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Two series of oxynitride glasses, RE–Si–Mg–O–N ( M = Mg , Al ; RE = La , Lu), have been studied by X-Ray photoelectron spectroscopy (XPS). The oxygen 1s photoelectron lineshape reveals a striking difference depending on the rare earth, both in the Mg series and in the Al series. Specifically, the oxygen 1s photoelectron lines of the La doped glasses are broader than the ones of the Lu doped glasses. This result is an experimental evidence that Lu has a larger affinity for oxygen versus nitrogen than La, as theoretically predicted by the first-principles calculations by Painter et al.

The aim of the present work was to evaluate the ability of reflection electron energy loss spectroscopy (REELS) as a non-destructive method to obtain depth profiles of dielectric constants, in a complementary way to transmission electron energy loss spectroscopy (TEELS). Two prototypical samples were used: a SrTiO3 single crystal and a 2 nm thick SiO2 layer on a Si single crystal. The single scattering cross-sections were decomposed into bulk, surface and interface contributions, with Drude–Lorentz parameters of the dielectric function fitted on available optical or TEELS data. We show that the evolution of the shape of the REELS single scattering cross-section with primary energy is qualitatively well reproduced by our model. In the case of the 2 nm SiO2 layer on Si, introducing the Si/SiO2 interface energy loss function contribution proved necessary to account for the interface plasmon peak observed, in very good agreement with reported TEELS measurements. REELS is well suited to studying dielectric properties of interfaces between a substrate and a nanometric layer.

The giant magnetoresistance (GMR) of multilayers in the current in plane (CIP) geometry can be understood by considering the spin-dependent scattering effects within the magnetic layers and at the non-magnetic/magnetic interfaces. In this paper we experimentally put on view the magnetoresistance effect created only by the spin-dependent reflection of electrons at the interfaces. Two epitaxial systems were studied, consisting of two ferrimagnetic insulators layers at low temperature (CoFe2O4 and Fe3O4) separated by a non-magnetic metallic layer (M = Au or Pt). The transport properties indicate that conduction of the CoFe2O4/M/Fe3O4 trilayers take place within the thin metallic layer. An optimal magnetoresistance up to 5% at 10 K associated to the switching from parallel to anti-parallel configuration of the magnetization of the two ferrite layers has been obtained. It is associated to the spin-dependent interfacial scattering contribution of the CIP-GMR. The augmentation of the GMR when decreasing the metallic layer thickness or increasing the oxide layers width confirms this GMR is essentially an interfacial phenomenon. The Pt-based system shows an additional Anisotropic Magnetoresistance (AMR), which is not evidenced in the Au-based trilayer. We suggest that this AMR comes from the polarization of the Pt layer by the ferrimagnetic oxides.

In this paper, a detailed study of the magnetic behaviour particularly at the Verwey transition is presented for some stoichiometric magnetite samples, as a function of their shape (film/bulk) and crystalline structure (single/polycrystal). Travelling solvent floating zone technique was first used to obtain crystalline bulk magnetite samples (1 1 0) oriented, while molecular beam epitaxy allowed us to grow a 68 nm Fe 3 O 4 (1 1 1) crystalline film on α -Al 2 O 3 (0 0 0 1) substrate. A 64 nm Fe 3 O 4 polycrystalline film grown on an Si/SiO 2 substrate using DC reactive magnetron sputtering method completed the sample series. An anomalous magnetisation behaviour has been evidenced in the Verwey temperature vicinity that is shown to depend on the applied magnetic field. A magneto-electronic model explains the origin of this behaviour. The maximum in the demagnetisation at the Verwey transition has been noticed very near 0.1 T (i.e. 90 kA/m, as found out on bulk magnetite). The amount and nature of structural defects have been proven to be determinant in the magnetisation process, and the approach to saturation is well described by the M = M S ( 1 - b / B n ) relation, with n = 2 in bulk magnetite crystal and n = 0.5 in epitaxial magnetite film, even across and below the Verwey transition after paraprocess corrections. These dependences exist in the 10–300 K temperature range, excluding isotropic point (near 130 K), and demonstrate that the antiphase boundaries (APBs) are responsible for the much more progressive approach to saturation observed in thin films than in bulk samples which is consistent with previous observations. On the epitaxial film, n is found to decrease down to 0.33 at isotropic point due to an enhanced super-exchange interaction through APBs.

A new pair of Kramers–Kronig (KK) dispersion relationships for the transformation of surface energy loss function Im[−1/(e + 1)] has been proposed. The validity of the new surface KK transform is confirmed, using both a Lorentz oscillator model and the surface energy loss functions determined from the experimental complex dielectric function of SrTiO3 and tungsten metal. The interband transition strength spectra (Jcv) have been derived either directly from the original complex dielectric function or from the derived dielectric function obtained from the KK transform of the surface energy loss function. The original Jcv trace and post-Jcv trace overlapped together for the three modes, indicating that the new surface Kramers–Kronig dispersion relationship is valid for the surface energy loss function. © 2004 Elsevier B.V. All rights reserved.

