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Abstract

The electronic structure of CoO is studied by resonant inelastic soft X-ray scattering spectroscopy using photon energies across the Co 2p absorption edges. The different spectral contributions from the energy-loss structures are identified as Raman scattering due to d-d and charge-transfer excitations. For excitation energies close to the L3 resonance, the spectral features are dominated by quartet-quartet and quartet-doublet transitions of the 3d7 configuration. At excitation energies corresponding to the satellites in the Co 2p X-ray absorption spectrum of CoO, the emission features are instead dominated by charge-transfer transitions to the 3d8L-1 final state. The spectra are interpreted and discussed with the support of simulations within the single impurity Anderson model with full multiplet effects which are found to yield consistent spectral functions to the experimental data.

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... 14,15 This same trend is visible in the RXES spectra as well, where the spectra of the high fluence sample appear as fluorescence at a constant emission energy, as expected for metallic systems, whereas those of the lowest fluence sample (L) show sharp d-d excitations tracking the excitation energy, again typically present in ionic Co materials such as oxides. 16,17 Thus, for low fluence implantation, we observe oxygen coordinated Co, while higher fluences show increasing metallic aggregation of Co (likely in the form of Co nanoclusters which have been previously produced in ion implantation studies 5,6 ). Also note that the stark variation in the intensity ratios for the L 2 and L 3 peaks in panel (d) provides further evidence for a metallic environment for higher fluence and oxygen coordination for lower fluence. ...
... Such excitations are captured by the hybridization terms of the SIAM, and the agreement with experiment is very good, again using the parameters of Table II. Here, we find Co 3d-O 2p hopping integral V e g with a magnitude very similar to what is found in monoxides, 11,16,17 indicating a significant covalent interaction between the Co ions and the O atoms of the SiO 2 . This will be further elucidated when discussing the O K edge results below. ...
... Very similar SIAM parameters are found for Mn compared to Co, with the primary difference being the charge transfer energy D. This trend is again consistent with differences between Co and Mn monoxides. 16,17,22 C. Effects on host electronic structure ...
Article
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Cobalt and manganese ions are implanted into SiO_2 over a wide range of concentrations. For low concentrations, the Co atoms occupy interstitial locations, coordinated with oxygen, while metallic Co clusters form at higher implantation concentrations. For all concentrations studied here, Mn ions remain in interstitial locations and do not cluster. Using resonant x-ray emission spectroscopy and Anderson impurity model calculations, we determine the strength of the covalent interaction between the interstitial ions and the SiO_2 valence band, finding it comparable to Mn and Co monoxides. Further, we find an increasing reduction in the SiO_2 electronic band gap for increasing implantation concentration, due primarily to the introduction of Mn-and Co-derived conduction band states. We also observe a strong increase in a band of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to oxygen deficient centers formed during implantation.
... Figure 2(b) shows Co L-edge XAS spectra of BFCTO and LBFCTO films. Similar to the XAS of Fe, two absorption edges from spin-orbit-split Co 2p core levels can be observed around 778 eV (L 3 ) and 794 eV (L 2 ) [25,26]. Strong multiplet effects due to Coulomb and exchange interactions are observed at the L 3 absorption edge. ...
... The RIXS data are all normalized to incident photon flux with the same acquisition time (10 min for each spectrum), and shown on an energy loss scale. The Co L-edge RIXS spectra of BFCTO and LBFCTO films are quite similar to that of CoO reported previously [25,32], indicating that we can interpret the Co RIXS data of the films based on FIG. 4. Co L-edge RIXS spectra of (a) BFCTO and (b) LBFCTO films at selected incident photon energies. The incident photon energies are displayed next to the corresponding RIXS spectra and are indicated by vertical sticks in Fig. 2(b). ...
... The assignments of these d-d excitations are also summarized in Table II. The broad dispersion around −7 eV can be attributed to O 2p to Co 3d charge-transfer excitations [25,26]. The crystal-field splitting energy 10Dq of BFCTO and LBFCTO films can be determined to be about 0.7 eV from the energy position of the first loss peak [32,33]. ...
Article
Aurivillius-phase oxides exhibit the coexistence of ferroelectricity (FE) and ferromagnetism (FM) above room temperature, and the FE and FM orders can be modified by various dopants. However, detailed examinations of their electronic structures in response to the dopants are still lacking. Here, the Bi6Fe2Ti3O18, Bi6FeCoTi3O18, and LaBi5FeCoTi3O18 epitaxial thin films were prepared and the systematic experimental and theoretical resonant inelastic x-ray scattering (RIXS) studies were performed at both Fe and Co L edges. The RIXS measurements reveal the modified electronic structures induced by the doping of Co and La. The RIXS simulations demonstrate that the electronic structure variations are the outcome of modified crystal-field parameters of FeO6 and CoO6 octahedra.
... The peaks around 777.6 eV, 779.2 eV, 782.2 eV and 794.0 eV in the spectrum of Co 3 O 4 found are consistent with earlier report [38]. In contrast, five peaks of CoO3 are observed near 775.6 eV, 777.0 eV, 778.2 eV, 780.5 eV and 792.7 eV, denoted as A, B, C, D and E (Fig. 7(d)), are in line with earlier reports on CoO [38,39]. Peaks B and E are assigned to the transitions from Co 2p 3/2 (L3) and 2p 1/2 (L2) electrons into the empty d-states, respectively. ...
... The Eg 1 and Eg 2 are associated with the Co 2+ transitions in octahedral crystal field and O 2− → Co 2+ processes, respectively [46,47]. However, the energy of these transitions are very close to the d-d excitations in CoO [39]. ...
... Fig. 5a shows the Co L-edge NEXAFS spectrum for Fe/MgO/Fe/Co multilayer with the spectral features occurring at 772 (A 2 ) and 776 eV (B 2 ). Spectral features at this edge are common in TEY and TFY mode and analogues to that of Co metal reported by various authors [33,34]. TEY and TFY NEXAFS spectra for this multilayer exhibit spectral features different from CoO [34]. ...
... Spectral features at this edge are common in TEY and TFY mode and analogues to that of Co metal reported by various authors [33,34]. TEY and TFY NEXAFS spectra for this multilayer exhibit spectral features different from CoO [34]. Angle dependent measurements collated in Fig. 5b (Fig. 6a). ...
Article
The present work investigates the local electronic structure of magnetic Fe/MgO/Fe/Co multilayer structure by using angle dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The multilayer stack was grown on Si(100) substrates using electron-beam evaporation. X-ray diffraction study reveals the polycrystalline nature of different layers in the stack. Transmission electron microscopy shows different layers separated with each other except for upper Fe and Co layers. Magnetic nature of deposited structure was confirmed using vibration sample magnetometer. While the Fe L-edge NEXAFS measurements with total electron yield mode exhibit the spectral features that are indicative for formation of FeOx, the total fluorescence yield mode measurements exhibit onset of metallic nature of the Fe layer. These measurements at the Co L-edge reflect the presence of metallic nature of cobalt. Mg K-edge measurements performed at different angles exhibit local electronic structure of MgO layers very much similar to that of bulk MgO. Further, O K-edge fine structure supports the results obtained from various edges for the elements in each layer.
... To properly understand and describe the CT excitations, it was recognized that taking into account only the ligand band structure leads to the wide band of CT excitations seen in the experiment (Fig. 41). Such an extension of the cluster approach leads to the SIAM with full multiplet effects, which is widely used in modern semiempirical simulations of RIXS (Magnuson, Butorin, Agui, and Nordgren, 2002;Magnuson, Butorin, Guo, and Nordgren, 2002;Ghiringhelli et al., 2005Ghiringhelli et al., , 2006Chiuzbăian et al., 2008) of the RIXS. The combination of the local density approximation to the density-functional theory and the dynamical mean-field theory (Hariki, Winder et al., 2020) eliminates most of the empirical parameters of the traditional cluster method. ...
