arXiv subjects

G. Shibata

Publications and source records attributed to G. Shibata.

At least 19 recordsLinked to original sources

arXiv2025

Orbital magnetic moments in FeCr2S4 studied by x-ray magnetic circular dichroism

We have investigated the element specific magnetic characteristics of single-crystal FeCr2S4 using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). We have found that the Fe L2,3-edge XAS spectra do not exhibit clear multiplet structures, indicating strong hybridization between the Fe 3d and S 3p orbitals, leading to delocalized rather than localized electronic states. The Fe 3d and Cr 3d spin moments are antiferromagnetically coupled, consistent with the Goodenough-Kanamori rule. The orbital magnetic moments of Fe and Cr are determined to be -0.23 and -0.017 {\mu}B/ion, respectively. The large orbital magnetic moment of Fe is due to the d6 configuration under the relatively weak tetrahedra crystal field at the Fe site, and the delocalized Fe electrons maintain the orbital degree of freedom in spite of their itinerant nature. To understand phenomena such as the gigantic Kerr rotation, it is essential to consider not only the orbital degrees of freedom but also the role of spin-orbit coupling, which induces a finite orbital magnetic moment through t2 and e level hybridization under the tetrahedral crystal field. This finite orbital moment serves as a direct indicator of spin-orbit interaction strength and links element-specific orbital magnetism to the large Kerr rotation. On the other hand, the octahedral crystal-field splitting of the Cr 3d level is large enough to result in the quenching of the orbital moment of the Cr ion in FeCr2S4.

cond-mat.str-el↗

arXiv2024

Spin-orbit-entangled electronic structure of Ba$_2$CaOsO$_6$ studied by O $K$-edge resonant inelastic X-ray scattering

Transition-metal ions with $5d^2$ electronic configuration in a cubic crystal field are prone to have a vanishing dipolar magnetic moment but finite higher-order multipolar moments, and they are expected to exhibit exotic physical properties. Through an investigation using resonant inelastic X-ray spectroscopy (RIXS), Raman spectroscopy, and theoretical ligand-field (LF) multiplet and $ab initio$ calculations, we fully characterized the local electronic structure of Ba$_2$CaOsO$_6$, particularly, the crystal-field symmetry of the 5$d^2$ electrons in this anomalous material. The low-energy multiplet excitations from RIXS at the oxygen $K$ edge and Raman-active phonons both show no splitting. These findings are consistent with the ground state of Os ions dominated by magnetic octupoles. Obtained parameters pave the way for further realistic microscopic studies of this highly unusual class of materials, advancing our understanding of spin-orbit physics in systems with higher-order multipoles.

cond-mat.str-el↗

arXiv2022

Cr doping-induced ferromagnetism in the spin-glass Cd1-xMnxTe studied by x-ray magnetic circular dichroism

The prototypical diluted magnetic semiconductor Cd1-xMnxTe is a spin glass (x<0.6) or an antiferromagnet (x>0.6), but becomes ferromagnetic upon doping with a small amount of Cr atoms substituting for Mn. In order to investigate the origin of the ferromagnetism in Cd1-x-yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Cr and Mn L2,3 edges. Thin films were grown by molecular beam epitaxy with a fixed Mn content of x = 0.2 and varying Cr content in the range of y = 0 - 0.04. Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the ferromagnetic or superparamagnetic components of Cr and Mn are aligned in the same directions. The magnetization of Mn increases with increasing Cr content. These results can be explained if ferromagnetic interaction exists between neighboring Mn and Cr ions although interaction between Mn atoms is largely antiferromagnetic. We conclude that each ferromagnetic or superparamagnetic cluster consists of ferromagnetically coupled several Cr and a much larger number of Mn ions.

cond-mat.str-el↗

arXiv2022

Magnetic Properties and Electronic Configurations of Mn Ions in the Diluted Magnetic Semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ Studied by X-ray Magnetic Circular Dichroism and Resonant Inelastic X-ray Scattering

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45μ_{\text{B}}$/Mn and the small orbital moment of $0.05μ_{\text{B}}$/Mn. This indicates that the Mn atoms are in the high-spin configurations of $d^{5}$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. RIXS spectra show broad peaks from 1 to 6 eV energy loss, which originate from the $d$-$d$ crystal field excitations of the Mn ions. Based on a comparison of the RIXS line shapes with those of Ga$_{1-x}$Mn$_{x}$As, we conclude that the ground state of Mn in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ consists not only of the charge-transferred $3d^{5}\underline{L}$ electron configuration ($\underline{L}$: ligand hole) with weakly bound holes as in Ga$_{1-x}$Mn$_{x}$As, but also of the pure $3d^{5}$ configuration with free holes.

cond-mat.str-el↗

arXiv2021

Magnetic anisotropy of the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ studied by angular-dependent XMCD