X-ray photoelectron spectroscopy (XPS) and time-resolved photoluminescence measurements have been performed on SON68 type nuclear glass and its leached product (gel) in order to characterize uranium environment in both materials. XPS measurements allow to detect two oxidation states in glass: about 20% of tetravalent uranium and 80% of hexavalent uranium, present under two types of environment. Comparison of XPS and time-resolved photoluminescence data allow to identify these U(VI) environments. In glass, they are respectively attributed to an uranate and an uranyl-type environment. In gels, U(IV) is oxidized in U(VI), the uranyl type environment is conserved whereas some amorphous uranyl hydroxides are formed. The U environment depends strongly on the leaching mode, in particular the leaching duration and water renewal rate.

A detailed study of the XPS binding energy shifts of Si 2p, O 1s and Zr 3d in a series of aluminosilicate glasses (a three oxide glass: SiO 2 –Al 2 O 3 –CaO, three four-oxide glasses: SiO 2 –Al 2 O 3 –CaO–TiO 2 , ZrO 2 or CeO 2 , along with a six-oxide glass SiO 2 –Al 2 O 3 –CaO–TiO 2 –ZrO 2 –CeO 2 ) is presented. Their composition is such that these glasses have the same mean electronegativity, so that no changes in the atomic charges is expected. The binding energy shifts are interpreted in terms of initial and final state effects, and the balance of charge transfer contribution and electrostatic effects is discussed. Referred to the ternary glass, the binding energy shifts of the Si 2p, O 1s and Zr 3d lines in the complex glasses are due to an initial state effect, as the extraatomic relaxation is similar along the glass series. These shifts originate from electrostatic Madelung effects, likely coming from a structural change induced by the presence of the other oxides TiO 2 , ZrO 2 and CeO 2 . These structural effects make it impossible to deduce information on the atomic charges directly from the binding energy shifts, even in a qualitative way.

This paper illustrates the potentialities and the innovative character of surface analysis techniques such as nuclear microprobe analysis (NMA) and X-ray photoelectron spectroscopy (XPS) in the research field of humic substances (HS), in particular to characterize HS/trace elements association. NMA showed its potentialities to characterize HS/ trace element associations at the microscopic scale and quantification of trace elements scavenged by humic acids (HA) and distribution and thickness of HA films sorbed onto silica surfaces after sorption tests. XPS was used to determine the chemical environment of iodine in natural HS. These techniques enabled us to establish association between HS colloids and numerous trace elements (both cations and anions, especially I) and to evidence a strong affinity of the smallest HA colloids (< 15 nm) for these elements. Moreover, associations between HS and I were clearly evidenced by NMA, through the study of the ternary system (SiO 2 /HA/I), as well as of natural HS. Chemical bonds implied in these associations may be of covalent character, as seen from XPS data. Combination of these techniques with Ion Coupled Plasma Mass Spectrometry (ICP-MS) was also necessary to analyze the aqueous solutions in contact with HS.

SummaryThe solubility behavior of uranium dioxide was studied under oxidizing and reducing conditions in simulated Belgian Boom clay water at 25 °C, on unirradiated fuel pellets. For a comparable period (1 month), the uranium concentration and the dissolution rate (10−12mol m−2s−1) in reduced medium were ∼30 times smaller than in oxidizing conditions (3 × 10−11mol m−2s−1). X-ray photoelectron spectroscopy analyses revealed that U(VI) and U(IV) oxidation states were present in the altered layer in both cases. In the case of the experiments performed in oxidizing conditions, this technique also revealed the formation of U-OH bonds. Although steady state was not reached, geochemical calculations allowed us to suggest a class of secondary phases able to control the uranium solubility in the long term.

In order to confine specific long-life actinides, originating from high-level nuclear wastes, lanthanum aluminosilicate glasses are considered. The aqueous corrosion behavior of these glasses is studied under static conditions ( T=96 ° C, duration between one and six months, glass surface area/leachate volume, S/V=0.3, 50 and 500 cm−1) by means of solution analysis (ICP-MS and spectrophotometry) and solid analysis (X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry). Tests performed at high S/V ratio lead to silicon releases of 10−3 mol l−1 while lanthanum releases reach 10−4.5 mol l−1. Phosphate anions are found to modify the dissolution, as an amorphous lanthanide phosphate is evidenced at the glass surface after the corrosion experiment. The lanthanide releases decrease in presence of phosphate, but the matrix dissolution process is not reduced.