Article
An overview of both experimental and theoretical results in the field of resonant scattering of tunable soft and hard x-ray radiation is presented, with a main focus on the closely related processes of resonant inelastic x-ray scattering (RIXS) and resonant Auger scattering (RAS). The review starts with an overview of fundamental dynamical aspects of RIXS illustrated for different systems. A detailed analysis of case studies with increasing complexity, considering both gas-phase and condensed matter (liquids and solids) applications, is given. In the review, the most important achievements in investigations of coupled electron-nuclear dynamics and structural aspects in studies of liquids and solids over the last two decades are outlined. To give a perspective on the insights from RIXS and RAS, the x-ray results are discussed against the background of complementary experimental techniques like vibrational infrared absorption and Raman spectroscopy, as well as small-angle x-ray and neutron scattering. Finally, recent achievements in time-resolved studies based on x-ray free-electron lasers are described.
... I of the Supplemental Material [23]). Spectral weights of excitations to the different multiplet states as marked in Fig. 1(c) are seen to be modulated with incident photon energy like previous reports [12,20,27,28]. What is unprecedented is the nearly 10 times increase in intensities of excitation to the spin-singlet multiplet state 1 A 1g , as one goes from the main Ni L 3 resonance peak (MP) to the satellite peak (SP) of XAS as shown in Fig. 1(d) [12]. ...
Article
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Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L3-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.
... In an effort to contribute to the debate about the origin of magnetism in LaCoO 3 , we employ 2p3d Resonant Inelastic X-ray Scattering (2p3d RIXS) to study the complex electronic configuration of the Co ions in a LaCoO 3 single crystal and strained thin films. 2p3d RIXS probes both the local [26][27][28][29][30][31][32][33][34] and collective excitations [35][36][37][38][39][40][41][42] , including small lattice distortions [43][44][45] . In the case of LaCoO 3 , the 2p3d RIXS process in the ionic limit can be described as 3d 6 > 2p 5 3d 7 > 3d 6 transitions of Co 3+ ions, which allows us to distinguish the spin state manifolds. ...
Article
We present 90 meV resolved Co 2p3d resonant inelastic x-ray scattering linear dichroism spectra of strained LaCoO3 films and a LaCoO3 single crystal. A polarization-dependent low-energy excitation is observed at ∼0.2eV on the tensile-strained LaCoO3/SrTiO3 film, while it is not observed in either bulk LaCoO3 or the compressive-strained LaCoO3/LaAlO3 film. Guided by cluster calculations, we are able to distinguish the spin-state manifolds close to their transition point of Co3+ ions in LaCoO3 systems. Through a polarization analysis, we show that the spin state can easily flip from a low-spin A1g1 state in an octahedral symmetry to the high-spin B2g5 or Eg5 states with a small tetragonal distortion. A mixture of spin states suggests that the high-spin Co3+ plays an important role in long-range ferromagnetic order on both tensile- and compressive-strained LaCoO3 films.
... The diagonalization ofĤ CEF results in an orbital triplet ground state ( 4 T 1 ), an excited orbital triplet ( 4 T 2 ), and an orbital singlet 4 A 2 , where ( 4 T 1 → 4 T 2 ) = 480B 4 and ( 4 T 2 → 4 A 2 ) = 600B 4 . The Stevens factor B 4 is related to the crystal-field splitting by 10Dq = 400B 4 [ Fig. 1(b)], where 10Dq was previously measured to be ∼1 eV [54,[68][69][70][71][72]. Since the 4 T 1 crystal-field ground state and 4 T 2 first excited state are separated by ∼1 eV, it is a valid approximation that the 4 T 1 ground state will exclusively determine the magnetic properties of CoO [73,74]. ...
Article
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CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported recently [P. M. Sarte et al., Phys. Rev. B 98, 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [R. A. Cowley et al., Phys. Rev. B 88, 205117 (2013)] results in significant mixing between jeff spin-orbit levels in the low-temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at ∼300K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl3 [W. J. L. Buyers et al., Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multilevel spin-orbit exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co2+ ions lying on a rocksalt lattice. This parametrization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low-energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high-energy transfers, where the neutron response decays faster with momentum than the Co2+ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high-energy transfers.
... eV and E = 794.1 eV are similar to the broad spectrum of metallic Co [69][70][71]. This suggests that the fine structures of Mn 2p → 3d transition should result from the localization of Mn 3d electrons rather than the oxidation of the surface [72][73][74]: otherwise, the multiplet structure of Co L 2,3 edges should be observed [75,76]. To qualitatively illustrate the features of the measured XAS, simulations were performed using the small cluster approach in the Xclaim software tool [77]. ...
Article
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A room-temperature skyrmion-hosting compound Co8Zn8Mn4 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 L2,3 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=20 K and T=120 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.
... The RIXS spectra were calculated using the CTM4XAS-55 MATLAB interfacing program based on Kramers−Heisenberg formula [21][22][23], from which the generated XAS absorption and XES emission files were obtained. Consequently, these files are utilized as input files for the CTM4RIXS010 program, which was used to generate the calculated RIXS matrix and visualize the dependence mapping representation. ...
Article
The resonant inelastic X-ray scattering (RIXS) is a powerful tool as a probe for the local electronic and magnetic structures and provides detailed knowledge on the 3d open shell valence state in solids and molecules. In this work, the electronic structure, charge transfer (CT), and d-d excitation studies in perovskite manganite: LaSr2Mn2O7 and La1.2(Ba, Ca)1.8Mn2O7 single crystals have been explored using X-ray absorption spectra (XAS) and RIXS. The incident photon energy was tuned in the vicinity of Mn L3,2 absorption edges (639.2, 650.2, and 660 eV) to induce the RIXS matrix. The various electronic structural parameters were estimated using the charge transfer multiplet (CTM) approach implemented in Cowan. It is observed that manganese ions are present in the predominant Mn3+/Mn4+ ground configurations. The pre-edge shoulder, at XAS Mn L3 edge of La1.2(Ba, Ca)1.8Mn2O7 single crystal, is indicative of enhanced Mn3+ valence state and increase the Mn3+/Mn4+ concentration ratio. The ground and excited states of Mn3+ (3d4) in octahedral (Oh) local symmetry are 2T1 (1D) and 2E(2D) respectively. The ground state of trivalent Mn3+ (5D) exists with a high spin (HS) active state (|t2g3 eg1>). We find that the separation between t2g and eg states and the d–d excitation is primarily influenced by CF with a weak involvement of CT.
... The 0.870 and 2.3 eV peaks are in agreement with x rays, but the 1.84 eV excitation was not resolved with nonresonant x rays and is forbidden in the dipolar approximation; however, it has been reported in other optical and x-ray studies such as RIXS and EELS that are sensitive to dipole forbidden cross sections. 38,[43][44][45][46][47] This result highlights the point that the 1.84 ± 0.03 eV excitation is dipole forbidden and confirms our claim that we are sampling a quadrupolar matrix element in our neutron scattering experiments. ...
