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Made available in DSpace on 2020-12-10T20:10:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-08-28 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are used for investigating heterostructure samples of GaAs/SnO2. XANES data are used for analyzing the local organization around Eu in the heterostructure formed by GaAs and Eu-doped SnO2. The differences between the XANES data for these samples and data obtained for Eu-doped SnO2 thin films, deposited on glass substrate, are assumed as responsible for the differences in the photoluminescence (PL) spectra concerning the Eu3+ emission, since films deposited on glass substrate do not present Eu3+ PL transitions until the annealing temperature is rather high. Eu3+ emission is explored using two different excitation sources: 350 nm from a Kr+ laser (above SnO2 energy bandgap) and 488 nm from an Ar+ laser (below SnO2 bandgap energy). The existence of more organized regions around the Eu3+ site observed for the heterostructure surface may be associated with the Eu3+ luminescent emission. The main and secondary features in the XANES show that there are differences in the average local Eu environment for the SnO2:Eu isolated thin films and heterostructures, being more organized in the latter. Electrical characterization evidences that the portion of the resistivity reduction that corresponds to photo-ionized intrabandgap states is responsible for the persistent photoconductivity phenomenon in the heterostructures. Sao Paulo State Univ UNESP, Postgrad Program Mat Sci & Technol POSMAT, Dept Phys FC, Bauru, SP, Brazil Univ Grenoble Alpes, Inst Neel, CNRS, F-38042 Grenoble, France Sao Paulo State Univ UNESP, Postgrad Program Mat Sci & Technol POSMAT, Dept Phys FC, Bauru, SP, Brazil CAPES: 88881.131882/2016-01 CAPES: 88887.375016/2019-00-CAPES-PRINT

The chemical stability of supported CoPt nanoparticles in out-of-equilibrium core-shell configurations was investigated mainly by anomalous grazing incidence small angle X-ray scattering (AGISAXS) in association with combined transmission electron microscopy and X-ray absorption spectroscopy. CoPt nanoparticles were prepared at room temperature by ultrahigh vacuum atom beam deposition using two different routes: simultaneous deposition of the two metals (CoPt) or sequential deposition. In this last case, Co deposition on a Pt-core (Pt@Co) and the reverse configuration (Co@Pt) are explored. In the Pt@Co case, our experimental analysis of 2.5 nm particles shows the stability of a Pt rich-core (80% Pt) surrounded by a two-monolayers-thick Co shell. In the reverse case, the core-shell structure is also stabilized, while the codeposited sample leads to an alloyed structure. These results suggest that the growth kinetics can trap the thermodynamically non-favorable core-shell structure even for this system which has a high alloying tendency. Besides the lack of atom mobility at room temperature, this stabilization can also be associated with core strain effects. Post thermal treatment of core-shell samples induces a structural transition from the core-shell configuration to the equilibrium alloyed configuration. This study demonstrates that the element-selective scattering technique, AGISAXS is highly efficient for the extraction of chemical segregation information from multi-component supported nanoparticles, such as core-shell structures, up to ultimate small sizes.

We investigated the molecular beam epitaxy growth, the structure and the magnetic properties of the exchange coupled CoO/Fe bilayers on Ag(0 0 1). In situ X-ray scattering shows that Fe grows in registry with Ag(0 0 1) with an out-of-plane lattice parameter that varies with the Fe layer total thickness. The growth of CoO to build an exchange coupled CoO/Fe system impacts the Fe layer even at room temperature. Two different bilayers grown under close conditions are studied. They differ by the ratio of the oxidized Fe layers over the 7 initial Fe monolayers (30% and 40% respectively). Low temperature exchange magnetic coupling with a blocking temperature T B of about 180 K–150 K and similar thermal behavior are observed for the two samples. We studied one sample with synchrotron X-ray resonant magnetic scattering confirming that the magnetization was perpendicular to the surface below T B . Noteworthy, we found that the magnetic easy axis was lying in-plane at room temperature, above T B . These results point to another example of perpendicular ferromagnetism in an ultra-thin film due to an exchange mechanism with an antiferromagnetic layer that rotates the magnetic easy axis from in-plane to perpendicular below T B .

Cobalt ferrite ultrathin films with the inverse spinel structure are among the best candidates for spin filtering at room temperature. High-quality epitaxial CoFe2O4 films about 4nm thick have been fabricated on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate and then treated twice with O-2, first at room temperature and then during annealing. The epitaxial orientation and the surface, interface and film structure were resolved using a combination of low-energy electron diffraction, scanning tunnelling microscopy, Auger electron spectroscopy and in situ grazing-incidence X-ray diffraction. A slight tetragonal distortion was observed, which should drive the easy magnetization axis in-plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in situ resonant X-ray diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using FDMNES, an ab initio program already extensively used to simulate X-ray absorption spectroscopy, and shows that the Co ions are predominantly located on octahedral sites with an inversion parameter of 0.88(5). Ex situ X-ray photoelectron spectroscopy gives an estimation in accordance with the values obtained through diffraction analysis.

