_2_のスピン分解光電子分光を行ったのでその結果を報告する。バルク由来のバンド構造においてスピン軌道相互作用の効果が観測されたことに加えて、ストライプ型の電荷軌道秩序を示す低温相において、特異なスピン分極を持つ表面状態が観測された。観測結果に基づいて、IrTe_2_のバルクおよび表面でのスピン軌道相互作用の効果を議論する。</p>

<p>SPring-8 BL07LSUにて分割型クロスアンジュレータと電磁石位相器を組み合わせ、唯一の軟X線高速連続偏光変調光源を実現した。さらにその光源を用いた軟X線領域における光学遅延変調法を世界で初めて開発した。この手法は磁性体の磁気円二色性（MCD）と旋光性を同時にかつ高精度に測定できる。本講演では新規光源と手法の詳細を述べると共に、それを用いた鉄系磁性体のMCD及び磁気光学カー効果（MOKE）測定の結果を報告する。</p>

Element-specific magnetic structures, particularly orbital magnetic moments, of spinel-type MnV2O4 were investigated using X-ray magnetic circular dichroism (XMCD). X-ray absorption and XMCD spectra clearly reveal that the Mn2+ (d(5)) and V3+ (d(2)) states are coupled antiferromagnetically. Analyses of XMCD spectra using magneto optical sum rules revealed that small but finite orbital magnetic moments remain in both V and Mn 3d states, which accounts for the antiferro-type orbital ordering in the V sites of MnV2O4 with coexisting complex and real orbital states. Additionally, the Cr doping effect in MnV2O4 was examined. The XMCD spectra of Cr3+ (d(3)) L-edges exhibited the substitution of Cr ions to the V sites ferromagnetically, with low conductivity through the suppression of the orbital ordering.

The substitution of hydride anions (H-) into transition metal oxides has recently become possible through topotactic reactions or high-pressure synthesis methods. However, the fabrication of oxyhydrides is still difficult because of their inherently less-stable frameworks. In this study, we successfully fabricated perovskite SrCoOxHy thin films via the topotactic hydride doping of brownmillerite SrCoO2.5 epitaxial thin films with CaH2. The perovskite-type cation framework was maintained during the topotactic treatment owing to epitaxial stabilization. Structural and chemical analyses accompanied by X-ray absorption spectroscopy measurements revealed that the doped hydride ions form a two-dimensional network of Co-H--Co bonds, in contrast to other reported perovskite oxyhydrides, SrMO3-xHx (M = Cr, Ti, V). The SrCoOxHy thin film exhibited insulating behavior and had a direct band gap of 2.1 eV. Thus, topotactic hydride doping of transition-metal-oxide thin films on suitable substrates is a promising method for the synthesis of new transition metal oxyhydrides. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Ultrafast magnetization reversal of a ferrimagnetic metallic alloy GdFeCo was investigated by time-resolved resonant magneto-optical Kerr effect measurements using a seeded free electron laser. The GdFeCo alloy was pumped by a linearly polarized optical laser pulse, and the following temporal evolution of the magnetization of Fe in GdFeCo was element-selectively traced by a probe free electron laser pulse with a photon energy tuned to the Fe M-edge. The results have been measured using rotating analyzer ellipsometry method and confirmed magnetization switching caused by ultrafast heating. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

We have studied the electronic structure of Li1-x[Mn0.5Ni0.5](1-x)O-2 (x = 0.00 and 0.05), one of the promising cathode materials for Li ion battery, by means of x-ray photoemission and absorption spectroscopy. The results show that the valences of Mn and Ni are basically 4+ and 2+, respectively. However, the Mn3+ component in the x = 0.00 sample gradually increases with the bulk sensitivity of the experiment, indicating that the Jahn-Teller active Mn3+ ions are introduced in the bulk due to the site exchange between Li and Ni. The Mn3+ component gets negligibly small in the x = 0.05 sample, which indicates that the excess Li suppresses the site exchange and removes the Jahn-Teller active Mn3+. (C) 2015 AIP Publishing LLC.

Using resonant soft-x-ray scattering as a function of both temperature and magnetic field, we reveal a large number of almost degenerate magnetic orders in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a so-called magnetic devil's staircase. It is demonstrated how a magnetic field induces transitions between different microscopic spin configurations, which is responsible for the magnetoresistance of SrCo6O11. This material therefore constitutes a unique combination of a magnetic devil's staircase and spin-valve effects, yielding a novel type of magnetoresistance system.

In 5d Ir oxides with an interplay of spin-orbit coupling and electron correlations, we have tailored a spin-orbital magnetic insulator out of a semimetal SrIrO3 by tuning the structure through superlattices [(SrIrO3)(m), SrTiO3] (m = 1, 2, 3, 4, and infinity). We observed the systematic decrease of the magnetic ordering temperature and the resistivity as a function of m. The transition from the semimetal to the insulator is found to be closely linked to the appearance of magnetism at m similar or equal to 3. Long range magnetic ordering was realized even in the m = 1 single layer superlattice, implying that the design and realization of novel electronic phases is feasible at the level of a single atomic layer in complex Ir oxides.

We investigated the electronic and magnetic properties of fully oxidized BaFeO3 thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for Fe4+, which showed good agreement with the experimental results. We also studied SrFeO3 thin films, which have an Fe4+ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that BaFeO3 thin films are insulating with large magnetization (1.7 mu(B)/formula unit) under similar to 1 T, using valence-band HAXPES and Fe 2p XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe 2p core-level HAXPES and Fe 2p XAS spectra of BaFeO3 and SrFeO3 thin films are quite similar, we compared the insulating BaFeO3 to metallic SrFeO3 thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of BaFeO3 is dominated by d(5)(L) under bar ((L) under bar: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O 2p character. We revealed that the differences of the electronic and magnetic properties between BaFeO3 and SrFeO3 arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.

We investigated the electronic and magnetic properties of fully oxidized BaFeO3 thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for Fe4+, which showed good agreement with the experimental results. We also studied SrFeO3 thin films, which have an Fe4+ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that BaFeO3 thin films are insulating with large magnetization (1.7 mu(B)/formula unit) under similar to 1 T, using valence-band HAXPES and Fe 2p XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe 2p core-level HAXPES and Fe 2p XAS spectra of BaFeO3 and SrFeO3 thin films are quite similar, we compared the insulating BaFeO3 to metallic SrFeO3 thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of BaFeO3 is dominated by d(5)(L) under bar ((L) under bar: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O 2p character. We revealed that the differences of the electronic and magnetic properties between BaFeO3 and SrFeO3 arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.