Article
Full-text available
Neutron scattering is used to investigate the single-ion spin and orbital excitations below the Mott-Hubbard gap in CoO. Three excitations are reported at 0.870±0.009 eV, 1.84±0.03, and 2.30±0.15 eV. These were parametrized within a weak crystal field scheme with an intraorbital exchange of J(dd)=1.3±0.2 eV and a crystal field splitting 10Dq=0.94±0.10 eV. A reduced spin-orbit coupling of λ=−0.016±0.003 eV is derived from dilute samples of Mg_{0.97}Co_{0.03}O, measured to remove effects due to spin exchange and structural distortion parameters which break the cubic phase degeneracy of the orbital excitations complicating the inelastic spectrum. The peak at 1.84 eV, while reported using resonant x-ray and optical techniques, was absent or weak for nonresonant x-ray experiments and overlaps with the expected position of a ^{4}A_{2} level. This transition is absent in the dipolar approximation but expected to have a finite quadrupolar matrix element that can be observed with neutron scattering techniques at larger momentum transfers. Our results agree with a crystal field analysis (in terms of Racah parameters and Tanabe-Sugano diagrams) and with previous calculations performed using local-density band theory for Mott insulating transition metal oxides. The results also demonstrate the use of neutron scattering for measuring dipole forbidden transitions in transition metal oxide systems.
... Å) Cu O bonds in the basal plane. As the Cu electron configurations fluctuate between 3d 10 , 3d 9 and 3d 8 in the ground state, as in the case of binary transition metal oxides [10] [11], the wave function contains all three configurations in addition to several charge-transfer configurations that were considered. ...
Article
We present a study on the high-Tc superconductor (HTSC) YBa2Cu3O7-x (YBCO) using polarization-dependent X-ray absorption and resonant inelastic X-ray scattering. High-resolution measurements using synchrotron-radiation are compared with calculations using a quasi-atomic multiplet approach performed at the Cu 2p3/2-edge of YBCO. We use a multiplet approach within the single impurity Anderson model to reproduce and understand the character of the localized low-energy excitations in YBCO. We observe a distinct peak at about 0.5 eV in O K RIXS. This peak shows dependence on doping, incident energy, and momentum transfer that suggests that it has a different origin than the previously discussed cuprate bi-magnons. Therefore, we assign it to multi-magnon excitations between the Zhang Rice bands and/or the Upper Hubbard bands, respectively.
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The electronic structure and magnetic properties of cobalt doped CdSe quantum dots (QDs) are studied using electron microscopy, soft x-ray spectroscopy, and magnetometery. Magnetometry measurements suggest these QDs are superparamagnetic, contrary to a spin-glass state observed in the bulk analogue. Electron microscopy shows well formed QDs, but with cobalt existing as doped into the QD and as unreacted species not contained in the QD. X-ray absorption measurements at the Co $L_3$-edge suggest that changes in spectra features as a function of particle size can be described considering combination of a cobalt ion in a tetrahedral crystal field and an octahedreally coordinated (impurity) phase. With decreasing particle sizes, the impurity phase increases, suggesting that small QDs can be difficult to dope.
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Soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have been performed on several nickel-containing bioinorganic complexes. RIXS spectral features are shown to be informative and diagnostic for the different oxidation states (Ni II vs Ni III) and spin states (high spin Ni II vs low spin Ni II) in these bioinorganic systems. In addition to the experimental results, multiplet simulation has also been performed to assist in understanding the observed XAS and RIXS features. These results demonstrate the power and complementarity of RIXS in identifying the electronic states for covalent and biorelevant complexes for the first time and pave the way for potential RIXS application to real biological systems.
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Co4Nb2O9 becomes antiferromagnetic (AFM) below 27.4 K with a spin-flop transition at a critical field, Hc, of 12 kOe. Room-temperature dielectric properties are dominated by finite electronic conductivity. Below 125 K, the charge carriers are frozen-out and the dielectric constant is controlled by the lattice phonons. A large (12%) spin flop-driven enhancement in dielectric constant is found in the very narrow temperature interval (Delta T = 1.6 K) in the vicinity of the AFM phase transition. Magneto-dielectric anomaly shows low-frequency dispersion; therefore, the H-induced changes in the phonon eigenfrequencies are unlikely. Other possible reasons for unusual magneto-dielectric effect in Co4Nb2O9 are discussed.
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Neutron scattering is used to investigate the single-ion spin and orbital excitations below the Mott-Hubbard gap in CoO. Three excitations are reported at 0.870 $\pm$ 0.009 eV, 1.84 $\pm$ 0.03, and 2.30 $\pm$ 0.15 eV. These were parameterized within a weak crystal field scheme with an intra-orbital exchange of $J(dd)$=1.3 $\pm$ 0.2 eV and a crystal field splitting 10Dq=0.94 $\pm$ 0.10 eV. A reduced spin-orbit coupling of \lambda=-0.016 $\pm$ 0.003 eV is derived from dilute samples of Mg$_{0.97}$Co$_{0.03}$O, measured to remove complications due to spin exchange and structural distortion parameters which split the cubic phase degeneracy of the orbital excitations complicating the inelastic spectrum. The 1.84 eV, while reported using resonant x-ray and optical techniques, was absent or weak for non resonant x-ray experiments and overlaps with the expected position of a $^{4}A_{2}$ level. This transition is absent in the dipolar approximation but expected to have a finite quadrupolar matrix element that can be observed with neutron scattering techniques at larger momentum transfers. Our results agree with a crystal field analysis (in terms of Racah parameters and Tanabe-Sugano diagrams) and with previous calculations performed using local-density band theory for Mott insulating transition metal oxides. The results also demonstrate the use of neutron scattering for measuring dipole forbidden transitions in transition metal oxide systems.
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Cu Kβ2,5 X-ray emission and resonant inelastic X-ray scattering measurements were performed on monovalent and divalent copper compounds. The data were compared with the results of local-density-approximation calculations. The methods were found to be efficient tools for studies of Cu 4p states in the valence band and for distinguishing between different monovalent copper compounds. This is of particular importance for the debate concerning copper corrosion in oxygen-free water.
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The magnetic ground state in the double perovskite system Sr2−xCaxCoOsO6 changes from an antiferromagnet (x=0), to a spin glass (x=1), to a ferrimagnet (x=2) as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of SrCaCoOsO6 and Ca2CoOsO6 are investigated and compared with Sr2CoOsO6. Both compounds adopt a monoclinic crystal structure with rock-salt ordering of Co2+ and Os6+ and a−a−b+ octahedral tilting, but the average Co–O–Os bond angle evolves from 158.0(3)∘ in SrCaCoOsO6 to 150.54(9)∘ in Ca2CoOsO6 as the smaller Ca2+ ion replaces Sr2+. While this change may seem minor, it has a profound effect on the magnetism, changing the magnetic ground state from antiferromagnetic in Sr2CoOsO6 (TN1=108K, TN2=70K), to a spin glass in SrCaCoOsO6 (Tf1=32K, Tf2=13K), to ferrimagnetic in Ca2CoOsO6 (TC=145K). In the first two compounds the observation of two transitions is consistent with weak coupling between the Co and Os sublattices.
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The temperature-dependent 1s2p resonant inelastic X-ray scattering (RIXS) spectra of CoO have been measured with 0.3 eV overall resolution, and the RIXS planes have been analyzed with multiplet calculations. The analysis of the high-resolution 1s2p RIXS plane allows a more detailed determination of the ground-state electronic structure, as compared to 1s X-ray absorption spectroscopy (XAS). The apparent absence of interference effects suggests that the lifetime broadening of the pre-edge states is (significantly) reduced from the edge lifetime broadening. The temperature-dependent RIXS planes are explained as a combination of the ground state and first excited state due to thermal population of the excited state, which are a result of the symmetry distortion, 3d spin?orbit coupling, and magnetic exchange interactions. No features due to charge transfer and nonlocal transitions are observed due to the relatively small cobalt?oxygen overlap as compared to higher valent systems. The successful determination of the spin state and crystal field parameters using hard X-ray experiments promises to make 1s2p RIXS a useful technique for in situ transition metal oxide studies.