International audience; We report on the crystallographic phases and their epitaxial relationships observed during the metal-insulator transition (MIT) of a VO2 film deposited on Al2O3(0001). A key feature of this work is to establish two possible in-plane epitaxial relationships for the VO2 film, introducing an “on-axis” and a “diagonal” model. These models have distinctive signatures in reciprocal spaces in the form of multiplet reflections and can, therefore, be easily differentiated by diffraction. They serve as a basis for interpreting the results gained by high-resolution X-ray diffraction, and complemented by Raman spectroscopy, on a 420-nm-thick VO2 film grown by microwave plasma-assisted reactive sputtering on Al2O3(0001). We address its orientation and follow its structure through the MIT. The film is oriented according to the diagonal model and exhibits a MIT involving three phases with different temperature domains of (co)existence. The room- and high-temperature phases are, respectively, identified as the monoclinic M1 and rutile R polymorphs of VO2. Additionally, we observe that when the sample resistivity starts to decrease, the insulating M1 phase turns into an intermediary phase, which then partially transforms into the metallic R phase when the MIT is complete. The intermediary phase is similar to the M2 polymorph of VO2, according to its lattice parameters, Raman signature, and its known proximity with the M1 and R phases in tensile conditions. The mixing of the M2 and R phases is still detected at 110∘C, i.e., far above the MIT; the M2 polymorph is being probably stabilized at the interface between the film and the substrate, where the interfacial strain is higher.

We report a detailed study of the Verwey transition in a magnetite ultrathin film (UTF) grown on Ag(001) using resonant x-ray scattering (RXS). RXS was measured at the Fe K-edge on the crystal truncation rod of the substrate, increasing the sensitivity to the film thanks to the cross-interference, thereby obtaining an x-ray phase-shift reference and a polarization analyzer. The spectra were interpreted with ad hoc calculations based on density functional theory within a surface-scattering formalism. We observed that the UTF has a relatively sharp transition temperature ${T}_{V}=120$ K and is remarkably close to the bulk temperature for such thickness. We determined the specific Fe stacking at the interface with the substrate below ${T}_{V}$, and detected a spectroscopic signal evolving with temperature from ${T}_{V}$ up to at least ${T}_{V}+80$ K, hinting that the RT crystallographic structure does not set at ${T}_{V}$ in the UTF.

International audience; We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

We studied by soft X-ray absorption spectroscopy the magnetization axis in a 4 nm thin CoO (111) layer exchange-coupled to an ultra thin L 1 0 PtFe layer with perpendicular magnetic anisotropy. The angular dependence of the linear magnetic dichroism at 10 K and the relative variations of the spectral features provide a full description of the spin orientation in this antiferromagnetic layer. The spins are found in the film plane, pointing along the 110 direction. This results is discussed in relation to the film strain and the preferential occupation of t 2 g orbitals. The strong orthogonal coupling between Co and Fe spins should be at the origin of the robustness of the exchange bias effect found in this bilayer system.

Epitaxial iron oxide layers with different orientations were grown on Ag(001) surface by choosing the appropriate preparation conditions. A film with a hexagonal surface mesh interpreted as (111)-oriented magnetite was formed by reactive deposition of iron in molecular oxygen at room temperature (RT), followed by annealing in UHV. Instead, highly ordered epitaxial layers with P4m symmetry were obtained by a three-step process, optimized through in situ experiments. Following this method, an ultrathin Fe layer was first grown in coherent epitaxy on the substrate and then dosed twice with O2, first at RT and next during annealing. A structural analysis combining low-energy electron diffraction, scanning tunneling microscopy, and accurate surface x-ray diffraction measurements confirmed that these films consist of (001)-oriented magnetite, although with a slight tetragonal distortion induced by the substrate constraints. Both its surface and interface are atomically sharp, an essential requirement for its integration into spintronic based devices., Beamtime is acknowledged at the French CRG BM32 beamline of the ESRF. We thank all the beamline staff and in particular O. Geaymond and O. Ulrich for their invaluable technical support. X. Torrelles thanks the MICINN (Spain) for financial support through project MAT2012- 38213-C02-02.

An energy-dispersive X-ray absorption spectroscopy beamline mainly dedicated to X-ray magnetic circular dichroism (XMCD) and material science under extreme conditions has been implemented in a bending-magnet port at the Brazilian Synchrotron Light Laboratory. Here the beamline technical characteristics are described, including the most important aspects of the mechanics, optical elements and detection set-up. The beamline performance is then illustrated through two case studies on strongly correlated transition metal oxides: an XMCD insight into the modifications of the magnetic properties of Cr-doped manganites and the structural deformation in nickel perovskites under high applied pressure., Facultad de Ciencias Exactas, Centro de Química Inorgánica

International audience; Ultradense macroscopic arrays of ferromagnetic alloy nanowires exhibit unique properties that make them attractive both for basic physics studies and for prospective nanodevice applications in various areas. We report here on the production of self-organized equiatomic FePt nanowires produced by glancing-angle ion-beam codeposition on alumina nanoripple patterns at room temperature and subsequent annealing at 600 °C. This study demonstrates that periodically aligned FePt nanowires with tunable size (∼10–20 nm width and ∼0.5–10 nm height) can be successfully grown as a consequence of shadowing effects and low mobility of Fe and Pt on the rippled alumina surface. Moreover, the structure and magnetic properties of the FePt nanowires, which undergo a phase transition from a disordered A1 (soft) structure to a partially ordered L10 (hard) structure, can be modified upon annealing. We show that this behavior can be further exploited to change the effective uniaxial anisotropy of the system, which is determined by a strong interplay between the shape and magnetocrystalline anisotropies of the nanowires.