The van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr $L_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be 3+ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin $t_{2g}$$^3$ configuration of the Cr$^{3+}$ ion. A large ($\sim 0.2$ eV) trigonal crystal-field splitting of the $t_{2g}$ level caused by the distortion of the CrTe$_6$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign {\it opposite} to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of CrI$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Extended superconducting dome of electron-doped cuprates after protect annealing revealed by ARPES

The electron-doped cuprates are usually characterized by a more robust antiferromagnetic phase and a much narrower superconducting (SC) dome than those of the hole-doped counterparts. Recently, bulk single crystals of Pr1.3-xLa0.7CexCuO4-δ (PLCCO) prepared by the protect annealing method have been studied extensively and revealed many intriguing properties that were different from those obtained from samples annealed by the conventional methods. Here, we report on a systematic angle-resolved photoemission spectroscopy study of PLCCO single crystals after protect annealing. The results indicate that the actual electron concentration (nFS ) estimated from the Fermi-surface area is significantly larger than the Ce concentration x and the new nFS-based SC dome of PLCCO is more extended towards the overdoped side than the x-based SC dome derived for samples prepared using the conventional annealing method.

cond-mat.supr-con↗

arXiv2018

Anisotropic spin-density distribution and magnetic anisotropy of strained La$_{1-x}$Sr$_x$MnO$_3$ thin films: Angle-dependent x-ray magnetic circular dichroism

Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic anisotropy of $L1_0$-ordered FePt thin films studied by Fe and Pt $L_{2,3}$-edges x-ray magnetic circular dichroism

The strong perpendicular magnetic anisotropy of $L{\rm1_0}$-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy (MAE). We have investigated the anisotropy of the orbital magnetic moments of Fe 3$d$ and Pt 5$d$ electrons in $L{\rm1_0}$-ordered FePt thin films by Fe and Pt $L_{2,3}$-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order $S$. Fe $L_{2,3}$-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it, and that the anisotropy of the orbital magnetic moment increased with $S$. Pt $L_{2,3}$-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe $L_{2,3}$-edge XMCD results although the anisotropy of the orbital magnetic moment increases with $S$ like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev ${\it et\ al.}$ [Phys. Rev. B $\bf{52}$, 13419 (1995).], which also predicts the dominant contributions of Pt 5$d$ to the magnetic anisotropy energy rather than Fe 3$d$ due to the strong spin-orbit coupling and the small spin splitting of the Pt 5$d$ bands in $L{\rm1_0}$-ordered FePt.

cond-mat.mtrl-sci↗

arXiv2015

Thickness-dependent magnetic properties and strain-induced orbital magnetic moment in SrRuO3 thin films

Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ~ 0.1μB/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

cond-mat.str-el↗

arXiv2015

Electronic and magnetic properties of off-stoichiometric Co$_\mathrm{2}$Mn$_β$Si/MgO interfaces studied by x-ray magnetic circular dichroism

We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co$_\mathrm{2}$Mn$_β$Si (CMS)/MgO ($β$=0.69, 0.99, 1.15 and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition ($β$) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, $m_\mathrm{spin}$(Mn), derived from XMCD weakly decreased with increasing Mn composition $β$ in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, $m_\mathrm{spin}$(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of Co$_\mathrm{Mn}$ antisites detrimental to the half-metallicity of CMS with increasing $β$. This finding is consistent with the higher TMR ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.

cond-mat.str-el↗

arXiv2015

Photoemission and X-ray Absorption Studies of the Diluted Magnetic Semiconductor Ba$_{1-y}$K$_{y}$(Zn$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ Isostructural to Fe-based Superconductors

The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states (PDOS) obtained by RPES shows a peak around 4 eV and relatively high between 0-2 eV below the Fermi level ($E_{F}$) with little contribution at $E_{F}$, similar to that of the archetypal DMS Ga$_{1-x}$Mn$_{x}$As. This energy level creates $d^{5}$ electron configuration with $S=5/2$ local magnetic moments at the Mn atoms. Hole carriers induced by K substitution for Ba atoms go into the top of the As $4p$ valence band and are weakly bound to the Mn local spins. The ferromagnetic correlation between the local spins mediated by the hole carriers induces ferromagnetism in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$

cond-mat.mtrl-sci↗

arXiv2014

Bond-Order and the Role of Ligand States in Stripe-Modulated IrTe2

The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$d$ $\rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3$d$ $\rightarrow$ 5$p$ resonance at the $Q\!=\!(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_s\sim$ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5$p$-Ir 5$d$ bond-density in driving the stripe-like order in IrTe$_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-T$_c$ superconducting cuprates suggests that the iridates may harbor similar exotic phases.

cond-mat.str-el↗

arXiv2013

Spectroscopic studies on the electronic and magnetic states of Co-doped perovskite manganite Pr0.8Ca0.2Mn1-yCoyO3 thin films