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The electronic structure of the kagome staircase compounds, Ni3V2O8 and Co3V2O8, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering (RIXS). Comparison between the two compounds, and with first principles band structure calculations and crystal-field multiplet models, provide unique insight into the electronic structure of the two materials. Whereas the location of the narrow (Ni,Co) d bands is predicted to be close to EF, we experimentally find they lie deeper in the occupied O 2p and unoccupied V 3d manifolds, and determine their energy via measured charge-transfer excitations. Additionally, we find evidence for a dd excitation at 1.5 eV in Ni3V2O8, suggesting the V d states may be weakly occupied in this compound, contrary to Co3V2O8. Good agreement is found between the crystal-field dd excitations observed in the experiment and predicted by atomic multiplet theory.
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We have performed 2p X-ray absorption (XAS) and 2p3d resonant X-ray emission (RXES) experiments on a CoO bulk single crystal as well as on 4.2 nm CoO nanocrystals. The single crystal data were measured with linearly polarized incident X-rays in the scattering and perpendicular to the scattering plane. An unprecedented total experimental resolution of 100 meV allowed the first X-ray observation of the CoO 4T1g(4F) manifold that occurs 120 meV above ground state. Detailed theoretical modeling was performed to assess the tetragonal crystal field splitting, spin–orbit, and superexchange parameters for both the single crystal and the nanocrystals. We show that 2p XAS is mainly sensitive to the octahedral field and 3d spin–orbit coupling, while the 4T1g(4F) manifold in 2p3d RXES probes the tetragonal distortion and the superexchange interactions with high sensitivity. We observe that the nanocrystals have a reduced cubic crystal field splitting and a broadened 4T1g(4F) RXES manifold that we ascribe to a larger average Co–O distance and an increased magnetic exchange in the nanocrystals. We demonstrate that further improvement of the RXES experimental resolution would not only allow for better disentanglement of the tetragonal distortion, spin–orbit, and superexchange interactions, but it would also allow observing anti-Stokes features in the RXES spectrum.
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The local structures of transition-metal dopant atoms in semiconductor host lattices of dilute magnetic semiconductors are determined using resonant inelastic x-ray scattering. Ligand field multiplet calculations are employed to analyze experimental data and show when dopant atoms substitute into the lattice and when they form metallic or oxide clusters. For ZnO doped with Mn, low-temperature ferromagnetism is shown to be intrinsic, but can be quenched by the formation of MnO clusters. For CeO 2 doped with Co, room-temperature ferromagnetism is due to clusters of Co, while substitution into the lattice leads to paramagnetic behavior.
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We demonstrate that magnetic properties of ultra-thin Co films adjacent to Gd2O3 gate oxides can be directly manipulated by voltage. The Co films can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to an oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. Consequently, an unprecedentedly large change of magnetic anisotropy energy up to 0.73 erg/cm2 has been realized in a nonvolatile manner using gate voltages of only a few volts. These results open a new route to achieve ultra-low energy magnetization manipulation in spintronic devices.
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The Co 2p3/2 X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate K5H[CoW12O40]·xH2O were measured. The low-energy dd transition features at 0.55 eV, unmeasurable with ultraviolet-visible (UV/vis) spectroscopy, were experimentally revealed in 2p3d RIXS spectra. RIXS simulations based on ligand-field multiplet theory were performed to assess the potential cobalt tetragonal symmetry distortion, which is described with the ligand-field parameters 10Dq (-0.54 eV), Ds (-0.08 eV), and Dt (0.005 eV). Because 2p3d RIXS probes not only the optical spin-allowed transitions but also the spin-forbidden transitions, we show that the current 2p3d RIXS simulation enables a series of dd feature assignments with higher accuracy than those from previous optical data. Furthermore, by wave-function decomposition analyses, we demonstrate the more realistic and detailed origins of a few lowest dd transitions using both one-electron-orbital and term-symbol descriptions.
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Magneto-transport study on the photolithographically-patterned anti-dot micro-arrays of cobalt upper-layer over a uniform thin nickel under-layer has been carried out. Circular anti-dots in rhomboidal symmetry were fabricated. The magnetoresistance (MR) curves were recorded for magnetically-easy and -hard axes of the bilayer structure at various temperatures. In addition to anisotropic MR, at low temperatures, the MR data shows anomalous behavior with several kinks and plateau regions, indicating complex reversal phenomena. Further, decrease in temperature results in increase of MR average peak height, double splitting of MR peaks and broadening of MR peaks. Additionally, the exchange bias behavior was found in the transverse MR curve; while no significant exchange effect was seen in the longitudinal MR curve. Using energy dispersive spectrum and X-ray absorption spectra taken at Co L-3,L-2-edge, the presence of CoO layer over the bilayer structure was detected; while the Ni under-layer remained deoxidized.
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Different layer thicknesses of Cobalt ranging from 2.6 {\AA} (1.5 ML) up to 55 {\AA} (30.5 ML) deposited on ferroelectric BaTiO$_3$ have been studied regarding their magnetic behavior. The layers have been characterized using XMCD spectroscopy at remanent magnetization. After careful data analysis the magnetic moments of the Cobalt could be determined using the sum rule formalism. There is a sudden and abrupt onset in magnetism starting at thicknesses of 9 {\AA} (5 ML) of Cobalt for measurements at 120 K and of 10 {\AA} (5.5 ML) if measured at room temperature. Initial island growth and subsequent coalescence of Co on BaTiO$_3$ is suggested to explain the sudden onset. In that context, no magnetically dead layers are observed.
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The electronic structure of material plays an important role in its functionality for different application which can be probed with synchrotron base spectroscopy technique. Here, various cobalt based compounds differ in crystal structure, ligands surrounding the central metal ion and morphology have been studied with soft X-ray absorption spectroscopy at the Co L - edge in order to measure the effect of these parameters on the electronic structure. A careful qualitative analysis of the spectral branching ratio and relative intensities of L3 and L2 peaks provide useful insight into the electronic properties of compounds such as CoO/Co(OH)2, CoCl2.6H2O / CoF2.4H2O, CoCl2 / CoF2, Co3O4 (bulk / nano / micro). For further detail analysis of the XAS spectra, quantitative analysis has been performed by fitting the spectral profile with simulated spectra for a single set of Co compounds (CoF2 and CoCl2) using crystal field atomic multiplet calculation.
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The mixed electronic ionic conducting materials (Ba0.5Sr0.5) (Co0.8Fe0.2)1-xNbxO3-δ with partial Nb substitution (x: 0.05, 0.10, 0.15, 0.20) for B cations (Co/Fe), synthesized using a solid state reaction method, have been studied by near edge X-ray absorption fine structure (NEXAFS). Co L2,3- absorption spectra of (Ba0.5Sr0.5) (Co0.8Fe0.2)1-xNbxO3-δ (BSCFN) powders were analyzed with the purpose to understand the influence of the Nb substitution on the atomic electronic structure of BSCFN. The joint analysis of the Co L2,3- absorption spectra reveals the presence of mixed oxidation states Co2+/Co3+ in all the studied BSCFN structures. It was established that the proportion of oxidation states Co2+/Co3+ and the corresponding coordinations of Co atoms depend on the content of Nb. In the 10% Nb substituted BSCF sample Co atoms mostly occur in the Co2+ oxidation state and are preferentially characterized by an octahedral coordination site. In all other structures Co atoms are rather characterized by Co2+/Co3+ oxidation states and occupy both octahedrally and tetrahedrally coordinated sites.