Wereport on x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine; structure (EXAFS) measurements of SmNiO3 from 20 Kto 600 Kand up to 38 GPa at the NiKand Sm; L3 edges. A multiple component pre-NiKedge tail is understood, originating from 1 s transitions to; 3d-4p states while a post-edge shoulder increases distinctively smoothly, at about the insulator to; metal phase transition (TIM), due to the reduction of electron–phonon interactions as the Ni 3d andO; 2p band overlap triggers the metallic phase. This effect is concomitant with pressure-induced Ni-O-Ni; angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group.; Room temperature pressure-dependent Ni white line peak energies have an abrupt∼3.10±0.04 GPa; valence discontinuity from non-equivalent Ni3+δ + Ni3–δ charge disproportionate net unresolved; absorber turning at∼TIM into Ni3+ of the orthorhombic Pbnm metal oxide phase. At 20 Kthe overall; white line response, still distinctive at TIM∼8.1±0.6 GPa is much smoother due to localization.; Octahedral bond contraction up to 38 GPa and at 300 Kand 20 Kshow breaks in its monotonic; increase at the different structural changes. The SmL3 edge does not show distinctive behaviors either; at 300 Kor 20 Kup to about 35 GPa but the perovskite Smcage, coordinated to eight oxygen atoms,; undergoes strong uneven bond contractions at intermediate pressures where we found the coexistence; of octahedral and rhombohedral superexchange angle distortions.Wefound that the white line; pressure-dependent anomaly may be used as an accurate alternative for delineating pressure–; temperature phase diagrams., Centro de Química Inorgánica

FePt and CoPt alloy films with FCT(0 0 1) structural phase were deposited by sputtering. The films were formed by periodically alternating monatomic layers deposition of Fe(Co) and Pt, on MgO(1 0 0) substrates, with and without a Pt buffer layer, at substrate temperatures (TS) ranging from 200 to 500 °C. The X-ray diffraction patterns show the formation of the FCT crystalline structure in FePt films deposited at T S ⩾ 2 0 0 ° C . Hysteresis measurements, using Vibrating Sample Magnetrometry, show that FePt and CoPt films with predominant FCT(0 0 1) phase are perpendicularly magnetized. The long range order parameter S = 0.5 8 demonstrates the good degree of chemical order in FePt film prepared at T S = 4 0 0 ° C . The Experimental results demonstrate that the alternate monatomic layer sputter deposition technique is very effective to grow Fe(Co)Pt ordered alloy at moderate TS.

Rare earth nickel perovskites (RNiO3) undergo a first-order metal to insulator transition as temperature decreases. They present also an unusual antiferromagnetic order in the insulating low temperature state. These transport and magnetic properties are strongly linked to the structure. Modifications in the macroscopic properties results from very subtle changes in the atomic arrangement and electronic states. We have used EXAFS spectroscopy to study the complex interplay among short-range structure and the transport and magnetic properties. The Ni K-edge EXAFS spectra of rare earth nickel perovskites (RNiO3) were measured at several temperatures. A splitting into two bond lengths in the Ni coordination shell, corresponding to two different Ni sites, has been observed in the insulating and metallic states of PrNiO3, NdNiO3 and EuNiO3. The existence of two Ni sites in these light rare earth compounds provides a common ground to understand the behavior of the whole family of nickelates. The splitting has been analyzed by the phase-derivative method, since standard fitting procedures were not able to produce reliable results. The phase derivative is a fit-independent method that can be applied to obtain quantitative structural parameters in EXAFS spectroscopy. It has proven to be a reliable and useful tool to be applied in cases where standard fitting procedures lead to unstable and unreliable parameters.

The anisotropy of the Fe orbital moment is investigated in epitaxial ultrathin ($l2\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$) FePt layers by angular dependent x-ray magnetic circular dichroism (XMCD) for two distinct epitaxial systems, CoO/FePt/Pt(001) and Pt/FePt/MnPt/Pt(001). Element selective hysteresis loops obtained from the XMCD signal at the Fe ${L}_{3}$ edge are used to quantify the anisotropy constants, which are consistent with the corresponding chemical order parameter. A careful procedure for XMCD sum rules data analysis is set up to avoid experimental overestimation of the orbital moment. The measured orbital moment of Fe is found comparable to theoretical values and shows a significant anisotropy. This orbital moment anisotropy is discussed in the light of the current theoretical understanding of magnetocrystalline anisotropy in ordered alloys.

We report on an angular resolved EXAFS study in the plane of La0.7Sr0.3MnO3 thin films epitaxially grown by pulsed laser deposition on slightly mismatched substrates. EXAFS data collected in plane for tensile substrate show a change in the Mn O average bond distance and the increase of Mn Mn length matching with the enlargement of the cell parameter. From these results, we conclude that there is no significant change in the tilt angle Mn O Mn. Our observations suggest that the distortion within the octahedra is a key parameter in the strain-dependence of the transport and magnetic properties in manganite films.

