The orbital contribution to the magnetic moment of the transition-metal ion in the isostructural weak ferromagnets $A{\mathrm{CO}}_{3}$ ($A=$ Mn,Co,Ni) and ${\mathrm{FeBO}}_{3}$ was investigated by a combination of first-principles calculations, nonresonant x-ray magnetic scattering, and x-ray magnetic circular dichroism. A nontrivial evolution of the orbital moment as a function of the $3d$ orbitals filling is revealed, with a particularly large value found in the Co member of the family. Here, the coupling between magnetic and lattice degrees of freedom produced by the spin-orbit interaction results in a large single-ion anisotropy and a peculiar magnetic-moment-induced electron cloud distortion, evidenced by the appearance of a subtle scattering amplitude at space-group-forbidden reflections and significant magnetostrictive effects. Our results, which complement a previous investigation on the sign of the Dzyaloshinskii-Moriya interaction across the series, highlight the importance of spin-orbit coupling in the physics of weak ferromagnets and prove the ability of modern first-principles calculations to predict the properties of materials where the Dzyaloshinskii-Moriya interaction is a fundamental ingredient of the magnetic Hamiltonian.
We have measured angular-dependent fluorescence-yield x-ray magnetic circular dichroism spectra on single crystals of the heterometallic $3d\ensuremath{-}4f$ 12-metallacrown-4 ${\mathrm{TbMn}}_{4}$ and ${\mathrm{DyMn}}_{4}$ complexes. Simulated spectra using crystal-field multiplet calculations reproduce the experimentally observed spectra. The orientation of the molecules causes linear dichroism spectra of the $4f$ absorption spectra. This natural linear dichroism shows the anisotropic charge distribution of the rare-earth $4f$ state in the tetragonal crystal field despite the small $4f$ crystal-field splitting. The magnetic moment of the molecule is dominated by the rare-earth moment revealing a considerably large contribution of orbital moment. From a sum-rule analysis of experimental and simulated x-ray magnetic circular dichroism, we determined corrected spin and orbital Dy moments at low temperature (14 K) within a magnetic field of 7 T. We find a significant angular dependence of the Dy magnetic moments, indicating the presence of fourth-order magnetic anisotropy.
A procedure to build the optical conductivity tensor that describes the full magneto-optical response of the system from experimental measurements is presented. Applied to the Fe L 2,3 -edge of a 38.85 nm Fe 3 O 4 /SrTiO 3 (001) thin-film, it is shown that the computed polarization dependence using the conductivity tensor is in excellent agreement with that experimentally measured. Furthermore, the magnetic field angular dependence is discussed using a set of fundamental spectra expanded on spherical harmonics. It is shown that the convergence of this expansion depends on the details of the ground state of the system in question and in particular on the valence-state spin–orbit coupling. While a cubic expansion up to the third order explains the angular-dependent X-ray magnetic linear dichroism of Fe 3+ well, higher-order terms are required for Fe 2+ when the orbital moment is not quenched.
The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as dd transitions. In this vein, intense interest has surrounded the appearance of sharp, below-band-gap optical transitions, i.e., excitons, within the magnetic phase of correlated Ni2+ van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in Ni2+ systems raises intriguing questions on the roles of long-range magnetic order and of metal-ligand charge transfer in the exciton nature, which inspired microscopic descriptions beyond typical dd excitations. Here we study the impact of charge transfer and magnetic order on the excitation spectrum of the nickel dihalides (NiX2, X=Cl, Br, and I) using Ni-L3 edge resonant inelastic x-ray scattering (RIXS). In all compounds, we detect sharp excitations, analogous to the recently reported excitons, and assign them to spin-singlet multiplets of octahedrally coordinated Ni2+ stabilized by intra-atomic Hund’s exchange. Additionally, we demonstrate that these excitons are dispersive using momentum-resolved RIXS. Our data evidence a ligand-mediated multiplet dispersion, which is tuned by the charge-transfer gap and independent of the presence of long-range magnetic order. This reveals the mechanisms governing nonlocal interactions of on-site dd excitations with the surrounding crystal or magnetic structure, in analogy to ground-state superexchange. These measurements thus establish the roles of magnetic order, self-doped ligand holes, and intersite-coupling mechanisms for the properties of dd excitations in charge-transfer insulators. Published by the American Physical Society 2024
Hematite (α-Fe2O3) is a photoelectrode for the water splitting process because of its relatively narrow bandgap and abundance in the earth's crust. In this study, the photoexcited state of a hematite thin film was investigated with femtosecond oxygen K-edge X-ray absorption spectroscopy (XAS) at the PAL-XFEL in order to follow the dynamics of its photoexcited states. The 200 fs decay time of the hole state in the valence band was observed via its corresponding XAS feature.
Abstract The systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.
Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L_{3}-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.
This metadata file gathers crystal-chemical, crystallographic and spectroscopic raw data supporting the manuscript entitled “Niobium speciation in minerals revealed by L2,3-edges XANES spectroscopy“ submitted to American Mineralogist. EMP_natural_references: spreadsheet compiling crystal-chemical data of the Nb geological references (columbite, fluorcalciopyrochlore, hydropyrochlore, latrappite, niocalite) obtained from electron microprobe analysis (EMP_crystal_chemistry_references.xls). Analyses were conducted on a CAMECA SX Five electron at the CAMPARIS facility (Sorbonne Universite, Paris). Two to seven spots were probed for each sample at 15 kV and 40 nA for the major elements and at 15 kV and 299 nA for minor and : trace elements. WDS analyses were performed using the following standards: albite for Na, diopside for Mg, Si and Ca, orthoclase for Al and K, MnTiO3 for Mn and Ti, hematite for Fe, BaSO4 for Ba; topaze for F, LiNbO3 for Nb, metallic Ta for Ta, SrSi for Sr, zircon for Zr, galena for Pb, monazite for Th, uraninite for U, and REE-bearing silicates for for Y, La, Ce, Nd.XANES_spectra: raw X-ray absorption data files (.dat). There are either one or two files per sample (_01/_02). Amounts of Nb in Ti oxide samples are mentionned in file titles (Nb1pc/Nb5pc/Nb10pc). Niobium L2,3-edges XANES spectra were recorded on the LUCIA beamline of the SOLEIL synchrotron radiation facility (Gif-sur-Yvette, France) operating with a storage ring current of 450 mA and energy of 2.75 GeV. A double crystal Si(111) monochromator crystal was used with an energy resolution of 0.25 eV at 2400 eV and calibrated at this energy using Nb2O5.XRD_geological_references: spreadsheet compiling crystal-cell parameters obtained from the reduction of single-crystal XRD data (geological_samples_crystal_cell_data.xls). The number of reflections indexed and teta range of measurement are specified. An Agilent Diffraction Xcalibur-S diffractometer equipped with a Sapphire CCD-detector with Mo K-α1 radiation (λ = 0.71073 Å, graphite monochromator) was used at room temperature. Data reduction, cell refinement and space group determination were performed using CrysAlisPro software. XRD_synthetic_samples: XRD data obtained for the synthetic samples (.csv/.xrdml). Amounts of Nb in Ti oxide samples are mentionned in file titles (Nb1pc/Nb5pc/Nb10pc). X-ray diffraction was performed with a PANALYTICAL X’pert Pro MPD diffractometer. Measurements were carried out in Bragg-Brentano geometry using a Co K-α anode in order to minimize the X-ray absorption of Fe in iron oxide synthetic compounds. Data were recorded with an X’Celerator detector between 3 ° 2θ and 90 ° 2θ with 0.017 ° steps. Incident beam mask was fixed at 20 mm and soller slits at 0.04 rad.
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