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Herein, we show the synthesis of high-capacity anode, InFeCoO4 spinel for lithium ion batteries (LIBs), by facile glycine-assisted chemical approach. The structure and morphology are evaluated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM) techniques, respectively. The pure phase formation of spinel InFeCoO4 is confirmed from XRD pattern, whereas the oxidation state of Co in 2+ is determined from XAS analysis. Electrochemical performance of InFeCoO4 in the half-cell configuration is evaluated by galvanostatic and cyclic voltammetry (CV) in the voltage window of 0.005–3.0 V vs. Li. When cycled at 60 mA g−1, it shows a high first cycle reversible capacity of 750 (±10) mA h g−1. However, slow capacity degradation is noticed upon cycling and reached 285 (±10) mA h g−1 after 40 cycles. An improved Li-storage performance is noticed under similar cycling condition, when the electrode is heat-treated. It shows first cycle reversible capacity of 880 (±10) mA h g−1 and reached 535 (±10) mA h g−1 after 40 cycles. The coulombic efficiency is >98 % during cycling. The improved Li-storage performance is possibly due to the distribution of PVDF (binder) in the active materials as well as better electrical contact after heat treatment.
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Cobalt-based catalysts are widely used to produce liquid fuels through the Fischer-Tropsch (FT) reaction. However, the cobalt nanocatalysts can exhibit intriguing size-dependent activity whose origin remains heavily debated. To shed light on this issue, the electronic structures of cobalt nanoparticles with size ranging from 4 to 10 nm are studied using soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopies. The RIXS measurements reveal the significant size-dependent d-d excitations, from which we determine that the crystal-field splitting energy 10Dq changes from 0.6 to 0.9 eV when the particle size is reduced from 10 to 4 nm. The finding that larger Co nanoparticles have smaller 10Dq value is further confirmed by the Co L-edge RIXS simulations with atomic multiplet code. Our RIXS results demonstrate a stronger Co-O bond in smaller Co nanoparticles, which brings in further insight into their size-dependent catalytic performance.
Article
The effect of Sn codoping on the local Co structural environments of the (In0.95-xSnxCo0.05)2O3 (x = 0, 0.02, 0.05, 0.08) films has been investigated in detail by x-ray absorption spectroscopy at Co K-edge and L-edge. The results show that the Sn codoping can remarkably influence the local Co structures of the films. For the films with low Sn concentration (x ≤ 0.02), most Co²⁺ atoms substitute for In³⁺ sites of In2O3 lattices, while a part of Co atoms form the precipitate of Co metal clusters. With further increasing Sn concentration (x > 0.02), the doped Co atoms are completely substitutionally incorporated into the In2O3 lattices. It can be concluded that the codoping of Co and Sn atoms forms p-n pairs of electronics and holes with opposite charge state, which can activate the substitution of Co atoms in In2O3 lattice and suppress the forming of metallic Co clusters. The p-n codoping method can provide a powerful guiding principle in designing the oxides based diluted magnetic semiconductors.
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In-situ carbon-thermal reduction of cobalt oxide nanoparticles supported on carbon nanotubes was studied by cobalt 2p3d resonant inelastic X-ray scattering (RIXS). The in-situ 2p X-ray absorption spectroscopy (XAS) and RIXS measurements were performed at 500oC, 600oC and 700oC, where four consistent excitations energies were used for RIXS acquisitions. After 700oC reduction, the XAS spectrum shows a cobalt metal-like shape, while the RIXS spectra reveal the minority cobalt monoxide phase. The holistic fit on both XAS and RIXS data reveals the respective contributions from metal and monoxide. We show that the relative precision to determine the monoxide content changes from ~5.6% in XAS results to better than 0.8% in the RIXS analysis, suggesting that RIXS is a useful tool to track the oxidation state of nanoparticles under in-situ conditions. We determined a relative radiative ratio (Ρ) factor of approximately 5, where this factor gives the ratio between the relative strengths of the radiative decay channels compared to the non-radiative channels in CoO and Co metal.
Article
A method for measuring resonant inelastic X-ray scattering based on the conversion of X-ray photons into photoelectrons is presented. The setup is compact, relies on commercially available detectors, and offers significant flexibility. This method is demonstrated at the Linac Coherent Light Source with ∼0.5eV resolution at the cobalt L3-edge, with signal rates comparable with traditional grating spectrometers.A description of the concept and first test of a photoelectron-based method for measuring inelastic X-ray scattering is given.
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We show that with 2p3d resonant inelastic X-ray scattering (RIXS) we can accurately determine the charge transfer parameters of CoF2, CoCl2, CoBr2 and CoS. The 160 meV resolution RIXS results are compared with charge transfer multiplet calculations. The improved resolution and the direct observation of the crystal field and charge transfer excitations allows the determination of more accurate parameters as could be derived from x-ray absorption and x-ray photoemission, both limited in resolution by their life time broadening. We derive the crystal field and charge transfer parameters of the Co²⁺ ions, which provides the nature of the ground state of the Co²⁺ ions with respect to symmetry and hybridization. In addition the increased spectral resolution allows the more accurate determination of the atomic Slater integrals. The results show that the crystal field energy decreases with increasing ligand covalency. The L2 edge RIXS spectra show that the intensity of the (Coster-Kronig induced) non-resonant x-ray emission is a measure of ligand covalency.
Article
Two isostructural cobalt containing polyoxometalate water oxidation catalysts, [Co4(H2O)2(α-PW9O34)2]¹⁰⁻ (Co4P2) and [Co4(H2O)2(α-VW9O34)2]¹⁰⁻ (Co4V2), exhibit large differences in their catalytic performance. The substitution of phosphorus centers in Co4P2 with redox-active vanadium centers in Co4V2 leads to electronic structure modifications. Evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS). The XAS and RIXS spectra determine the ground state electronic structure of the cobalt and vanadium sites in both Co4P2 and Co4V2. High-energy resolution RIXS results reveal that Co4P2 possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band. The differences in electronic structures offer insights into the enhanced catalysis of Co4P2.
Article
A theoretical framework is developed for better understanding the time-dependent soft-x-ray response of dissipative quantum many-body systems. It is shown how x-ray absorption and resonant inelastic x-ray scattering (RIXS) at transition-metal L edges can provide insight into ultrafast intersystem crossings of importance for energy conversion, ultrafast magnetism, and catalysis. The photoinduced doublet-to-quartet spin crossover on cobalt in Fe-Co Prussian blue analogs is used as a model system to demonstrate how the x-ray response is affected by the nonequilibrium dynamics on a femtosecond time scale. Changes in local spin and symmetry and the underlying mechanism are reflected in strong broadenings, a collapse of clear selection rules during the intersystem crossing, fluctuations in the isotropic branching ratio in x-ray absorption, crystal-field collapse and/or oscillations, and time-dependent anti-Stokes processes in RIXS.
Article
The valence states, the distribution of Co ions, and defect structures in the Co-doped ITO films with Co concentrations of 5–13 at.% were examined by X-ray absorption spectroscopy (XAS) at Co, K, and L-edges. The structural analyses and ab initio calculations reveal that the Co atoms are substantially incorporated into the ITO lattice and form cobalt–vacancy complexes, while partial formation of Co⁰ species is observed for all the films. The analyses of Co–K edge XAS reveal that the Co–O bond length RCo–O is shortened and the corresponding Debye–Waller factor (σ²) obviously increases with Co doping, implying the relaxation of oxygen environment around the substitutional Co ions. The qualitative fitting of Co L3-edge XAS further confirms the coexistence of Co⁰ and Co²⁺ in the films. The Co atoms mainly occupy the substitutional sites of In2O3 lattices with the metallic Co clusters being about 20–43 at.% for the 5, 7, and 8.5 at.% Co-doped ITO films. However, a significant fraction (∼57 at.%) of metallic Co clusters is found in the 13 at.% Co-doped ITO film.