A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the whole series of the substituted solids. XRD patterns demonstrate that the order around Fe remains typical of a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the incorporation of Cr causes a slight decrease in the cell volume with the c-cell parameter following the Vegard’s law. This decrease is accompanied by changes in opposite directions of the various Me-Me distances. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom changes slightly upon Cr substitution: Measurements in the Cr K-edge show that the Cr environment remains unchanged in the whole series. All the observed trends in both average Rietveld and local EXAFS distances can be traced back to the differences in the coordination polyhedra around Cr and Fe. The polyhedron around Cr is more symmetric and can be described as Cr(OH0.5)6 as opposed to the polyhedron around Fe that contains two distinct sets of ligands, FeO3(OH)3. The effects caused by substitution are governed by this difference, rather than by the smaller size of Cr(III) as compared to Fe(III). Simultaneous use of XAS and Rietveld refinement of XRD data permits tracing the trends in the average long range ordering (Me-Me distances) to local changes in distances and angles when Cr3+ substitutes Fe3+ in goethite. Complex changes in the various interatomic distances and angles may result in deceivingly simple long-range trends. These trends are therefore of limited value as probes for the atomic scale changes. On the other hand, XAS provide direct information on the fundamental, atomic-scale changes.

We report on polarized X-ray absorption spectroscopy (P-XAS) study of La 0.7 Sr 0.3 MnO 3 (LSMO) films, epitaxially grown by laser ablation on tensile (SrTiO 3 ) and compressive (LaAlO 3 ) substrates. In-plane and out-of-plane bond information was obtained by setting the angle between electric field vector and film surface close to 0 and 90°, respectively. Measurements show significant modifications in the average MnO 6 octahedron around manganese atoms in the film plane for tensile and compressive substrates. The modifications of the XANES spectra were correlated to a modification in the average MnO distance and a distortion of the MnO 6 octahedra. Ab initio calculations using the full multiple scattering approach confirm the structural model of distorted octahedron.

Human dental calculi are biological calcium phosphates, which consist of an organic phase and an inorganic or mineral phase. In the latter phase, spectrochemical analyses have revealed the presence of several different magnesium and calcium phosphates. As the crystalline structure of the calculus passes through several stages during its allocation in the mouth, special attention is paid to some elements, such as zinc, that can modify the mineralization process. Several in vitro studies relating to the dental calculus mineralization process have been performed so far, but there is a lack of data obtained from biologically synthesized samples. The knowledge of the zinc distribution and incorporation in biological calcium phosphates is of great interest in providing more information about the biological process of calculus formation. In this paper we present surveys of the elemental distribution and incorporation of zinc in human dental calculus, by using a combination of different techniques: x-ray microfluorescence using synchrotron radiation, scanning electron microscopy and x-ray absorption spectroscopy. One-dimensional x-ray microfluorescence of zinc and magnesium measurement shows that there is a high accumulation of both elements in the sub-gingival region of the calculus and a strong correlation of their spatial distribution. Experimental Ca/P molar ratios were determined by energy-dispersive spectroscopy to identify different calcium phosphate phases, the sub-gingival region being composed of a mixture of highly and poorly calcified phosphates and the supra-gingival region composed mainly of carbonated hydroxyapatite. Finally, x-ray absorption measurements were carried out at the zinc K edge on synthetic and biological samples. The Zn—O distance and coordination number of the synthetic samples and the supra-gingival calculus show that zinc is adsorbed on these structures, whereas in the sub-gingival samples it is allocated in a cation site. The results are indicative of the active participation of zinc in the calcification process of sub-gingival calculus. Copyright © 2003 John Wiley & Sons, Ltd.

Goethite (α-FeOOH) is abundant at the Earth’s surface and has the unusual capacity to adsorb and fix ions from migrating solutions. Understanding the mechanisms by which foreign elements are incorporated into natural goethite has implications for environmental and mining problems. X-ray absorption spectroscopy (XAS) was used to obtain structural information on the local environment around Ni in natural Ni-containing goethite (1.8–4.1 mol% Ni) from Vermelho lateritic deposit of Serra dos Carajas (Brazil) and in synthetic analogues. The data were collected at the LNLS XAS beam line at the Ni and Fe K -edges, at room temperature, and at the Ni K -edge at 8 K. Nickel was found in essentially the same environment in all natural and synthetic samples, with negligible thermal disorder. The coordination polyhedron is a tetragonal dipyramid of oxygen atoms showing that Ni preserves its usual local symmetry. This finding is compatible with a model in which substitution of Ni for Fe is accompanied by a proton capture resulting in NiO2(OH)4 octahedra. The polyhedral linkages are similar to that of pure a-FeOOH, consisting of four shared edges at about the same metal-metal distances, as in the pure mineral. The third and longest metal-metal distance is about 6% larger than the expected corner-sharing distance in the α-FeOOH structure, showing that incorporation of Ni locally distorts and opens the structure.