The aim of this work was to get a better understanding of the O 1s binding energies Eb(O1s) in complex glasses of interest in nuclear waste storage in relationship with the mean charge on the oxygen atoms, relevant to acidic/basic properties of the glass. Three different glasses were considered: an aluminum borosilicate, a lanthanum aluminosilicate and a calcium aluminosilicate. The O1s binding energies were measured for each glass, along with individual simple oxides. The oxygen charges were estimated by an empirical method based on the electronegativity equalization principle. We show that for the first two glasses, the O 1s binding energy shift is driven by the changes in the oxygen charge induced by mixing the oxides to obtain the glass For the latter glass however, Madelung effects are of prime importance and the Eb(O1s) value of the glass cannot be predicted from the values in the individual oxides.

A mixed zirconium–lanthanum phosphate La1/3Zr2(PO4)3, noted LaZrP, is studied as a potential host for actinides issued from high-level nuclear wastes. Chemical durability is evaluated and compared with two other phases: a monazite phase LaPO4, noted LaP, and a mixed oxide phase La0.1ZrO2.15, noted LaZrO. Leaching tests are performed and followed by solution and solid analyses. Static tests are performed with a low ratio between surface of ceramic and volume of solution, S/V (about 0.1 cm −1 ). For LaZrP, the phosphate release rate decreases from 10−2 to 10 −3 g / m 2 / day , while zirconium and lanthanum releases remain very low ( −5 g / m 2 / day ). For the LaP phase, the phosphate release is about 10 times lower than for LaZrP and remains stationary, while the rate of the lanthanum release is below 10 −6 g / m 2 / day . Tests performed at high S/V (about 20 cm −1 ) reveal that the lanthanum release rates after 2 weeks were, respectively, at 10−9, 10−6 and 10 −4 g / m 2 / day for LaP, LaZrP and LaZrO phases. It is shown that NZP or monazite type ceramics are highly resistant to the leaching process.

The growth of Fe 3 O 4 on α-Al 2 O 3 (0 0 0 1) by atomic-oxygen-assisted molecular beam epitaxy has been studied by real time reflection high energy electron diffraction, low energy electron diffraction and X-ray photoelectron spectroscopy. The in-plane lattice parameter has been followed in situ during growth. For thickness below 1.5 nm, an iron oxide consisting on an expanded oxygen sub-lattice with an incomplete occupation of the Fe sites, is formed. This phase seems to be specific for a given thickness and independent of the growth conditions. For thickness above 1.5 nm, the long-range order of the Fe cations develops simultaneously with a decrease of the oxygen sub-lattice expansion. From this stage, Fe 3 O 4 (1 1 1) is stabilised.

The solubility behavior of uranium dioxide was studied under oxidizing conditions in the Belgian Boom clay water at 25°C up to 323 days. X-ray photoelectron spectroscopy analyses reveal that both U(VI) and U(IV) oxidation states are present in the altered layer. The U(VI)/U(IV) ratio in this altered layer increases up to 43 days and then remains constant throughout the whole test. In the leachate the uranium concentration also increases up to 43 days and then stabilizes. It is rather unlikely that the altered layer formed on the surface controls thermodynamically the uranium solubility. It is rather probable that the system has reached a pseudo-equilibrium governed by kinetics.

This paper presents a report on the XPS coherent binding energy (BE) data basis for iron and sulfur reference compounds that are representative of oxidation products formed onto the pyrite reactive surface. This work is unique for various reasons: (1) no recent (published after 1980) BE data basis with similar normalization method has been found in the literature; (2) the coupling of data collected from Fe and S photoelectron peaks and from charge transfer satellites of Fe allows for the simultaneous determination of the oxidation state and chemical environment; (3) the depth heterogeneities of samples can be taken into account by comparing the data obtained with Fe2p and Fe3p peaks, whose associated BEs are widely divergent. In the second part of the paper, the data in BEs obtained from reference compounds have been exploited to show the surface species of an oxidized pyrite in a HCO3− medium (10−2 M).

This paper illustrates the coupling of Neutron Activation Analysis (NAA), Proton Induced X-ray Emission (PIXE) and X-ray Photoelectron Spectroscopy (XPS) to quantify iodine contents in natural humic substances from different geochemical origins and to determine its chemical environment. Non negligible contents of iodine from some hundreds of μg/g to more than one weight per cent have been quantified in the humic substances, and iodine seems to be attached to the organic matter by covalent bonds.