Article
We have observed that the Rh substitution for Co in LaCo0.5Rh0.5O3 leads to the spectral feature at the Co−L2,3 absorption edge taken at 300 K similar to that in LaCoO3 taken at 650 K, in which the spin state of magnetic Co3+ ions has been controversially discussed in the past decades and can be easily clarified by studying LaCo0.5Rh0.5O3 without worrying about oxygen loss induced at high temperature. Our combined experimental and theoretical x-ray absorption spectroscopy (XAS) at the Co−L2,3 and the experimental Rh−L2,3 edges indicated a nearly 1:1 mixture of high-spin (HS) and low-spin (LS) Co3+(3d6) and a LS Rh3+(4d6) in contrast to the Co2+/Rh4+ state found in Ca3CoRhO6 at room temperature. Upon cooling only a small portion of the HS Co3+ ions was converted to a LS state until 10 K in LaCo0.5Rh0.5O3. The Co-Kβ x-ray emission spectra revealed a gradual spin-state transition from a mixed LS/HS at ambient pressure to a complete LS state of Co3+ ions up to 14 GPa. The theoretical and experimental intensity ratio I(L3)/I(L2) on the Co−L2,3 edges and a comparison between the difference spectrum of Kβ x-ray emission of LaCo0.5Rh0.5O3 taken at ambient pressure (AP) and 14 GPa and that of Sr2CoRuO6 taken at AP and 39.6 GPa exclude the intermediate spin state of Co3+ in LaCo0.5Rh0.5O3.
Article
We report on the electronic structure of cobalt(II) tris-2,2′-bipyridine and cobalt(III) tris-2,2′-bipyridine in aqueous solution using resonant inelastic X-ray scattering (RIXS) spectroscopy at the Co L-edge and N K-edge resonances. Partial fluorescence yield X-ray absorption spectra at both edges were obtained by signal integration of the respective RIXS spectra. Experiments are complemented by calculations of the X-ray absorption spectra for high- and low-spin configurations using density functional theory/restricted open-shell configuration interaction singles and time-dependent density functional theory methods. We find that linear combinations of the simulated X-ray absorption spectra for different spin states reproduce the experimental spectra. Best agreement is obtained for measurements at the Co L-edge, for both samples. For cobalt(II) tris-2,2′-bipyridine, our combined experimental and computational study reveals 40% low-spin and 60% high-spin state components. Much stronger low-spin character is found for cobalt(III) tris-2,2′-bipyridine, 80% low spin and 20% high spin. Prominent energy-loss features in the Co RIXS spectra are indicative of d-d excitations and charge-transfer excitations due to strong mixing between metal and ligand orbitals in both complexes. Analysis of N 1s RIXS data reveals the emission from metal dominated orbitals in the valence region, supporting the strong metal-ligand mixing.
Article
In my work, magnetic and structural transitions of three categories of compounds are investigated by density functional calculations under local spin density approximation (LSDA). The first compound is Rh2MnGe with full Heusler structure at ambient condition. However, the structure is unstable at T=0 according to our calculations. A more stable structure we found is tetragonal one with either extension or compression along c-axis. The electronic reason of this distortion is the band Jahn-Teller effect where the Jahn-Teller active states are 4d states of Rh which is accidently put at the Fermi level by spin splitting. Then, magnetic moment behavior under pressure in itinerant compounds is investigated in four cubic Laves phase compounds (YFe2, ZrFe2, HfFe2, and LuFe2). The magnetic spin moment is decreased under pressure. A magnetic collapse where the spin moment vanishes is predicted under pressure around 20 GPa for Zr and Hf compounds, 40 GPa for Y and Lu compounds. The behavior of the magnetic moment is the result of competition between magnetic exchange interactions and kinetic energy during the compression of the volume, as described by the Stoner model. The last material investigated is CoO using LDSA+U in order to describe the strong Coulombic interaction of the transition metal ion. The pressure induced a magnetic transition, which was discovered in experiments, is explained by the competition between ligand field splitting and exchange energy. The ligand field splitting is increased under pressure, and suppresses the intraatomic exchange. As a result, the spin state changes from high spin to low spin, and at last to nonmagnetic state.
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We have performed resonance photoemission, angle-resolved photoemission, and core-level photoemission studies of single-crystalline CoO. On the one hand, strong correlation effects among the {ital d} electrons are observed, as signaled by a strong reduction of Co 3{ital d} bandwidths and satellites in both the valence band and the cation core levels. On the other hand, the oxygen states are found to be very bandlike, as indicated by strong dispersions of oxygen states in the valence band and the lack of oxygen satellites. We give estimations of 30 and 1.5 for {ital U}/{ital W} (Coulomb interaction divided by bandwidth) ratio of Co 3{ital d} bands and O 2{ital p} bands, respectively. By comparing the experimental and theoretical {ital E} versus {bold k} relation, we show that the density-functional band calculation works well for the oxygen bands but not for the Co bands. We argue that CoO is not a band insulator, but a charge-transfer insulator. We have also observed the effects of local magnetic order on the electronic structure. Finally, we suggest a guideline on calculating the band structure of CoO: introducing a mechanism that reduces the Co 3{ital d} bandwidth by 25% while still retaining the other essential features of the band calculation.
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The electronic structure of LixCo1-xO (0.01≤x≤0.2), LiCoO2, and Co3O4(1% Li) has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and x-ray-absorption spectroscopy. The experimental results are compared with model cluster calculations. We find that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like. The first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character. Its local symmetry corresponds to 3T1g, similar to an intermediate-spin Co3+ state. For x≤0.2 the local Co electronic structure is similar to that of CoO. However, LiCoO2 has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a Co3+ low-spin ground state. LiCoO2 is an insulator with a gap of 2.7 eV. From a comparison of the XPS and BIS CoO spectra to the cluster calculations, we find values for U (=5.3 eV), Δ (=5.5 eV), and (pdσ) (=1.3 eV).
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X-ray fluorescence spectroscopy with monochromatic photon excitation is presented as a tool for studies of charge-transfer excitations in correlated systems, using CeO2 and UO3 as examples. Ce 4f-->3d and U 5f-->3d x-ray fluorescence, with excitation near the 3d thresholds, probes states as eigenvalues for the ground state Hamiltonian from the Anderson impurity model. Sweeping the excitation energy across 3d absorption edges enhances contributions of different electronic configurations to fluorescence so that observed resonances indicate the charge-transfer origin of the absorption satellites.
Article
We investigate the Co 2p core-level X-ray absorption (XAS) and photoemission spectra (XPS) in CoO on the basis of a CoO6 cluster model, where Co 3d-O 2p hybridization and multiplet coupling are fully taken into account. We pay particular attention to the effects of multiplet coupling on the XPS and those of Co 3d-O 2p hybridization on the XAS, because those points have been disregarded in the analyses made so far by many authors. From the combined analysis, we show that the XPS and XAS provide us with complementary information on the electronic states of CoO.
Article
Intra-atomic d-d transitions in NiO(100) and CoO(100) have been investigated with angle-resolved electron energy loss spectroscopy (EELS) at primary energies close to the metal 3s excitation threshold. Electron exchange scattering was found to be resonantly enhanced at the 3s threshold due to the temporary formation of a negative ion core state and its subsequent decay via Auger-like transitions. In both oxides the threshold is lowered several eV due to a strong electron- core hole interaction. Angle-dependent studies reveal a different dependency of exchange processes on the scattering angle as compared with nonresonant measurements and reveal a different angle dependence for each specific d-d transition. It is suggested that in these oxides large-angle single-step inelastic scattering contributes significantly to the EELS measurements in reflection mode.