We have investigated the electronic and magnetic properties of Co-doped Pr0.8Ca0.2MnO3 thin films using various spectroscopic techniques. X-ray absorption and hard x-ray photoemission spectroscopy revealed that the substituted Co ions are in the divalent state, resulting in hole doping on the Mn atoms. Studies of element-selective magnetic properties by x-ray magnetic circular dichroism found a large orbital magnetic moment for the Co ions. These spectroscopic studies reveal that the substituted Co ions play several roles of hole doping for Mn, ferromagnetic superexchange coupling between the Co2+ and Mn4+ ions, and orbital magnetism of the Co2+ ions. Competition among these complex interactions produces the unique electronic and magnetic behaviors including enhanced coercivity of the Co-doped Pr0.8Ca0.2MnO3.

cond-mat.str-el↗

arXiv2013

Orbital magnetic moment and coercivity of SiO$_{2}$-coated FePt nanoparticles studied by x-ray magnetic circular dichroism

We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1$_{0}$-ordered phase coated with SiO$_{2}$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{\rm 2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{\rm orb}$) to the spin magnetic moment ($M_{\rm spin}$) of Fe to be $M_{\rm orb}/M_{\rm spin}$ = 0.08. This $M_{\rm orb}/M_{\rm spin}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($\sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1$_{0}$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_{\rm c}$) was found to be as large as 1.8 T at room temperature, $\sim$3 times larger than the thin film value and $\sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles with the $H_{\rm c}$ distributed from 1 T to 5 T.

cond-mat.mtrl-sci↗

arXiv2013

Phase diagram of Ca$_{1-x}$Ce$_x$MnO$_3$ thin films studied by X-ray magnetic circular dichroism

In the perovskite-type Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO), one can control the transport and magnetic properties through varying Ce content. In the case of thin films, the properties can also be controlled by epitaxial strain from the substrate through changing it such as YAlO$_{3}$ (YAO), NdAlO$_{3}$ (NAO), and LaSrAlO$_{4}$ (LSAO). However, one cannot measure the magnetization of thin films on NAO substrates by conventional magnetization measurements because of the strong paramagnetic signals from the Nd$^{3+}$ ions. In order to eliminate the influence of Nd$^{3+}$ and to identify magnetic phases of the CCMO thin films, we have performed element-selective X-ray magnetic circular dichroism (XMCD) measurements of the Mn 2{\it p} core level. By studying the anisotropy of the XMCD intensity, we could unambiguously determine the magnetic phase diagram of the CCMO thin films.

cond-mat.str-el↗

arXiv2013

Role of doped Ru in coercivity-enhanced La$_{0.6}$Sr$_{0.4}MnO$_3$ thin film studied by x-ray magnetic circular dichroism

The coercivity of La$_{1-x}$Sr$_x$MnO$_3$ thin films can be enhanced by Ru substitution for Mn. In order to elucidate its mechanism, we performed soft x-ray absorption and magnetic circular dichroism measurements at the Ru M$_{2,3}$ and Mn L$_{2,3}$ edges. We found that the spin direction of Ru and Mn are opposite and that Ru has a finite orbital magnetic moment. Cluster-model analysis indicated that the finite orbital magnetic moment as well as the reduced spin moment of Ru result from local lattice distortion caused by epitaxial strain from the SrTiO$_3$ substrate in the presence of spin-orbit interaction.

cond-mat.mtrl-sci↗

arXiv2013

Enhanced ferromagnetic moment in Co-doped BiFeO3 thin films studied by soft X-ray circular dichroism

BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe$_{1-x}$Co$_x$O$_3$ ({\it x} = 0 to 0.30) (BFCO) thin films grown on LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe$^{3+}$ and Co$^{3+}$ states. The XMCD results showed that the Fe ions show ferromagnetism while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic $γ$-Fe$_2$O$_3$ which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.

cond-mat.mtrl-sci↗

arXiv2013

Absence of Superconductivity in the "hole-doped" Fe pnictide Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$: Photoemission and X-ray Absorption Spectroscopy Studies

We have studied the electronic structure of Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ ($x$=0.08), which fails to become a superconductor in spite of the formal hole doping like Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, by photoemission spectroscopy and X-ray absorption spectroscopy (XAS). With decreasing temperature, a transition from the paramagnetic phase to the antiferromagnetic phase was clearly observed by angle-resolved photoemission spectroscopy. XAS results indicated that the substituted Mn atoms form a strongly hybridized ground state. Resonance-photoemission spectra at the Mn $L_{3}$ edge revealed that the Mn 3d partial density of states is distributed over a wide energy range of 2-13 eV below the Fermi level ($E_F$), with little contribution around $E_F$. This indicates that the dopant Mn 3$d$ states are localized in spite of the strong Mn 3d-As $4p$ hybridization and split into the occupied and unoccupied parts due to the on-site Coulomb and exchange interaction. The absence of superconductivity in Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ can thus be ascribed both to the absence of carrier doping in the FeAs plane, and to the strong stabilizaiton of the antiferromagnetic order by the Mn impurities.

cond-mat.supr-con↗