The As sorption capacity of a natural Mn and Fe mineral-containing sample from the Iron Quadrangle province, Brazil, was investigated. A detailed mineralogical identification was obtained by combining X-ray diffraction analyses (with Rietveld refinement), X-ray fluorescence spectroscopy, optical microscopy, and scanning electron microscopy coupled with X-ray energy dispersive spectrometry-EDS. The oxidation state of the adsorbed As species was determined by X-ray absorption near edge structure spectroscopy. The results demonstrate that the presence of naturally occurring Mn oxides promotes the effective oxidation of As (III) to As(V). Also, the Mn minerals show a significant uptake of both the trivalent and pentavalent As species. This study demonstrates that the combined influences of As(III) depletion by oxidation and adsorption on a natural oxide sample consisting of Mn minerals and Fe oxides may effectively contribute to the reduction the As concentration in waters.

Irreversible photoexpansion and photobleaching effect has been observed in amorphous Ga10Ge25S65 glass when its surface was exposed to light with energy greater than the band gap, 3.52 eV. The magnitude of the expansion of GaGeS glasses depends on the exposure conditions. Extended X-ray absorption fine structure (EXAFS) spectroscopy and Rutherford backscattering spectrometry (RBS) have been used to identify the chemical nature of the glass samples before and after illumination. The quantitative analysis of the EXAFS data leads to a two-shell model of 0.5 oxygen atoms at 2.01 A and 3.6 sulfur atoms at a 2.20 A. RBS technique demonstrated that chemical composition of the glass surface after irradiation is oxygen rich. The existence of GeO bonds in the glass after illumination was also confirmed by infrared measurements.

We discuss the specific features of the K-edge X-ray absorption spectroscopy (XAS) at the Fe and Co K-edges in Fe x Co 1− x /NM discontinuous multilayers with NM=Ag, Cu, and Al 2 O 3 . Thermal treatments lead to the breakup of the layers forming a heterogeneous alloy with magnetic clusters of the FeCo embedded in a nonmagnetic (NM) matrix. Stabilization of the BCC phase of the FeCo alloy is directly evidenced by XAS. Differences among Fe and Co K-edge signatures indicate a strong dependence on the chemical nature of the absorbing atom for annealed samples. The amplitude of the extended X-ray absorption fine structure signal higher at the Co K-edge than at the Fe one reflect a smaller structural disorder in the Co environment. This behavior depends neither on the composition of the alloy nor on the NM material. In the case of Al 2 O 3 a higher disorder around Fe atoms is observed also in the as-deposited sample. All these results indicate that the Fe segregates at the interface of the magnetic clusters/NM matrix after thermal annealing.

In contrast with crystals, glass structure cannot be unambiguously determined by experimental techniques, and molecular dynamics (MD) simulations are often used to model such systems. Ion–ion interactions are modeled by empirical potentials, which are validated by comparison between the structural information extracted from experimental results and the simulated structure. Bond valence (BV) theory is often used to check the reliability of experimentally determined crystal structures. However, it has not been applied in a similar fashion to the results of MD simulations of disordered systems. To illustrate the use of BV theory in silicate glasses, BV calculations were performed in a 1500 atom-3D-periodic MD simulation box of a glass of composition CaO–FeO–2SiO2 (mol%) with two sets of Born–Mayer–Huggins potentials. The structural models are analyzed from the point of viewof BV theory. The BV sums around the 1500 atoms showthat the BV model is globally verified. In addition, w e showthat the BV sums are sensitive to changes in MD potential parameters and can thus be used to test the plausibility of the MD model of a silicate glass. This newstrategy opens possibilities in the derivation of more robust and constrained structural models of silicate glasses and melts. 2002 Elsevier Science B.V. All rights reserved.

We report X-ray absorption spectroscopy study of (80− x )Ga 2 S 3 –20La 2 S 3 – x CsCl glasses, with x =0–40. The addition of CsCl to Ga 2 S 3 –La 2 S 3 glasses increases their thermal stability making them suitable a host-matrix for luminescent rare earth ions. The gallium-based network is composed of almost regular tetrahedra of sulfur atoms connected by corners. The average local lanthanum site is a distorted polyhedron of sulfur atoms at a distance of 2.96±0.01 A, with a coordination number close to 7. There is no alteration in the local environment of these elements in the doped glasses with respect to the undoped one detected in the X-ray absorption spectra. The chlorine and cesium sites, as observed in the X-ray absorption spectra, are similar to that in CsCl. These experimental observations are compatible with a model in which these elements are incorporated in network ionic vacancies in the form of CsCl n units.

Irreversible photoexpansion effect has been observed in amorphous Ga10Ge25S65 glasses when its surface was exposed to light with energy greater than the band gap, 3.52 eV. A volume change of about 5% was reached in bulk samples by controlling illumination time and the laser power density. To understand the atomic scale processes of the photoexpansion effect, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used as a local probe of the germanium environment in the glass samples before and after illumination. Modifications are observed in the average coordination shell around Ge atoms in the illuminated sample compared to the non-illuminated one. For the non-illuminated sample, the Ge coordination shell is described by a distorted tetrahedron of sulfur atoms at around 2.20 A. After illumination, the EXAFS signal can be explained by introducing an additional contribution to this average environment. Based on an analysis of the EXAFS data we proposed a two-shell model of 0.5 oxygen atoms at 2.01 A and 3.6 sulfur atoms at a 2.20 A. The existence of Ge–O bonds in the glass after illumination was confirmed by infrared measurements.