INTRODUCTION Saturation effects related to indirect detection of x-ray absorption spectra, like total electron yield (TEY) or fluorescence yield measurements, are of interest for applications of linear and circular dichroism (respectively XMLD and XMCD). XMCD studies on Fe, Co and Ni using the L-edge TEY spectra have shown that saturation can cause errors in excess of 100% on the extracted magnetic moment values [1]. In TEY measurements, the intensity is given by

The aqueous corrosion behavior of lanthanide aluminosilicate glasses has been studied under static conditions (T=96°C, duration=1 and 3 months, glass surface area/leachate volume, S/V=0.3 cm−1) by means of solution and solid analyses. It was found that these glasses exhibit a high chemical durability. The influence of yttrium, magnesium and nitrogen, which are supposed to improve the mechanical properties, on the chemical durability, has been investigated. After a one-month experiment, lanthanum and yttrium releases were found to be about 10−7 mol l−1, while silicon and aluminum releases were about 10−5 mol l−1. Yttrium seems to improve the chemical durability. The presence of nitrogen does not seem to modify the glass constituents releases, but seems to improve the surface state of the altered glass. XPS experiments reveal that lanthanum and yttrium are more concentrated near the surface (20–30 A) of the glass after the leaching test.

The growth of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ on $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ by atomic oxygen---molecular-beam epitaxy has been studied by real time reflection high-energy electron diffraction, low energy electron diffraction, and x-ray photoelectron spectroscopy. For equivalent thicknesses below 2 ML, epitaxial islands of an uncommon iron oxide are evidenced. This metastable FeO(111)-like phase exhibits a strongly expanded in-plane parameter, and contains ferric ${\mathrm{Fe}}^{3+}$ ions instead of ferrous ${\mathrm{Fe}}^{2+}$ ones. This phase seems to be specific to a given thickness (\ensuremath{\sim}2 ML), independently of the substrate. For higher equivalent thicknesses, epitaxial islands of hematite $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ begin to nucleate. The amount of the hematite phase increases whereas that of the FeO(111)-like phase decreases with deposition time. For a deposit equivalent to one hematite unit cell, islands coalesce and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ grows in a two-dimensional mode, with the hematite bulk in-plane parameter.

The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO2 and CeO2 are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific atomic arrangement and electronic structure. The structures at the edge originate from oxygen 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighbouring shells of the central oxygen atoms.

An α-Fe2O3 (0001) film epitaxially grown on an α-Al2O3(0001) substrate has been used for studying the polarization dependence of the XAS O–K edge. Experimentally, strong differences between the spectra recorded at θ=90° and θ=25° photon polarization orientations with respect to the surface are observed. Multiple scattering calculations allow one to explain the physical origin of each angle-dependent feature, in terms of single and multiple scattering events occurring in and out of the film surface plane.

This study presents a comparison of the experimental O K-edge absorption spectrum of haematite (α-Fe2O3) with a theoretical spectrum obtained from all-electron ab initio periodic Hartree-Fock calculations of the antiferromagnetic R3 structure. There is good overall accord between the two spectra and agreement to within about 0.2eV for the major peak-to-peak separations. From a consideration of the empty p density of states (DOS), calculations predict the first 20eV of the absorption to result almost entirely from excitations of the type |O 1s〉 → |O np〉 with negligible participation of the Fe p states. The lower part of the spectrum is attributed to O p states hybridized with Fe d states while, at energies greater than 5eV above the absorption edge, in the region of the broad absorption at about 11 eV, calculations suggest that the predominant hybridization is with the Fe s states. Important differences are found between the near-edge empty p DOS of the antiferromagnetic R3 and ferromagneti...

X-ray absorption spectroscopy (XAS) is a powerful tool for investigating the local atomic and electronic structure around selected types of atoms in materials, independently of the presence of long or medium range order. We show on several examples, how XAS can bring useful information in the case of ceramic materials, when combined to theoretical modelling of the absorption spectra. Local structure around Ce doping atoms in Y 2 O 3 , structural modifications of PbO during the α–β phase transformation are presented first. We illustrate in the case of iron oxides, the possibilities offered by XAS at different edges (Fe and O K, Fe L 23 ) in determining both the valence and site symmetry of the iron atoms. We conclude with the perspectives provided by the third generation synchrotron sources in the field of microanalysis by XAS.

We report on two independent measurements of the spin polarization at the Fe 3 O 4 / γ - Al 2 O 3 interface. Fe 3 O 4 / γ - Al 2 O 3 (1 1 1) bilayers have been epitaxially grown by oxygen-assisted molecular beam epitaxy onto α - Al 2 O 3 (0 0 0 1) substrates and some were covered ex situ by a cobalt electrode. Spin resolved photoemission yield to a negative spin polarization ( ≃ - 40 %) while transport measurements on Fe 3 O 4 / γ - Al 2 O 3 / Co magnetic tunnel junctions show a positive TMR, which suggest that the tunnel spin polarization at the Fe 3 O 4 / γ - Al 2 O 3 interface is positive.

We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al2O3) by Grazing Incidence X-ray Scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.

