Edge X-ray Absorption Spectroscopy Spectra Research Articles

Texture, electronic and magnetic states of Mn atoms in polycrystalline MnSi thin films facing a c-sapphire substrate studied by soft X-ray magnetic circular dichroism

Polycrystalline MnSi is a highly efficient magnetic skyrmionic material and information carriers for future spintronic devices, deposited on a c-cut sapphire substrate by magnetron sputtering with a low base pressure. Due to their fascinating crystal structures and spin textures these Mn-based silicides are most extensively studied now a days. In order to study the appearance of any impure phases like MnxSiy, hysteresis behavior and the inception of ferromagnetic/superparamagnetic domains present in the MnSi film, we have performed X-ray diffraction (XRD), XRD pole figures in stereographic projection, vibrating sample magnetometry and powerful element specific X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques. The room temperature hysteresis curve points towards the phenomenon of long-range ferromagnetic ordering. The Mn L2,3 edge XAS spectra did not show a Mn2+-like multiplet structure. The overall magnetization increases with the increase in applied magnetic field from 0.1 T to 2 T on MnSi thin film. The observed spin and orbital magnetic moments ∼ 0.03 ̶ 0.1 μB and 0.01 to 0.04 μB, respectively.

Effect of substrate on interfacial electronic properties of ferrocene thin films

Ferrocene molecular thin films on conducting substrates have potential for novel molecular electronic devices. Based on X-ray absorption spectroscopy and photoemission spectroscopic techniques, we have analyzed electronic properties of ferrocene thin films nano-junctions on layered highly oriented pyrolytic graphite (HOPG) and epitaxially grown graphene on Ni(1 1 1) substrates. Fe L edge analysis indicates that layered structure of HOPG has significant impact on room temperature desorption of ferrocene as compared to grapheme on Ni(1 1 1) substrate. C-K edge XAS spectrum reveal that ferrocene molecules do not have preferred orientation for adsorption on grapheme grown on Ni(1 1 1) substrate. In addition, X-ray absorption spectroscopy (XAS) reveals that there is no change in iron atom oxidation state suggesting that ferroence interaction is feeble with grapheme on Ni(1 1 1) substrate and HOPG.

Degradation Mechanism of Ni-Rich Layered Cathode Materials during High Voltage Cycling Revealed By Multi-Model Synchrotron X-Ray Imaging and Spectroscopy Techniques

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, these materials are suffering severe capacity fade caused by surface reconstruction, unstable cathode-electrolyte interface (CEI) formation and micro crack formation during cycling, especially when a high cut off charge voltage is applied (such as 4.6 and 4.8V). To understand the origin of this capacity fade, state-of-the-art diagnostic tools are applied to understand their structure, chemical and morphological properties of LiNi0.94Co0.06O2 (NC) and LiNi0.92Co0.06Al0.02O2 (NCA) cathode materials during cycling. Electrochemical measurement results show that the NCA cathode delivered higher capacity and stable cyclic performance even cycled at a high cut of voltage of 4.8V. In situ X-ray absorption spectroscopy (XAS) results of NC and NCA electrodes measured during the first and 51st charge/discharge cycling between the voltage range of 2.8-4.4V showed that both Ni and Co contribute to the capacity by going through the Ni3+/Ni4+ and Co3+/Co4+ redox couples. The Ni-K edge XAS spectra of NC cathode charged to 4.8V showed slightly reduction of Ni4+ during the high voltage charging. While, XAS spectra of NCA cathode did not showed reduction of Ni4+ when the cell charged to the higher voltage. More interestingly, C, F, O, Ni and Co soft XAS spectroscopy of NC and NCA electrodes at the different state of charge provided more comprehensive insight into surface and bulk chemical properties. The results showed that much thicker CEI layer formed on the NC surface compared with the NCA cathode, indicated that Al-doping effectively reduced the side reaction of cathode materials with the electrolyte. We have also compared the morphological and chemical evolution of NC and NCA secondary particles using full-filed in-situ transmission X-ray microscopy (TXM) during the initial cycle. Results will be presented at the meeting. Acknowledgement This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704.