Rare earth nickel oxide perovskites (R NiO 3, R=rare earth) have, except for LaNiO 3, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T MI ) increases as the R-ion becomes smaller. They present also, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T N ) is close to T MI , while for heavier R-ions (e.g. Eu, Sm), T MI and T N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Although R NiO 3 perovskites are placed in the boundary of the Mott–Hubbard and charge transfer regimes, there are several evidences pointing to a charge transfer gap, mainly controlled by ligand-to-metal charge transfer energy, and thus strongly dependent on hybridization. Ni L-edge absorption spectroscopy (transition 2p → 3d) gives direct information on the density of Ni 3d empty states, and in particular on the multiplet splitting and hybridization between Ni 3d and O 2p bands. Here we present Ni L3 and L2 absorption spectra measured for NdNiO 3 and EuNiO 3 (T MI = 200 and 480 K). At room temperature, dramatic differences are observed between EuNiO 3 (insulating) and NdNiO 3 (metallic). The normalized spectra give evidence for a higher density of 3d unoccupied states and a larger multiplet splitting in EuNiO 3. Both effects might be correlated to a decrease in hybridization. The same behavior is observed for NdNiO 3 as it is cooled down to the insulating phase (T < 200 K), revealing that in these compounds the opening of the gap is directly related to the degree of hybridization.

The LNLS XAS beamline has been operating for external users since July 1997. Many facilities and improvements have been progressively added to it, extending the range of applications. Here, a technical description of the main beamline components is given, and results concerning important points, such as available flux at low and high energies, harmonic contamination, energy resolution and stability, are presented. Some key results are given to demonstrate the beamline performance and limitations. It is shown that the beamline can cover a large energy range, starting from the rather low energy of 2.3 keV up to 25 keV.

X-ray absorption fine structure spectroscopy and density functional calculations were used to determine the structural model of arsenic surface complex on gibbsite mineral. The structural environment of arsenic at the solid surface may determine its potential for remobilization and stability. Data were collected for arsenate adsorbed on gibbsite surface at pH 5.5. The X-ray absorption fine structure spectroscopy results showed that As(V) forms an inner sphere bidentate binuclear complex on the surface of Al oxyhydroxyl octaedra. Quantitative results showed an average interatomic As(V)-Al distance of 3.19 ± 0.05 A and a coordination number of 1.3 ± 1.0 atoms. Four different adsorption sites in which arsenate can interact with gibbsite have been studied using density functional calculations, i.e., bidentate binuclear complex, bidentate mononuclear complex, monodentate mononuclear complex, and monodentate binuclear complex. The density functional calculations confirm that the most stable structure predicted for As(V)-gibbsite system is the bidentate-binuclear complex.

The mixed oxides SiO 2 /TiO 2 and SiO 2 /TiO 2 /Sb 2 O 5 were prepared with different Ti and Sb amounts and investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) at the Ti K edge. There is no evidence for structural modification of titanium environment in the samples having between 5.6 and 20.3 wt% of TiO 2 . All the experimental evidence indicates that titanium has a local environment close to that of TiO 2 anatase. XAS show that this similarity is extends up to the next neighbouring Ti shells. XPS analysis shows that Ti and Sb are present as TiO 2 and Sb 2 O 5 .

The understanding of the structural effects of iron in amorphous systems is of primary importance in the design of technological glasses and for rationalising the dynamic properties of volcanic processes and glass formation. In the present work, the local and medium-range environment of iron in the totally reduced oxide glass CaFeSi2O6 are studied by combining extended X-ray absorption fine structure (EXAFS) spectroscopy and molecular dynamics (MD) simulations. Ferrous iron is located in sites with a distribution of Fe–O distances, with typical geometries close to a tetrahedron and to a triangular bipyramid. At medium-range order, the presence of calcium and iron as next nearest neighbours is found in the experimental data and in the simulations. The iron polyhedra are apex-connected to the silicate network, while edge-linked to each other and to calcium-polyhedra. The occurrence frequency of these edge-sharing polyhedra is that expected for a random distribution or a limited segregation of iron in the glass.

The local environment of titanium in glasses of the system TiO2–BaO–ZnO–B2O3 was studied by x-ray absorption spectroscopy. In all glasses, for TiO2 content varying from 35 to 49 mol%, the titanium atoms are mainly fourfold coordinated network formers. For the glass with the highest titanium content, about 20% of the titanium enters into a sixfold environment, indicating the presence of precrystalline nuclei. These nuclei are responsible for the decrease of the optical performance of the glass above threshold titanium content, prior to any observable crystallization.

5 pages; International audience; We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3 nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe double layer. The exchange bias shift is preserved up to the N'eel antiferromagnetic ordering temperature of TN = 293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain-induced distortion and strengthened interaction between the two well-ordered spin layers.

The emergence of a ferromagnetic component in ${\text{LaMnO}}_{3}$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr $K$ edges. The local magnetic moment strengths for Mn and Cr are proportional to each other and follow the macroscopic magnetization. The net ferromagnetic components of ${\text{Cr}}^{3+}$ and ${\text{Mn}}^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of ${\text{Cr}}^{3+}$ ions up to $x=0.15$ does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.