High-resolution x-ray and neutron scattering studies on powder samples of pure and Zr-doped (5 at.%) show that mesoscopic compositional inhomogeneities exist in the doped compound which are unlikely to be revealed by conventional diffraction methods. Even if the structural refinement of the doped compound gives quite good reliability factors within a single-phase model, a more careful analysis of the data proves the existence of two phases. The main phase (90%) has a composition and a structure very close to those of pure . The phase richer in Zr (10%) segregates and Zr atoms show site selectivity. The abnormally large value of the thermal displacement parameter of the site affected by the Zr substitution ( at 294 K) is interpreted on the basis of a split-atom model characterized by an off-site displacement of 0.38 A. Additional O atoms occupy the vacancies in the structure and provide the charge balance for the Zr dopants.

α-Fe 2 O 3 , FeO and Fe 3 O 4 compounds are characterized by X-ray Absorption Spectroscopy (XAS). The experimental Fe L 2,3 spectra are accounted for thanks to a configuration interaction calculation (Cl). The contributions of the various iron site symmetries and valencies are discriminated and the nature of insulating gap is deduced for each compound. The oxygen K-edge spectra are successfully interpreted by full multiple scattering (MS) calculations. Using increasing cluster sizes around the excited atom in the MS simulations, the features present on the spectra of each iron oxide are linked to its specific atomic arrangement and electronic structure.

\ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$,${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, and FeO compounds are characterized by means of x-ray-absorption near-edge-structure spectroscopy at the oxygen K edge. Using increasing cluster sizes around the excited atom in the full multiple-scattering simulations, we are able to link the features present in the spectra of each iron oxide to its specific atomic arrangement and electronic structure. The prepeak structure is successfully reproduced and interpreted as transitions from the oxygen 1s core state to antibonding oxygen 2p states hybridized with Fe 3d orbitals. Their intensity and shape depends on the Fe site symmetry, the occupation number of the d levels, and the O-Fe bond length of each different iron oxide. Higher lying spectral features are shown to be related to scattering of the photoelectron by a particular oxygen shell and an extended x-ray absorption fine structure--like relation is established between their energy position and the distance of the corresponding shell from the photoabsorber.

The purpose of the present work is to revisit the (1 X 2) reconstruction of the α-Fe 2 O 3 (1012) surface, in order to answer the questions of composition and in-depth extension of the reconstructed layers, which still remained. We have thus studied by LEED (low energy electron diffraction) and AES (Auger electron spectroscopy) the modifications of the surface structure and composition as a function of different in situ (T, p o2 ) treatments. The extension in depth of the (1 X 2) reconstruction increases with UHV annealing temperature (from 400°C to 1000°C, p o2 = 10 -10 mbar). Extending over several repeat units of the (1012) stacking sequence, the reconstructed layer reaches about 30 A depth after 900°C annealing. A model of an ordered oxygen deficient layer in which one half of the octahedral iron sites is replaced by ordered tetrahedral ones, is proposed.

The electronic structure of the α-Al 2 O 3 (0001) surface is calculated using a self-consistent tight-binding method, taking into account both the covalent and the electrostatic effects. For the ideal Al-terminated surface, a surface state is present in the band gap, 2 eV below the bottom of the conduction band (CBm). When the surface is relaxed, this surface state is pushed into the conduction band, 2.7 eV above CBm. We show that this 4.7 eV shift induced by the surface relaxation is strongly related to an enhancement of the AlO charge transfer on the ideal surface, which shows an increased ionicity compared to the relaxed one. As regards the energy position of this surface state with respect to the bulk band gap, the relative importance of the bond breaking on covalent effects and on the effective atomic levels energy positions is discussed in comparison with previous published theoretical works.

We have studied the first stages of the growth of copper on two different α-Al 2 O 3 (0001) alumina substrates, presenting respectively a (1 × 1) and a (√31 × √31) R ± 9° LEED (low energy electron diffraction) pattern. The study was realised by means of surface sensitive X-ray absorption spectroscopy (SEXAFS) at the CuK edge. For equal amounts of deposited copper (50% of an equivalent monolayer), we found quite different copper/alumina interfaces as a function of the substrate reconstruction. In the case of the (1 × 1) surface, we noticed the coexistence of copper dimers and very small copper nanoclusters (about 1 shell) weakly bonded to the substrate via the Al surface atoms. In the case of the (√31 × √31) R ± 9° reconstructed substrate, we detected slightly bigger Cu nanoclusters than those present on the (1 × 1) surface. We also observe that some Cu atoms are incorporated to the substrate, occupying the vacancies of the reconstructed surface. No CuO bonds are detected in both interfaces. This result can be understood in the case of the aluminium rich (√31 × √31) R ± 9° surface. However, it is more surprising in the case of the (1 × 1) surface and supports the hypothesis of an Al-terminated (1 × 1) alumina substrate.