Electronic structure investigation of a charge density wave coupled to a metal-to-metal transition in Ce3Co4Sn13

We study the electronic structure of the skutterudite ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$, which is known to exhibit a charge density wave (CDW) transition, at temperature ${T}_{CDW}\ensuremath{\sim}160$ K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$. The Co $2p$ and Sn $3p$ core level spectra show small but finite shifts in binding energy positions across ${T}_{CDW}$ while Ce $3d$ core level spectra do not show any change across the transition. The Ce ${M}_{4,5}$-edge XAS spectrum compared with calculations indicate a typical trivalent ionic ${\mathrm{Ce}}^{3+}$ spectral shape, ruling out Kondo screening in ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$. In contrast, the Co ${L}_{2,3}$-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic co-ordination. Temperature dependent XAS across the Co ${L}_{2,3}$-edge shows a small shift across ${T}_{CDW}$, consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across ${T}_{CDW}$, indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below ${T}_{CDW}$. The Co $3d$ states are observed at a binding energy of $\ensuremath{\sim}2$ eV while the Ce $4f$ states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$.

Electronic structure by X-ray absorption spectroscopy and observation of field induced unusually slow spin relaxation from magnetic properties in pyrochlore Eu2−xFexTi2O7

X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were investigated, where the rare earth Eu (4f) was replaced with transition metal Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the valence states of Eu and Fe ions are formally trivalent while that of Ti ions are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy octahedral sites with distorted Oh symmetry which is triggered by the presence of vacant 8a anionic site adjacent to TiO6 octahedra. Further study with Fe doping revealed that it essentially reduces the octahedral distortion by introducing anionic disorder (migration of 48f oxygen ions to 8a site). Analysis of O K edge XAS spectra further confirmed the Fe substitution causing the systematic change in the ligand (O2-) coordination of the Ti4+ cations. On the other hand, a new field induced transition (with Fe doping) at low temperature T* (4 K<T*< 8 K) in ac susceptibility with unusually slow spin relaxation was observed. The transition shifted towards higher temperatures both with increasing applied field and Fe concentration. However, the single ion spin freezing (Tf ~35 K) appears to be suppressed with Fe substitution. Interestingly, small amount of Fe3+ ion substitution showed significant enhancement in the dc magnetization at lower temperatures (<100 K). Analysis further indicated rise of dipolar FM exchange interaction with Fe doping.

Electronic structure studies of Fe doped Eu 2 O 3 thin film using x-ray absorption and resonant photoemission spectroscopy

Herein, we have investigated the electronic properties of Fe doped Eu2O3 thin film using x-ray absorption spectroscopy (XAS), valence band spectroscopy and resonant photoemission spectroscopy (RPES). The Fe L3, 2 edge XAS spectra confirms the “3+” ionic state of the Fe in the film. The valence band spectrum of Fe doped film shows an enhancement in the intensity of feature “A” close to Fermi level and origin of another feature “E” at 11.1 eV. The valence band spectroscopy measurements have been performed at the energies in the Eu 4d → 4f and Fe 3p → 3d photo absorption region that clearly disentangles the features in valence band to be of either Eu and Fe hybridized with O or to be of Eu and O only.

Inhomogeneous Degradation Phenomena of NMC Cathode in 18650-Type Lithium-Ion Battery: A Soft X-Ray Absorption Spectroscopic Investigation

