Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.     

Structural,magnetic and XPS studies of Sn0.95Co0.05O2-0.05 and Sn0.95Fe0.05O2-0.05 nanoparticles

Structural, microstructural, X-ray photoemission spectra (XPS) and magnetic properties of transition metal ion [5 mol% of Co (SC5) and Fe (SF5)]-doped SnO₂ nanoparticles have been studied. The SC5 and SF5 nanoparticles were synthesized by a chemical route using polyvinyl alcohol as surfactant. The doped SnO₂ crystallites were found to exhibit a tetragonal rutile structure and the average grains size was measured by the Scherer relation of X-ray diffraction. Transmission electron micrographs showed that the average grain size of SC5 is smaller than SF5. SC5 nanoparticles showed strong ferromagnetic behaviour but SF5 exhibited an F-centre exchange (FCE) mechanism. Temperature-dependent magnetization showed the values of phase transition temperature. XPS confirmed the presence of Sn–O–Co and Sn–O–Fe bonds in these SC5 and SF5 nanoparticles. The oxidation states of Sn, Co and Fe were found to be +4, +2 and +2, respectively, while the core level XPS peaks of Sn 3d, O 1s, Co 2p and Fe 2p were analyzed.     

Soft X-ray magnetic circular dichroism of Heusler-type alloy Co2MnGe

Co and Mn 2p core absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectra have been measured for the ferromagnetic ternary alloy Co₂MnGe. The observed Co 2p XAS spectrum can be understood on the basis of the unoccupied Co 3d partial density of states, whereas the overall features of the Mn 2p XAS and XMCD spectra have been partly reproduced by the Mn 2p⁵3d⁶ final state multiplets. We have found that the orbital polarization of the Co 3d and even the Mn 3d states are recognizable, which suggests that a spin-orbit coupling should be taken into account in the energy band structure in order to reproduce the half metallic nature of this alloy.     

X-ray photoelectron spectroscopy study of sputter-annealed Ni2.1Mn0.9Ga surface

The modifications in surface composition of Ni_2.1Mn_0.9Ga ferromagnetic shape memory alloy have been investigated using X-ray photoelectron spectroscopy (XPS) under various sputtering and annealing conditions. XPS core-level spectra show that sputtering makes the surface Ni rich. However, by annealing, the Mn content at the surface increases and at about 390 °C the bulk composition is restored. The valence band spectra show evidence of Ni related extra states for the sputtered surface, which decrease with annealing. This behavior is in agreement with the change in surface composition derived from the core-level spectra.     

Multiple scattering approach to Co K-edge XMCD and XANES spectra in Gd–Co alloys

We have measured and analyzed Co K-edge X-ray absorption near edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) near edge spectra in crystalline and amorphous GdCo₂ alloys. We have used a semi-relativistic full multiple scattering approach to the analyses of the XMCD spectra. A general formula is obtained which is applicable to randomly oriented systems in space. Useful information is obtained on both the electronic and geometric structure around a Co atom. The difference in XANES and XMCD spectra between crystalline and amorphous GdCo₂ is well explained by models referring to the anomalous X-ray scattering result, where three Gd atoms in the second shell are removed away. The calculated XANES are not so sensitive to the electronic structure, whereas the calculated XMCD spectra are rather sensitive to the spin polarization on Co atoms. The result shows that the spin polarization on Co atoms in GdCo₂ alloys is smaller than that in Co metals.     