We have studied the formation of the Ce Y 2 O 3 interface by X-ray photoelectron spectroscopy (XPS) from a Ce vapor deposi onto a polycrystalline stoichiometric Y2O3 sample. The substrate temperature ranged between the room temperature and 850°C, which is above the cerium melting point (795°C). The shape of the Ce 3d photoelectron lines shows that even at room temperature, an interfacial compound Ce2O3−x forms over a few atomic layers. When the substrate temperature increases, the thickness of this oxidized layer increases. A model based on a diffusion mechanism of oxygen atoms coming from the oxide substrate is proposed to account for the experimental results.

The influence of local order around the iron site on Fe 2p x-ray-absorption spectra is studied for \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, FeO, ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, and \ensuremath{\gamma}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. The experimental spectra are accounted for thanks to a configuration-interaction calculation of Fe ${\mathit{L}}_{2,3}$ x-ray-absorption edge shapes, explicitly taking into account the exact first-neighbor surrounding of the cation acting through crystal-field splitting and hopping terms. The peak's splittings and intensities are well reproduced: the contributions of the various iron site symmetries and valencies are discriminated. The nature of insulating gap for each compound is deduced.

Despite the extremely broad technical applications of the Si/SiO 2 structure, the equilibrium wetting properties of silicon oxide on silicon are poorly understood. Here, we produce new results in which a solid-state buffer method is used to systematically titrate oxygen activity about the Si/SiO 2 coexistence value. The equilibrium morphology at the Si(001) surface over >8 decades of PO 2 about coexistence is revealed to be a uniform sub-stoichiometric SiO x film of sub-nanometer thickness, coexisting with secondary island structures which coarsen with annealing time. A new thermodynamic method using chemical potential to stabilize and control surficial oxides in nanoscale devices is suggested.

Symposium on Nanoscale Tailoring of Defect Structures for Optimized Functional and Multifunctional Oxide Films held at the EMRS 2007, Strasbourg, FRANCE, 2007; International audience; Using surface X-ray diffraction, we investigated 20 nm thick alpha-Fe2O3(0 0 0 1) thin films deposited on alpha-Al2O3(0001) and Pt(111) single crystals. The films were grown in identical conditions by atomic oxygen assisted molecular beam epitaxy techniques. Both substrates offer close lattice parameter misfits. On sapphire an isostructural epitaxial relationship is observed and a 30 degrees in plane rotation of the lattice for Pt(111). The crystalline quality of the film deposited on Pt(111) is much better and contained less parasitic contributions. The improved crystalline quality of alpha-Fe2O3(0001) layers on Pt(111) is attributed to the presence of a very well ordered interfacial dislocation network which is missing when alpha-Al2O3 is used as substrate. (C) 2007 Elsevier B.V. All rights reserved.

We report on the antiphase boundaries network of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ thin films. $5\text{\ensuremath{-}}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\text{\ensuremath{-}}\mathrm{nm}$-thick samples were epitaxially grown by molecular beam epitaxy onto $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ substrates. The magnetic properties of the samples have been interpreted within the framework of a one-dimensional model of antiphase boundary (APB), which predicts that the magnetization is given by ${M}_{\ensuremath{\infty}}(1\ensuremath{-}b∕\sqrt{H})$ in the approach to saturation regime. Transmission electron micrographs of several samples were used to extract the statistical parameters of the APB network, particular emphasis being put on the relevance and statistical significance of the studied parameters. The mean antiphase domain size $\overline{D}$, as the antiphase boundaries characteristic length ${l}_{0}$ extracted from a fractal analysis, vary as the square root of film thickness/deposition time and are within the $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ range. The APB density was found to vary as $1∕{l}_{0}$ as expected from the fractal dimensions of the network. The dependency of the $b$ parameter of the magnetic model on the APB density is finally analyzed in the light of micromagnetic simulations of chains including finite size antiphase domains and two APBs.

The discovery of giant magnetoresistance in Fe/ Cr multilayers 1 has been followed by a large research effort for almost two decades aiming at developing new materials suitable for spintronics. The interest of a given material is mainly expressed by its spin polarization P =N↑ �N↓ /N↑ +N↓, N↑ N↓ standing for majority spin minority spin electron density of states at the Fermi level. This property is particularly important for magnetic tunnel junctions stacks of two magnetic electrodes separated by a thin insulating layer. 2 Indeed, the tunnel magnetoresistance TMR, corresponding to the variation of resistance between parallel and antiparallel configurations of the electrode magnetizations, is given in a simple view by the Julliere formula: 3 TMR =2 P1P2 / 1� P1P2, P1 and P2 being the spin polarizations of the two electrodes. Magnetite Fe3O4 is thus expected to be an interesting electrode material, for band structure calculations predict a half metallic behavior P = �100%, the con