IntroductionEstablishment of lifetime prediction methodology for Li-ion batteries (LIBs) is strongly required in order to prolong the lifetime of LIB and to use LIBs in safe. While “square root (SQRT) law” is proposed to predict the lifetime of LIBs [1] based on the formation of an irreversible solid electrolyte interphase (SEI) at anode surface and the electrochemically deactivation of cathode active materials, the SQRT law cannot describe the degradation behavior precisely in while period of LIB operation, especially near the end stage of degradation. In addition, detailed mechanism of degradation also remains unknown. Previously, we reported the detailed structure of SEIs in heavily deteriorated anodes by the Hard X-ray photoemission spectroscopy (HAXPES) and discuss the degradation mechanism focused on inhomogeneous SEI growth [2].In this study, we also investigated the detailed structure of surface in heavily deteriorated cathodes by the soft X-ray absorption spectroscopy (XAS) and then, we discuss the inhomogeneous degradation phenomena of cathode. ExperimentalA commercial 18650-type lithium ion battery cell, which consists of a graphite anode, Li(Ni1-x-y Mn x Co y )O2 cathode, and LiPF6 in a mixture solvent of EC, PC, EMC and DMC as electrolyte, was used in this study. The Cells were charged and discharged with 1C rate (0, 100, 200 and 500cycles) at 25ºC. After the cycle tests, the battery cells were disassembled in an argon glove box, in which the oxygen and water contents were maintained below 1 ppm. The electrodes were washed with highly purified DMC to remove the residual electrolyte components and then, the electrodes were vacuum-dried to evaporate the solvents. All samples were transferred to the analysis chamber by using a transfer vessel.The XAS measurements were carried out at SR Center of Ritsumeikan University on a grating beamline BL-11. We simultaneously obtained the XAS spectra in three different modes, a partial electron yield (PEY) mode, a total electron yield (TEY) mode and a fluorescence yield (FY), whose detection depth correspond to ~ 2 nm, ~ 20 nm and ~ 200 nm, respectively. Results and discussionFigure 1(a) shows the change of discharge capacity retention as a function of SQRT of the number of cycle. Although discharge capacity retention was proportional to SQRT up to 300cycles, sudden reduction was observed around 300cycles. We show the discharge curve of the reassembled coin-type cells operated in a voltage region of 4.2-2.6 V for MNC cathode at a constant current of 0.1 C at 25 °C in Fig. 1(b). In these measurements, we applied the cathode at the inside and outside of the sheet and we observed drastic fade of the cathode capacity at the outside of sheet after 500 cycled.Figure 1 (c), (d) and (e) show Mn L 2,3 –edge, Co L 2,3- edge and Ni L 2,3 –edge XAS spectra of cathode active materials in discharge state (3.6 V) taken by PEY mode, respectively. We can easily find that ratio of lower valence state in each XAS spectra (Mn: ~640 eV, Co: ~779 eV, Ni: ~854 eV) is higher at the outside of cathode sheet after 500 cycled, especially in Mn L 2,3 –edge XAS spectra. On the other hands, we observed the no difference in XAS spectra at the outside and inside of cathode sheet taken by FY mode (shown in Fig. 1(f)). These indicate that the surface of the cathode active materials at the outside of sheet strongly degenerated and the Mn ions were selectively influenced. We also show the Ni L 2,3 –edge XAS spectra in charged state (4.2 V) taken by PEY and FY mode in Fig. 1(g) and Fig. 1(h), respectively. In the NMC cathode, it is well known that only the Ni ions change in valence form Ni2+ to Ni 4+ during the charging process to 4.2V. However, we observed the no change in valence state of Ni ions at the surface and the outside of cathode sheet, indicating the existence of deactivated regions. In addition, the change in valence state of Ni ions at the outside of cathode sheet is smaller than that of the inside at bulky regions.From these results, the sudden reduction of discharge capacity retention was thought to be caused by the deactivation of active materials at the outside of cathode sheet, especially at the surface, in addition to the irreversible loss of Li due to rapid SEI growth at the inside of anode sheet [2]. Acknowledgement The synchrotron experiments were carried out on beamline BL-11 at SR center of Ritsumeikan University and we thank K. Yamanaka for his help in XAS measurements. Figure 1

Site-Selective and Element-Selective Analysis of Nickel Substituted Li-Rich Layered Material Using By X-Ray Diffraction and Absorption Spectroscopy