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The photoemission process and principles are briefly reviewed. The electron spectrometer, the specimen preparation chamber and, in particular, the associated ultra-high vacuum system and the technique for obtaining the ultimate vacuum, as well as the in situ preparation techniques, are described in detail. Some applications and their results are given.
Article
The oxygen 1s and cobalt 2p X-ray absorption spectra of CoO, Li-doped CoO and LiCoO2 have been measured with 0.1 eV resolution. The cobalt 2p spectra are analysed with a ligand-field multiplet model and the inclusion of chase-transfer effects is discussed. The oxygen 1s spectra are interpreted as transitions to empty oxygen p states and it is concluded that the effects of correlations in the 3d band possibly are too small to be detectable. The symmetries and the electronic configurations of the cobalt ions in the oxides are determined. It is concluded that, in contrast to for example NiO and La2CuO4, the doping-induced states are possibly of 1A1 symmetry, which would imply that the quasi-particles have spin 3/2 and are most likely trapped.
Article
The optical absorption of single-crystal CoO and MnO both above and below the Néel temperature is reported and interpreted. The absorption spectrum in each case is similar to that found in dilute paramagnetic salts of the same ions. Tanabe and Sugano's strong-field theory was used to interpret the data. In CoO the tetragonal distortion of the cubic lattice present in the antiferromagnetic phase was experimentally observed. An estimate of the strength of the tetragonal field is given. It is also found that the temperature dependence of the absorption coefficient in CoO is in accord with a second order electric dipole-phonon transition mechanism.
Article
The relationship between the magnetic state of divalent Co and Fe ions in the octahedral crystal field and the isotropic L2,3 X-ray absorption spectroscopy (XAS) spectrum is discussed with switching on and off the 3d spin-orbit interaction on the basis of an atomic multiplet calculation. The existence of orbital magnetic moment Morb of ˜1µ B is shown to manifest especially in L2 region as a narrow spectrum compared with that for Morb = 0 for both ions.A deviation of L3: L2 braching ratio from the statistical ratio 2 is also shown to be closely related with the existence of Morb for both ions. In the light of our findings, the magnetic state of both ions in CoO and FeO is discussed. The role of isotropic core XAS as a means to determine Morb in systems with no net magnetization is stressed.
Article
Valence-band photoemission studies have been performed on a CoO(100) single crystal as it is slowly oxidized under 1×10-4 Torr O2 at 623 K, eventually forming a Co3O4 epitaxial film. The most significant changes occur in 3d-related features, with the peak located at the top of the valence band sharpening and shifting to lower binding energies as the spinel oxide forms. Constant initial-state measurements indicate that Co3O4 contains admixture from neighboring O 2p in its 3d band, as observed for CoO and other monoxide charge-transfer insulators. Unlike the rocksalt monoxides, which have only a single type of cobalt (Co2+) located in octahedral lattice sites, the spinel Co3O4 has both octahedral Co3+ and tetrahedral Co2+ sublattices. The peak at the top of the Co3O4 valence band results from the 3d6L final state of the octahedral Co3+ 3d band. Although some states derived from tetrahedral Co2+ may be present at the top of the valence band, the greatest contribution from the tetrahedral Co2+ sublattice appears at approximately -3.8 eV, overlapping with O 2p derived features of the spectrum. The Co3+ 3dn-1 satellite is much less intense in Co3O4 than in CoO, as is observed for the analogous structure in the cobalt 2p core spectra. An oxygen 2p-derived structure remains fairly constant throughout the oxidation process, with the exception of an intermediate species, which imparts a broad, humplike appearance centered at -5.3 eV to the O 2p region and disappears as oxidation to Co3O44 is completed. The origin of the feature is not clear; however it is most likely due either to an adsorbate or to a defectlike intermediate in the oxidation process.
Article
Total and partial atomic-level widths of the K-, L-, M-, and N-levels of the elements (covering Z up to 120 for K- and L-levels) are presented in graphic form. Graphs are based on theoretical predictions; total widths are natural level widths as governed by the rates of the various deexcitation processes for a single hole in that level. The drawings make possible, throughout the periodic table, a quick survey of the magnitude and trends of the total level widths, rates and yields of radiative, Auger, and Coster-Kronig processes,and widths of x-ray, Auger, and Coster-Kronig lines.
Article
Electron-exchange processes in dipole-forbidden d-d excitations of the 3d transition-metal oxides NiO, CoO, and MnO have been investigated by spin-polarized electron-energy-loss spectroscopy over a wide primary energy range and for different scattering geometries. At primary energies corresponding to certain excitation thresholds, the d-d excitations in CoO and MnO show resonant enhancement, similar to the resonances previously found for NiO. The resonance around 38 eV, which is assigned to simultaneous excitations within the d-multiplet and oxygen excitations, occurs at nearly identical primary energy, due to identical energetic positions of the oxygen levels in the three oxides. The metal 3s-3d resonance is shifted toward lower energy in CoO and MnO, according to the lower binding energies of the 3s levels. In resonance, the d-d excitations are highly exchange dominated. Off resonance, a strong superposition with direct dipole-scattering processes has been found for the dominant multiplicity-conserving transitions in NiO and CoO. Excitation by direct dipole scattering is completely missing in the spin-forbidden, multiplicity-changing transitions in MnO.
Article
The interatomic coupling of magnetic moments in the insulating antiferromagnetic transition-metal compounds MnO, MnS, and NiO is calculated using a theory based on the itinerant-electron picture (energy-band theory and the local-spin-density treatment of exchange and correlation). Calculated values of these "Heisenberg" coupling constants agree with measured values to the extent that can be expected in light of the approximations required to execute the calculations. The calculations emphasize the importance of covalent interactions between the metal d states and the anion p states. These interactions are spin conserving and fundamentally nonmagnetic; they enter the coupling of the magnetic moments because the intra-atomic spin splitting of the metal d shell makes the covalent interactions dependent on the relative angle of the two magnetic moments.
Article
The hopping-matrix elements Vm(ɛ) between the conduction states with energy ɛ and the different crystal-field-split 4f states m are calculated for CeCu2Si2. With use of the Anderson impurity model, the effect of this m dependence on the static T=0 K susceptibility is studied. We also take into account that the one-particle hopping-matrix elements entering in the f0→f1 and the f1→f2 hopping processes are different (configuration dependence). It is concluded that for CeCu2Si2 larger hopping-matrix elements are needed for the description of thermodynamic properties than for spectroscopic properties. These results are consistent with renormalization effects, which the Coulomb interaction between the 4f and conduction electrons is expected to cause.
Article
The augmented-plane-wave (APW) method is applied to calculate the nonmagnetic band structures for the 3d transition-series monoxides CaO, TiO, VO, MnO, FeO, CoO, and NiO, all of which form with the rocksalt structure. The APW energy-band results at seven symmetry points in the fcc Brillouin zone are fitted with the Slater and Koster linear-combination-of-atomic-orbitals (LCAO) interpolation method involving nonorthogonal orbitals. A nonlinear least-squares fitting procedure is applied to determine 16 two-center energy and overlap LCAO parameters which characterize the oxygen 2s-2p and transition-metal 3d energy bands and their interactions. It is found that both the APW results and the corresponding LCAO parameters exhibit systematic variations across this series of compounds. These calculations predict that: (a) CaO is an insulator with a 10-eV band gap; (b) the metallic oxides TiO and VO have 3d bandwidths of about 0.5 Ry, which are two and a half times larger than those for the antiferromagnetic insulators MnO, FeO, CoO, and NiO; (c) the energy of the 3d band decreases systematically (relative to the bottom of the transition-metal 4s-4p band) as the nuclear charge is increased, causing the metal s-d bands to overlap for the early but not the later members of this series.