We study the magnetic behavior of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ thin films with thicknesses between $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The films are epitaxially grown on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ single crystals by atomic-oxygen-assisted molecular beam epitaxy. The ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ thin films exhibit high structural order with sharp interfaces and low roughness and exhibit a Verwey transition for thicknesses above $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. However, the samples have magnetic properties that deviate from the bulk ones. The magnetic moment varies between $2.4{\ensuremath{\mu}}_{B}$ for $5\text{\ensuremath{-}}\mathrm{nm}$-thick film and $3.2{\ensuremath{\mu}}_{B}$ for $50\text{\ensuremath{-}}\mathrm{nm}$-thick film in a field of $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, which is lower than that of bulk samples ($4.1{\ensuremath{\mu}}_{B}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ formula). Still the magnetic saturation is never reached, even in fields as large as $2\phantom{\rule{0.3em}{0ex}}T$. The thinner the film, the slower the approach to saturation. Structural analysis, performed using high-resolution transmission electron microscopy, reveals the presence of antiphase boundaries (APB's), the density of which decreases when the thickness increases. Using a model of ferromagnetic domains separated by antiferromagnetically sharp interfaces, we show that the slow approach to saturation observed in the films as a function of thickness is driven by the APB density.

The temperature-dependent Verwey transition in a 500 A (111) thin film of Fe3O4 (magnetite) has been studied using soft-x-ray emission spectroscopy at room temperature and below the transition temperature TV. The Fe L2,3 x-ray emission spectra show an increase in the intensity of the L2 emission relative to the L3 emission below TV. This is independent of the excitation energy and is attributed to a metal–insulator transition across TV. Comparison of the Fe L3 emission and O Kα spectra with LDA band structure calculations supports the suggestion of charge ordering in Fe3O4 at low temperature.

ACE; The chemical stability of a CeYSiAlO glass was examined after leaching with bidistilled water (autoclave: T=90$^\circ$) during 1 month. The initial $S/V$ ratio was approximately 0.06 cm$^{−1}$ and the pH was free to drift during the experiments (initial pH $\simeq$ 5.5). The leached samples were analyzed using scanning electron microscopy coupled with energy disperse X-ray spectrometer, X-ray photoelectron spectrometry and X-ray diffraction. Dissolution kinetics were also followed by inductively coupled plasma mass spectrometry and spectrophotometry analyses of the liquid medium. The results indicate that Ce is oxidized during leaching which results in the formation of a thick amorphous Ce(IV)-containing layer, possibly CeO$_2$ with Y and probably −OH (and/or H$_2$O) species. It is hypothesized that the low solubility of Ce(IV) oxide (and/or hydroxide) is mainly responsible for the formation of this layer, thus, preventing the release of Y and Ce and to a lesser extent of Si and Al after a few days of leaching.

Characterization of thin surficial films of oxides has become the focus of increased interest due to their applications in microelectronics. The ability to experimentally determine the electronic structure and optical properties of oxide materials permits the direct study of the interband transitions from the valence to the conduction band states. In the past years there has been much progress in the quantitative analysis of transmission electron energy loss spectroscopy (TEELS) in the electron microscopeHere we employed reflection electron energy loss function (REELS) as well as vacuum ultraviolet (VUV) spectroscopy to determine the dielectric functions of oxide materials, i.e. Al2O3, ZrO2 and SrTiO3. The two main steps in the analysis are the removal of the effects of multiple scattering from the REELS spectra followed by application of the Kramers-Kronig dispersion transforms to the single scattering energy loss function to determine the conjugate optical variable and then the complex dielectric function. The surface and bulk plasma resonance spectra for these oxide materials have been determined from VUV and REELS, along with the influence of primary electron energy on the REELS results. The relative contribution of surface and bulk plasmon oscillation in REELS has been investigated. Comparison with VUV results and existing TEELS results indicate that Kramers-Kronig analysis can also be applied to REELS spectra and the corresponding conjugate optical properties can be obtained. Quantitative studies of the electronic structure and optical properties of thin surficial films using VUV and REELS or TEELS, represent a new avenue to determine the properties of these increasingly important films.

X-ray magnetic linear dichroism (XMLD) at the Fe ${L}_{2}$ absorption edge has been used for investigating the magnetic properties of thin epitaxial $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ layers (20 and 80 \AA{} thick). When cooling below 263 K, bulk hematite undergoes a 90\ifmmode^\circ\else\textdegree\fi{} rotation of the antiferromagnetic axes from perpendicular to parallel to the c axis (Morin transition). The ${L}_{2}\ensuremath{-}\mathrm{XMLD}$ results indicate that the 80-\AA{}-thick layer does not undergo such a transition at least down to 125 K. This absence is attributed to important magnetoelastic interactions due to the 0.3% expansion of the lattice parameter in the c plane. In the case of a 20-\AA{}-thick layer, we do not detect any ${L}_{2}\ensuremath{-}\mathrm{XMLD}$ signal, regardless of the temperature. The absence of antiferromagnetic ordering might be caused by the combined effect of several factors: finite size effects, changes in the local crystal field of the ${\mathrm{Fe}}^{3+}$ ion, and magnetoelastic interactions, coming from the 0.8% expansion of the lattice parameter in the c-plane.