1. Introduction Li-rich type (manganese) oxides are one of the most featured cathodes for lithium ion batteries in recent years, owing to their high capacity and cyclability. In these cathodes, partial substitution of manganese by other transition metals such as Ni and Co has been proposed and shown to be effective in improving the performance, [1] however, the role of such metals in the battery performance has not been clarified in detail.We have synthesized Ni ion substituted Li excess manganese oxides (Li2Mn1-xNixO3) as a simple model of Li2MeO3 phase in solid solution cathode to understand the effect of introduced Ni by using the combination of in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction spectroscopy (XDS) analysis. Diffraction anomalous fine structure (DAFS) spectra are achieved by XDS analysis, that there were quantitatively showed potential-dependent transbilayer migration of nickel and manganese and their reversibility. 2. Experimental procedure The active material of nickel substituted Li2MnO3 (Li2Mn1-x NixO3, x = 0 ~ 0.25) was prepared from solid phase calcination at 1173 K for 12 h in air. The working electrode consisted of active material (80 wt.%), acetylene black (10 wt.%) and polyvinylidene difluoride (PVdF) binder (10 wt.%), coated on aluminum current foil.The electrochemical measurements of the electrode were employed using aluminum pouch type cells, metallic lithium as counter and reference electrodes, 1 mol dm-3 LiPF6 in a 3:7 mixture of EC/DMC, and a polyolefin film as a separator. The electrochemical charge and discharge tests were performed at room temperature and the potential range was between 2.0 and 4.8 V.XAS measurements were performed by the three-electrode cell with charging and discharging processes (in Operando). The XAS measurements of the Ni-K and Mn-K edge region were performed using a solid-state detector at beamlines BL28XU in SPring-8 (Hyogo, Japan).The XDS measurements were conducted at beamline BL28XU, SPring-8, Japan. A two-dimensional Pilatus detector (Dectris Co., Ltd.) was mounted onto an arm of a multi-axis goniometer and was used to measure the diffraction profile, and two ionization chamber was used for the absorption correction of the DAFS spectra. 3. Results and Discussion The obtained X-ray absorption near edge structure (XANES) region spectra at N-K energy region of Li2Mn1-x NixO3 (x = 0.25) in the 1st charging and discharging processes are shown in Fig. 1. The edge energy was shifted to high energy, which is suggested that Ni element function as a charge compensation species. However the end of charging region around 4.7 V to 4.8 V was shifted to low energy as seen in previous XAS report [2], which means the reduction of nickel on average in this region.As our group have published before,[3] XANES-like spectra only from selected sites (i.e. DAFS spectra) can be extracted from the XDS analysis. Fig. 2 shows the DAFS spectra of initial sample. The Ni-K edge XANES-like spectra shows that there was cation mixing in the lithium layer of the pristine material. In contrast, Mn-K edge XANES-like spectra shows that manganese mostly occupied in transition metal layer. XDS analysis of the fully charged sample indicated that Ni element further migrated to the lithium layer.These analytical approaches are important to understand the metal migration behavior as a function of operation potential together with the charge compensation mechanism during electrochemical charging and discharging.

Localized and mixed valence state of Ce4fin superconducting and ferromagneticCeO1−xFxBiS2revealed by x-ray absorption and photoemission spectroscopy

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce $4d$-$4f$ resonant photoemission spectroscopy (PES) on single crystals of CeO$_{1-x}$F$_x$BiS$_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In the Ce $L_3$-edge XAS, mixed valence of Ce was found in the $x=0.0$ sample and the F-doping suppresses it, which is consistent with the results on polycrystalline samples. As for the resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and $0.5$ systems respectively form a flat band at 1.0 eV and 1.4 eV below the Fermi level and there is no contribution to the Fermi surfaces. Interestingly, Ce valence in CeOBiS$_2$ deviates from Ce$^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in CeOBiS$_2$ is mixed with the unoccupied Bi $6p_z$, which is consistent with the previous local structural study. Based on the analysis of the Ce $L_3$-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller increasing the number of Ce $4f$ electrons upon the F substitution for O.

An in situ resonant photoemission and x-ray absorption study of the BiFeO3 thin film

Multiferroic bismuth ferrite (BiFeO3) thin films were prepared using the pulsed laser deposition (PLD) technique. The electronic structure of the film was studied in situ using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that the Fe ions were initially in a+3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field divides the unoccupied Fe 3d state into t2g↓and eg↓states. Valence band Fe 2p–3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays an important role in the multiferroic properties of BiFeO3 thin films.