An annealing-driven transition from a chemically disordered $A1$-like structure to a chemically ordered $L{1}_{0}$-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios $c/a$ are inferred. This ratio is found to be remarkably different depending on the approach used ($c/ag1$ from x-ray absorption at the Co $K$ edge and $c/a\ensuremath{\simeq}0.94$ from the Pt dominated x-ray diffraction). A first-principles theoretical study accurately explains this astonishing feature and provides a detailed understanding of the element-specific local relaxations, which are crucial in these nanoalloys.

A dilute system of CdSxSe1 − x nanocrystals has been studied by means of small-angle X-ray scattering. Information about the surface state has been obtained, taking advantage of the large-q part of the scattering curve. The presence of edges at the surface of the nanoparticles is evidenced by the limit at zero of the chord-distribution function. On the other hand, the information obtained from the high-q part of the scattering curve is compared to that deduced from the whole spectrum. This comparison shows that a polyhedral model is needed to account for the surface of the scattering particles. The particle-size distribution, evaluated assuming edged particles, is in good agreement with the expected distribution and yields a satisfactory simulated spectrum. Some properties of the particle structure can be suggested from the presence of the edges.

In Cd(S,Se)‐doped glasses the optical properties are strongly dependent on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on both stoichiometry and size is difficult to obtain in crystals smaller than 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation of amplitude of this intensity with the energy of the x ray—‘‘the anomalous effect’’—near the selenium edge is related to stoichiometry. Anomalous small‐angle x‐ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lower than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540–650 °C. The samples treated at temperatures above 580 °C contain crystals with size larger than 4 nm. For all these samples the anomalous effect has nearly the same amplitude, and we found the stoichiometry x=0....

A comparison is made between X-ray absorption spectra at the sulphur K edge for CdS in wurtzite and zincblende structures. Differences between the polarization dependences of the X-ray spectra are expected due to the differences in symmetry for the two modifications. Multiple-scattering calculations in both structures are compared to published spectra and allow us to interpret a strong dichroic effect in the wurtzite structure. The intensity of a specific resonance is related to the middle-range order around sulphur atoms and potential applications of this fingerprint analysis are proposed.

We report on an angular resolved x-ray absorption spectroscopy study of the local atomic structure around the manganese ions in La0.7Sr0.3MnO3 thin films epitaxially grown on tensile and compressive substrates. Ab initio calculations provide strong support to the analysis of the experimental data and make possible the unambiguous derivation of a model of local distortion around the manganese atoms, without modification of the tilt angle Mn–O–Mn, among the octahedra. This distortion, tending to localize the charge carriers, is the driving parameter in the modifications of the magnetic and transport properties observed in thin films with respect to bulk systems.

10; International audience; Ultrathin MnPt films have been grown on Pt(001) single crystals by alternate deposition of Mn and Pt and studied in situ by grazing incidence x-ray diffraction. The growth mode and chemical order strongly depend on the deposition conditions. At room temperature, a 3D growth mode for Pt leads to a disordered and rough MnPt film. After annealing at 770 K, chemical order develops with the tetragonal axis oriented in the surface plane. At 570 K, Pt grows in a quasi layer-by-layer mode. For all depositions, and even after annealing, Mn remains essentially at the surface with negligible diffusion into bulk. As a consequence, the film deposited at 570 K is chemically ordered, with the tetragonal axis oriented mainly perpendicular to the surface. Coupled to ferromagnetic layers with in-plane (out-of-plane) anisotropy, these ultrathin (≈3 nm) MnPt films exhibit exchange bias properties and enhanced coercivity at low temperature.

http://link.aip.org/link/?APL/100/132403; International audience; The influence of interfacial oxidation on the magnetic behaviour of CoO covered Fe/Ag(001) is reported. Coverage with CoO causes the formation of a mixed Fe2O3-Fe3O4 interfacial oxide layer. The depth of the Fe-oxide varies with the thickness of pre-covered Fe and above 8 monolayers (MLs) of Fe the oxide depth becomes constant at 2 ML. Differences in exchange bias and coercivity obtained from magnetic field and zero field cooling nearly vanish above 8ML Fe thickness, showing a direct correlation between the magnetic behaviour of Fe and structure of the interfacial Fe-oxide layer

International audience; A formalism to simulate the reflectivity of electromagnetic waves by magnetic materials is presented with an application. The formalism is applicable to retrieve the magnetic moment density along the depth of magnetic materials with arbitrary magnetic moment direction using matricial algebra, including roughness between layers. The reflectivity is derived following a classical description with Maxwell equations and a permittivity built from the quantum scattering amplitude. Approximations on the relative power of the Thomson scattering and the magnetic terms are trackable in order to evaluate the validity of the formalism on a case-by-case basis, from the optical light regime up to soft and hard x-rays. The originality of the present formalism lies in the use of eigen-waves throughout the treatment. An application to a Fe thin film illustrates the methodology.