The degree of ionic/covalent character in oxides has a great influence on the electronic structure and the material's properties. A simple phenomenological rule is currently used to predict the evolution of covalence/ionicity in mixed oxides compared to the parent ones, and is also widely used to interpret the x-ray photoelectron spectroscopy (XPS) binding-energy shifts of the cations in terms of charge transfer. We test the validity of this simple rule and its application to XPS of mixed oxides with a prototypical system: zircon ${\mathrm{ZrSiO}}_{4}$ and parent oxides ${\mathrm{ZrO}}_{2}$ and ${\mathrm{SiO}}_{2}.$ The ionic charges on Si, Zr, and O were extracted from the density functional theory in the local density approximation calculations in the plane-wave formalism. In agreement with the predictions of the phenomenological rule, the most ionic cation (Zr) becomes more ionic in ${\mathrm{ZrSiO}}_{4}$ than in ${\mathrm{ZrO}}_{2},$ while the more covalent one (Si) experiences a corresponding increase in covalence with respect to ${\mathrm{SiO}}_{2}.$ The XPS chemical shifts of the O $1s,$ Si $2p,$ and Zr ${3d}_{5/2}$ photoelectron lines in the three oxides were measured and the respective contributions of charge transfer and electrostatic effects (initial state), as well as extra-atomic relaxation effects (final state) evaluated. The validity of the phenomenological rule of mixed oxides used in x-ray electron spectroscopy as well as the opportunity to use the $\mathrm{O}1s$ binding-energy shifts to derive a scale of covalence in silicates is discussed.

Nanometric films of iron oxides (Fe3O4, α and γ Fe2O3) of high crystalline order and purity are epitaxially grown on α-A12O3(0001) by atomic oxygen assisted MBE. A complete characterization of the films structure has been performed by in situ LEED and RHEED, and ex situ GIXRD using synchrotron radiation. The films grown at room temperature and post annealed at 400°C and 700°C (po2=10−6 Torr) are respectively metastable γ-Fe2O3 (111) and α-Fe2O3 (0001). For a substrate temperature of 450°C during growth, Fe3O4 (111) is directly obtained. GIXRD shows an in-plane expansion of the films, which decreases with thickness (0.8 and 0.2% for film thickness of 20 and 80 Å, respectively).

Reflection Electron Energy Loss Spectroscopy (REELS) is a valuable tool to characterize the electronic structure of insulating surfaces and nanometric films. The REELS inelastic single scattering cross section obtained after removing multiple losses is related to the complex dielectric function in the surface layers. Moreover the probing depth can be varied by changing the primary energy of electrons. One can expect in essence to obtain non-destructive REELS depth profiles, where the evolution of the REELS spectrum translates to the in-depth evolution of the dielectric constant. We present a REELS study of pure and Nb-doped SrTiO3 single crystals, with primary energies ranging from 200 eV to 1500 eV so as to vary the probing depth between 0.7 and 3 nm. The resulting variations in the shape of the spectra are qualitatively interpreted by considering the energy loss functions corresponding to a double interface SrTiO3/carbon monolayer/vacuum, on the basis of the experimental dielectric function derived from TEELS measurements on strontium titanate and amorphous carbon.

We present an in-depth study of the magnetotransport properties of epitaxial Fe3O4 films as a function of film thickness. The films, grown on α-Al2O3(0001) single crystals by atomic-oxygen assisted molecular beam epitaxy, exhibit high structural order and abrupt interfaces. These films contain antiphase boundaries (APBs), the density of which is strongly dependent on film thickness. A series of resistivity and magnetoresistance measurements demonstrate a systematic evolution of these properties with decreasing film thickness, revealing the impact of APBs on the transport properties in the films. We present a model based on the spin-polarized transport across an antiferromagnetically coupled APB in order to successfully reproduce our experimental data over a large range of applied magnetic fields. The comparison of this model with experimental results further clarifies the mechanism of the anomalous magnetotransport behavior in Fe3O4.

We report on the elaboration and physical properties of fully epitaxial Fe3O4∕Al2O3 bilayers to be included in magnetic tunnel junctions. Uncovered Fe3O4(111) single layers and bilayers were epitaxially grown onto sapphire (0001) substrates. Appropriate growth conditions lead to a stoichiometric Fe3O4 layer both at the Fe3O4∕Al2O3 interface and on the whole thickness, as checked by in situ x-ray photoelectron spectroscopy and by the observation of the Verwey transition, respectively. Transmission electron microscopy demonstrates the epitaxial growth of a γ-Al2O3 layer on top of Fe3O4(111), which insulating properties have been monitored by conductive tip atomic force microscopy.