The optical absorption associated with charge carriers in NiO and CoO has been obtained from transmission measurements on pure and Li-doped crystals in the wavelength region 05 to 700 μm, at temperatures between 100 and 500°K. The absorption increases smoothly with frequency to a maximum near 1 ev and then shows a further increase extending to 2 ev and higher. In the region from 05 to about 4 μm the absorption increases slowly with temperature and correlates with the Li concentration. In the far infra-red the absorption σFI is independent of frequency and, for T <or= 300-400 °K, σFI > σdc. The activation energy associated with σFI is smaller than the d.c. value. The behaviour between 1 and 700 μm is very similar to that predicted by the theories of Klinger, Reik et al. and Bogomolov et al. for non-diagonal (hopping) transitions by small polarons. Similar absorption in TiO2 and other 3d oxides between 1 and 10 μm has been attributed to a hopping component in the free-carrier mobility. In NiO and CoO, however, we conclude that the absorption is primarily due to small-radius polarons hopping around Li centres with high-temperature activation energies Ea of 02-025 ev and 03-04 ev respectively. Above 400°K in NiO, σFI -> σdc and electrical studies indicate that about 20-30% of the carriers are ionized. No new features appear in the absorption but the above Ea values give an upper limit for the hopping energy of any non-diagonal component in the d.c. mobility. We infer a relaxation time of less than 10-13s for the diagonal component implying a bandwidth of at least 5 mev. Recent d.c. mobility data support a band picture, but a closer analysis indicates that an activated mobility with Ea less, similar 02 ev cannot be excluded.
Article
Electron-exchange processes in excitations within the d-multiplet of the TM-ions in NiO〈100〉 and CoO〈100〉 had been examined by spin-polarized electron energy-loss spectroscopy. The excitations at surface and bulk ions in NiO show a completely different scattering geometry dependence, providing the possibility of distinguishing between both. In addition to the well-known 0.6 eV surface state, the loss-structure at 2.1 eV excitation energy was identified as surface excitation. In CoO, no surface d-d transitions have been found. Different spin-flip intensities in the excitations of the bulk Co-ions had been used to identify multiplicity changing and conserving transitions definitely. By use of the previously found resonance primary energies and suitable scattering geometries, new d-d excitations had been measured in NiO as well as in CoO.
Article
Wenn ein Atom äußerer monochromatischer Strahlung von der Frequenz ν ausgesetzt ist, sendet es nicht nur sekundäre monochromatische Kugelwellen von der Frequenz ν aus, die mit der einfallenden Strahlung kohärent sind, sondern das Korrespondenzprinzip verlangt, daß im allgemeinen auch noch Kugelwellen von anderen Frequenzen ausgesandt werden. Diese Frequenzen sind alle von der Form ∣ν±ν*∣, wo hν* den Energieunterschied des Atoms im betrachteten und in irgend einem anderen Zustand bezeichnet. Die nicht kohärente Streustrahlung entspricht zum Teil gewissen Prozessen, die kürzlich von Smekal ins Auge gefaßt wurden anläßlich Betrachtungen, die an die Vorstellung von Lichtquanten anknüpfen. In der Abhandlung wird gezeigt, wie sich eine wellentheoretische Analyse der streuenden Wirkung des Atoms an der Hand des Korrespondenzprinzips m ungezwungener und scheinbar eindeutiger Weise durchführen läßt. Die Ausführungen bauen durchaus weiter auf der Auffassung der Verbindung der Wellenstrahlung des Atoms mit den stationären Zuständen, die in einer neuen Arbeit von Bohr, Kramers und Slater vertreten ist, und die Folgerungen, wenn sie sich bestätigen sollten, dürften eine interessante Stütze für diese Auffassung bilden.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced. © 1999 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoemission spectra; Fine structure; Core line spectra; One-electron removal valence band spectra.
Article
The total density of occupied states in the valence band of CoO and Co3O4 is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the dn −1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co3O4. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co3O4. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.
Article
The conditions necessary in metals for the presence or absence of localized moments on solute ions containing inner shell electrons are analyzed. A self-consistent Hartree-Fock treatment shows that there is a sharp transition between the magnetic state and the nonmagnetic state, depending on the density of states of free electrons, the $s$-${}d$ admixture matrix elements, and the Coulomb correlation integral in the $d$ shell; that in the magnetic state the $d$ polarization can be reduced rather severely to nonintegral values, without appreciable free electron polarization because of a compensation effect; and that in the nonmagnetic state the virtual localized $d$ level tends to lie near the Fermi surface. It is emphasized that the condition for the magnetic state depends on the Coulomb (i.e., exchange self-energy) integral, and that the usual type of exchange alone is not large enough in $d$-shell ions to allow magnetic moments to be present. We show that the susceptibility and specific heat due to the inner shell electrons show strongly contrasting behavior even in the nonmagnetic state. A calculation including degenerate $d$ orbitals and $d$-${}d$ exchange shows that the orbital angular momentum can be quenched, even when localized spin moments exist, and even on an isolated magnetic atom, by kinetic energy effects.
Article
We measured the Mn {ital L}{sub {alpha},{beta}} x-ray fluorescence spectra of MnO excited by selected photon energies near the {ital L}{sub 2,3} absorption edges. The resulting resonant inelastic x-ray scattering spectra probe low-lying electronic excited states, due to {ital dd} and charge-transfer excitations. Using a two-step model and a purely atomic approximation, we reproduce both energies and varying intensities of {ital dd} excitations relative to the electronic recombination peak. Our results show that strongly varying line shapes in resonant x-ray emission need not be due to channel interference effects. {copyright} {ital 1996 The American Physical Society.}
Article
We study the renormalization of hopping integrals between localized and delocalized levels due to a Coulomb interaction between these two types of levels. We find that the renormalization enhances the effective hopping matrix elements, due to effects of the type that enhance x-ray spectra at the threshold. This effect tends to make the hopping integrals appear larger for thermodynamic properties than for spectroscopic properties.
Article
A method to determine the absorption coefficient near the onset of core-electron transitions for concentrated samples using fluorescence-yield (FY) detection is presented. Measuring the FY signal for different experimental geometries, we are able to calculate the true absorption coefficient. Thus we are able to correct fully for saturation effects present in FY spectra of concentrated samples. The technique is demonstrated for Co and a buried layer of CoSi2.
Article
The localized character of the 3d electrons in the antiferromagnetic oxides NiO(100) and CoO(100) has been studied with electron energy-loss spectroscopy at primary energies between 20 and 1200 eV. The spectra of both charge-transfer compounds exhibit weak but sharp loss structures within the insulating gap region due to crystal-field excitations of the 3dn configuration (dn-->dn*). These parity forbidden d-d excitations are strongly enhanced at low primary energies due to exchange scattering. Their observed loss intensity depends on the momentum transfer at the scattering process as is shown by angle-dependent measurements. At NiO(100) a surface d state is found that is due to the lower symmetry of the ligand field at the surface. The measurements demonstrate the existence of spin-forbidden exchange excitations 3A2g-->(1Eg,1T1g) in NiO(100) and reveal their excitation energies. A resonant enhancement of all intra-atomic d-d transitions in NiO(100) is found at the Ni 3s excitation threshold. Temperature-dependent measurements up to the Néel point show only little influence of the antiferromagnetic ordering on the EELS spectra of NiO(100). The different shapes of the absorption edges due to interatomic d-d transitions across the insulating gaps of NiO(100) and CoO(100) are discussed.
Article
We have performed angle-resolved photoemission experiments and density-functional band calculations for NiO and CoO. The measured oxygen bands are in excellent agreement with the calculation, but the measured cation 3{ital d} dispersion is only about 25% of the calculated one. We have also demonstrated the effects of the antiferromagnetic order on the electronic structure of these materials.
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