Anderson's impurity-model analysis on CeO1-xFxBiS2

We have investigated the impact of F-doing on CeO1-xFxBiS2 in terms of the electronic-structural parameters of Anderson's impurity-model analysis. It was recently reported using Ce L3-edge x-ray absorption spectroscopy (XAS) that CeOBiS2 falls in the Ce valence fluctuation regime and the F-doping drives the system into the Kondo regime. The Ce L3- edge XAS spectra with the various F-doping levels can be reproduced by adjusting the transfer integral in the Anderson's impurity model. The present analysis indicates that the F-doping to the system corresponds to the decrease of the Ce-Bi transfer integral.

Pressure-Induced Valence Change and Semiconductor–Metal Transition in PbCrO3

Pressure-induced valence change and semiconductor–metal transition were revealed in perovskite compound PbCrO3 by X-ray absorption spectroscopy (XAS) and resistance measurements, respectively. The Cr L2,3 edge XAS spectra indicate a charge disproportionation of Cr (3Cr4+ → 2Cr3+ + Cr6+) at ambient pressure, suggesting that the ambient-pressure phase of PbCrO3 has complex local structure and cannot be explained using the simple cubic perovskite structure (Pm-3m). Upon compression up to 4.2 GPa, Cr3+ and Cr6+ ions were converted to Cr4+ via charge transfer, associated with semiconductor–metal transition. The high-pressure metal phase is determined to be cubic perovskite structure (Pm-3m). Compared with the previous X-ray diffraction experiment, the large volume collapse (∼9.8% at 1.6 GPa) may be related to valence change and semiconductor–metal transition.

Role of the Ce valence in the coexistence of superconductivity and ferromagnetism ofCeO1−xFxBiS2revealed by CeL3-edge x-ray absorption spectroscopy

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.

Electronic structure of the hole-doped delafossite oxides CuCr1−xMgxO2

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1\ensuremath{-}x}$Mg${}_{x}$O${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d\ensuremath{-}$O $2p\ensuremath{-}$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3{d}^{10}$) and Cu${}^{2+}$ ($3{d}^{9}$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s\ensuremath{-}$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

X-ray absorption and magnetic circular dichroism characterization of Fe-doped [formula omitted] thin films

We report x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements of both paramagnetic and ferromagnetic Ti1−xFexO2−δ thin films. Fe L3,2 edge XAS spectra showed that the Fe ions in our samples were in mixed valence states and that the Fe2+/Fe3+ ratio decreased with increasing oxygen partial pressure. Finite XMCD signals were observed only for the Fe2+ state in the ferromagnetic sample. These findings indicate that the observed ferromagnetism in Ti1−xFexO2−δ originates from ferromagnetic interactions between Fe2+ local spins.

Element-specific and bulk magnetism, electronic, and crystal structures of La0.70Ca0.30Mn1−xCrxO3

The magnetic interactions in La${}_{0.70}$Ca${}_{0.30}$Mn${}_{1\ensuremath{-}x}$Cr${}_{x}$O${}_{3}$ ($x=0.15$, 0.50, and 0.70) are investigated by x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), high-resolution x-ray powder diffraction, and bulk magnetization measurements. XAS in the Mn and Cr ${L}_{2,3}$ edges support stable single valent Cr${}^{3+}$ ions and a varying Mn valence state with $x$, while the O $K$ edge XAS spectrum reveals local maxima in the O $2p$ density of states close to the Fermi level due to mixing with Mn and Cr $3d$ states. A robust antiferromagnetic state is found for $x=0.70$ below ${T}_{N}=258$ K. For $x=0.15$, combined XMCD and bulk magnetization measurements indicate a fully polarized ferrimagnetic state for the Mn and Cr spins below ${T}_{c}=224$ K. For $x=0.50$, a reduced ferrimagnetic component dominated by Mn spins is present below ${T}_{c}=154$ K. No evidence of lattice anomalies due to cooperative charge and orbital orderings is found by x-ray diffraction for all samples. The magnetic properties of this system are rationalized in terms of a competition of ferromagnetic Mn-Mn double exchange and antiferromagnetic Cr-Cr and Cr-Mn superexchange interactions.