Tuning the coercive field of Ni and CuNi thin films with the embedding of Co nanoparticles: An element-specific study

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Characterization by electrolyte electroreflectance and X-ray photoelectron spectroscopy of amorphous Ni59Nb40Pt1−xSnx alloys and their activation by HF solutions

The influence of chemical etching with HF on the nature of the surface of amorphous Ni₅₉Nb₄₀Pt_1−xSn_x alloys has been studied in situ by electrolyte electroreflectance (EER) and ex situ by X-ray photoelectron spectroscopy (XPS). The EER spectrum of the untreated alloy in 0.5 M H₂SO₄ shows a bipolar band, which disappears after the HF treatment yielding a structureless EER spectrum similar to that of Pt, but reappears after several hours in the 0.5 M H₂SO₄ electrolyte. This process of dissolution by HF of an oxide species and its reappearance after a few hours cannot be followed by XPS, since the time interval between sample withdrawal from the electrolyte and actual measurement is of a few hours as well. XPS spectra showed the presence of metallic Nb before and after the HF treatment, and that niobium pentoxide was the main species in the as-quenched alloy, but that after treatment with HF it became a minor component, the main one being NbO. The main effect of the HF treatment is to produce a platinum enrichment of the surface, as unequivocally determined by cyclic voltammetry, XPS and EER. After Ar sputtering for 9 min the XPS spectrum of the untreated alloy showed metallic Nb only, while in the HF-treated alloy the peaks of metallic Nb were swamped by those of NbO and some Nb₂O₅. We interpret this difference as being due to the formation by the HF attack of a porous Nb film which becomes oxidized in the electrolyte and/or during transfer to the spectrometer, and so thick that it is not eliminated by Ar sputtering for 9 min.     

Structural and magnetic properties of <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript>–FeCoPt nanoparticles prepared by rf-sputtering

L1₀ ordered Fe–Co–Pt magnetic nanoparticles were fabricated using radio frequency sputtering at 613 K on single-crystalline NaCl and MgO substrates. The growth of particles was studied by varying the sputtering gas pressure and sputtering duration. High-resolution electron microscopy and electron diffraction studies revealed a formation of particles with L1₀-related variant domains. The structure and magnetic studies showed the preferential in-plane formation of the magnetic easy axis. The coercivity of the films increased with annealing at 773 K. The Co addition to the FePt alloy increased the saturation magnetization to 1300 emu/cm³.     

Surface characterization study of a Pd/Co3O4 methane oxidation catalyst

A surface characterization study using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on a 5 wt.% Pd/Co₃O₄ methane oxidation catalyst before and after exposure to a mixture of CH₄ and O₂ in N₂ at 250 °C for a period of 6 days. The primary peaks observed in the XPS survey spectra are the Co 2p, Pd 3d, O 1s and C 1s, along with Co, Pd and O Auger peaks. High-resolution Pd 3d spectra reveal that Pd exists on the surface predominantly as PdO, with no apparent change in chemical state during reaction. High-resolution XPS Co 2p and O 1s spectra reveal an accumulation of CoOOH and a depletion of CoO in the near-surface region during reaction. ISS analysis with intermittent 1-keV Ar⁺ sputtering was used to obtain depth profiles from the catalyst before and after reaction. The results indicate that the Pd/Co concentration ratio decreases with sputtering and that this ratio is larger for the as-prepared catalyst indicating that morphological changes occur during reaction. The ISS depth profile spectra obtained from the catalyst after reaction indicates the presence of an oxyhydroxide layer throughout the near-surface region. This observation is consistent with the XPS data indicating accumulation of hydroxide and oxyhydroxide species at the surface during reaction.Based on these data and the results of related studies, a reaction mechanism is proposed. In this mechanism, methane dissociatively chemisorbs to form a surface methoxy species and CoOOH. The remaining hydrogen atoms are stripped from the methoxy species leaving an active adsorbed C species which reacts with surface oxygen and a hydroxyl group to form an adsorbed bicarbonate ion which then decomposes to form CO₂ and a surface hydroxyl group. These hydroxyl groups also react to form H₂O and then more O₂ adsorbs dissociatively at the vacant sites.     

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.     