We report here on an X-ray absorption study of La 0.7 Sr 0.3 MnO 3 films epitaxially grown on SrTiO 3 substrate. The local organization around Mn in oriented films with 600 A in thickness was investigated by polarized Extended X-ray Absorption Fine Structure. The angle between electric field vector and film surface was set equal to 5° and 70° to investigate almost independently the contribution of the manganese neighbors situated in and out of the film plane. The first neighboring shell oxygen is found to be the same in both geometries, but small changes in the next neighboring contribution are observed. These changes are associated with variation in the Mn–Mn bond length. A small in-plane elongation (3%) is observed in the constrained films with respect to the unconstrained case.

The assumption of a random distribution of chalcogen atoms in the wurtzite structure is usually made to interpret the properties of ternary CdSxSe1-x compounds. However, there has not yet been any experimental evaluation of this distribution. The authors report here the results of low-temperature X-ray absorption measurements (4.5 K) at the selenium K edge in CdSxSe1-x compounds for x=0, 0.2, 0.5 and 0.8. These measurements make a quantitative analysis of the next-nearest neighbours up to the fourth-neighbour shell around selenium atoms possible. The numbers of sulphur and selenium second neighbours of selenium atoms for each ternary compound are found to be proportional to the respective molar ratio. The analysis can be made with the best accuracy for x=0.5, and no clustering is observed. As maximum clustering is expected for this value of x, it is concluded that derivation from the random distribution of selenium atoms is very small in CdSxSe1-x.

The average sizes of Cd(S,Se) quantum dots in glasses have been measured for the same samples by using three methods: high‐resolution transmission electron microscopy, low‐frequency inelastic Raman scattering, and small‐angle x‐ray scattering. Three samples with crystal sizes of 5.5–8, 9–12, and 17 nm have been studied. For each method the interaction mode and the underlying assumptions in measuring the size are discussed. The correlation between the measurements is good and points out a narrow size distribution. The discrepancies observed for the smallest sizes are discussed with respect to the probe and the interactions used by each technique. They may be ascribed to structural effects.

We report on an X-ray absorption study on the temperature dependence of the local organisation around Mn 3+ in a high-quality LaMnO 3.00 ceramics. The structural transition is clearly evidenced by modifications on the EXAFS spectra across the Jahn–Teller transition. These modifications are associated with alterations in the MnO 6 polyhedral arrangement. There is no experimental evidence of any reduction of the static disorder in the coordination shell around the Mn 3+ at high temperature. This result supports the hypothesis that the Jahn–Teller transition would correspond to a change from a static and cooperative distortion to a dynamic one, without remotion of the local distortions present in the MnO 6 below the JT temperature.

In silicate glasses containing small amounts of cadmium, sulphur and selenium, appropriate heat treatments can induce the formation of Cd(S,Se) nanocrystals giving rise to important linear and non-linear optical properties. In the present work, X-ray absorption spectroscopy (EXAFS and edge structure) is used to provide information on the environment of selenium in these glasses before any heat treatment. In zinc-containing glasses, the selenium atoms are not incorporated in the glass network. Their environment is close to that in zinc selenide but probably not unique. In zinc-free glasses, the selenium atoms are threefold coordinated with oxygen atoms at a distance of 1.69 A and they are fully inserted in the glass network.

International audience; Electronic and magnetic properties of nanoscale materials are closely related to the atomic arrangement at the interface shared by different chemical elements. A very precise knowledge of the surface/interface structure is then essential to properly interpret the new properties coming out. Of a particular interest is the relationship between structure, morphology and magnetic properties of exchanged-coupled interfaces in ferromagnetic (FM) and antiferromagnetic (AF) materials. The interaction at the AF/FM interface modifies the magnetic switching properties of the FM film, which turn out to be a usefull property on new magnetic devices technology. We present here an investigation of the buried exchange-coupled interface [NiO/CoO]/[PtCo] grown on a Pt(111) single crystal. The magneto-optical Kerr effect reveals a strong coupling at the interface, by an increasing coercivity, and a spin reorientation of the FM film when ordering occurs in the AF layer. The combination of grazing incidence X-ray diffraction, X-ray reflectivity and soft X-ray resonant magnetic scattering yields a comprehensive description of the system.

International audience; The present study aims, by using X-ray Absorption Spectroscopy, to describe the local process towards the pressure-induced abrupt changes in the resistivity in the Jahn-Teller distorted LaMnO3 doped compounds. Local environment of manganese atoms under pressure up to 37 GPa has been studied by X-ray absorption spectroscopy. We identify successive steps towards the pressure-induced abrupt changes in the resistivity. These steps are associated to local processes such as the opening of the inter-octahedra tilt angles and the average symmetrization of the Mn local environment. All modifications present a hysteretic behavior when the pressure is released, pointing to the coexistence of MnO6 distorted and undistorted units.

International audience; X-ray absorption spectroscopy (XAS) has been used to clarify the thicknessdependent magnetic properties in nanometric CoPt films. We get benefit from the variation of the sampling depth with the grazing angle to investigate the variations of the local order within the film. In order to properly resconstruct the 3D information the experiments were performed either in the in-plane as in the out-of-plane geometries and supported by ab initio calculations. A depth dependence in the chemical order is revealed and the magnetic behavior is interpreted within this framework.