O-vacancies in transition metal (TM) oxides: Coordination and local site symmetry of transition and negative ion states in TM 2O 3 and TMO 2 oxides

Removal of neutral O-atoms from a transition metal (TM) oxide results in two electrons residing within the vacated site. Two-electron multiplet theory has been used to develop a d 2 equivalent model. When applied to tetragonal HfO 2, the two electrons of neutral vacancy in the d 2 equivalent limit reside on two Hf d-state orbitals in a high-spin configuration. Transition energies and negative ion state features are detected in transition metal (TM) oxides by X-ray absorption spectroscopy (XAS) in the O K pre-edge/sub-band gap energy regime, and are compared with the model. These transitions are from the O 1s 2 doubly occupied core level approximately 543 eV below vacuum, and terminate in empty virtual bound states. In addition these strong spin-allowed transitions, spectra include weaker features corresponding to either spin-forbidden transitions in the d 2 model, or alternatively to a near-degeneracy with a singlet ground state with weak Hf-atom pair bonds. The analysis of O K edge XAS spectra identify a significant distinction between multiplet (i) triplet transition energies for neutral O-vacancies that are not active as electron or hole traps, (ii) O-vacancy negative ion states that are active as electron traps in Poole–Frenkel conduction or trap-assisted tunneling (TAT), and (iii) O-vacancy occupied ground states that can trap holes.

Direct observation of the pressure-induced charge redistribution inBiNiO3by x-ray absorption spectroscopy

To investigate the change in the electronic structure of ${\text{BiNiO}}_{3}$ accompanied by metal-insulator (MI) transition, we measured x-ray absorption spectroscopy (XAS) spectra at the $\text{Ni}\text{ }K$ and $\text{Bi}\text{ }L$ edges under various pressures up to 6 GPa. Both $\text{Bi}\text{ }{L}_{3}$ and $\text{Ni}\text{ }K$ edge XAS spectra clearly change at 4 GPa, indicating the electronic state in Bi and Ni ion changes. A quantitative analysis of the $\text{Ni}\text{ }K$ edge spectra in the pre-edge region based on the charge-transfer cluster model including multiplet terms revealed that the electronic configuration changes from ${d}^{8}$ in the insulating phase to $56%\text{ }{d}^{7}+44%\text{ }{d}^{8}\underset{̱}{L}$ in the metal phase. From these results, we concluded that the MI transition in ${\text{BiNiO}}_{3}$ is induced by the collapse of charge-transfer gap and is governed by the redistribution of $\text{O}\text{ }2p$ ligand holes.

X-ray Absorption Spectroscopic Studies of Pulsed-Laser-Deposited Thin Films of Fe3O4 on Si(111) Substrate

We report on the growth and the characterization of magnetite thin films grown on Si(111) substrates by using pulsed laser deposition at different substrate temperatures (TS = 350, 450 and 550 ◦C) by performing X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments. The O K− and Fe L3,2−edge XAS spectra reveal that the couplings of the O 2p with Fe 3d orbitals highly rely on the growth processes. The XMCD data of well-characterized thin films exhibit characteristic contributions from Fe A ions in tetrahedral sites and Fe 2+ B and Fe B ions in octahedral sites and have spectral features similar to those of the Fe3O4 single crystal. The temperature dependence of the XMCD line shape features remain unchanged below and above the Verwey transition. These investigations demonstrate the experimental conditions for controlled growth of Fe3O4 thin films on Si(111) substrates and suggest that the Fe3O4/Si(111) might a promising spintronic materials for future technology.

Defect induced ferromagnetism in Co-doped ZnO thin films

We present a study on the structural, magnetic, and optical properties, as well as the electronic structure of Co-doped ZnO films prepared by magnetron sputtering. Magnetization measurements performed at different temperatures indicate ferromagnetic and paramagnetic behavior for the samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show only paramagnetic behavior corroborating that the presence of oxygen-related defects is essential for ferromagnetism in Zn1-xCoxO. X-ray absorption spectroscopy (XAS) at the Co L2,3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co2+ (d7) state under tetrahedral symmetry indicating a proper incorporation in the ZnO host lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model.