Electronic structure,optical dielectric constant and born effective charge of EuAlO3

EuAlO₃ (EAO) is synthesized by the sol–gel process. The Rietveld refinement of the X-ray diffraction data shows that the material has orthorhombic structure with Pbnm space group. The density functional theory calculations are initiated with the experimental lattice parameters. The full potential linearized augmented plane wave method and projector augmented wave method are used to investigate the ground state properties of EAO. An indirect band gap of 1.8 eV is observed with the valence band maximum at the Γ point and the conduction band minimum at the R point. The X-ray photoemission spectroscopy (XPS) spectra of EAO are obtained in the energy window of 0–1000 eV. Using the electronic density of states, the valence band (VB) spectrum of EAO is generated and compared with the observed VB-XPS spectrum. The optical dielectric constant and the refractive index of the material are calculated for the photon energy radiation. The optical properties show a considerable anisotropy in the material. The Born effective charge of various elements and the dielectric tensor of EAO have been calculated.     

Oleic acid coating on the monodisperse magnetite nanoparticles

Monodisperse magnetite nanoparticles provide a more factual model to study the interface interactions between the surfactants and magnetic nanoparticles. Monodisperse magnetite nanoparticles of 7 and 19 nm coated with oleic acid (OA) were prepared by the seed-mediated high temperature thermal decomposition of iron(III) acetylacetonate (Fe(acac)₃) precursor method. Fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS) reveal that the OA molecules were adsorbed on the magnetic nanoparticles by chemisorption way. Analyses of transmission electron microscopy (TEM) shows the OA provided the particles with better isolation and dispersibility. Thermogravimetric analysis (TGA) measurement results suggest that there were two kinds of different binding energies between the OA molecules and the magnetic nanoparticles. The cover density of OA molecules on the particle surface was significantly various with the size of magnetite nanoparticles. Magnetic measurements of the magnetite nanoparticles show the surface coating reduced the interactions among the nanoparticles.     

XMCD study of the local magnetic and structural properties of microcrystalline NdFeB-based alloys

X-ray Magnetic Circular Dichroism (XMCD) technique was used to investigate local magnetic properties of microcrystalline Nd_10.4Zr_4.0Fe_79.2B_6.4 samples, oriented along either easy or hard magnetization direction. The Nd L _2,3 and Fe K edge XMCD spectra were measured at room temperature under a magnetic field of T. A very strong dependence of XMCD spectra on the sample orientation has been observed at the NdL _2,3-edges, whereas the Fe K-edge XMCD spectra are found to be practically isotropic. This result indicates that magnetic anisotropy of NdFeB-based alloys originates from the Nd sublattice. In addition, element selective magnetization curves have been recorded by measuring the intensity of XMCD signals as a function of an applied magnetic field up to T. To find a correlation between local and macroscopic magnetic properties of studied samples we compared these data with magnetization curves, measured by vibrating sample magnetometer up to T. Results are important for understanding the origin of high-coercivity state in NdFeB-based intermetallic compounds.     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000     

Threshold photoemission spectroscopy in solids

Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed.     

X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 (x = 0–0.05) thin films grown by pulsed laser ablation

The electronic and magnetic properties of well characterized Mo_1???xFe_xO₂ (x = 0–0.5) thin films that show ferromagnetism at room temperature (RT) have been investigated by the means of near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments at the O K-, Fe L-, and Mo M-edges. The NEXAFS spectra at O K- and Mo M_3,2 -edges show a strong hybridization of O 2p-4d Mo orbitals, and Mo ions change their symmetry with the substitution of Fe ions into MoO₂ matrix. The Fe 2p NEXAFS/XMCD spectra exhibit multiple absorption peaks and an appreciable XMCD signal that persists even at RT. These results demonstrate that Fe is in a mixed valence state of Fe^2?+?–Fe^3?+?, substituting at the Mo site and that the Fe^2?+?/3?+? ions are ferromagnetically polarized.     

Field dependence of spin and orbital moments of Fe in L10 FePt magnetic thin films

The field dependence of spin and orbital magnetic moments of Fe in L1₀ FePt magnetic thin films was investigated using X-ray magnetic circular dichroism (XMCD). The spin and orbital moments were calculated using the sum rules; it was found that the spin and orbital moment of Fe in L1₀ FePt films are ∼2.5 and 0.2 μ_B, respectively. The relative XMCD asymmetry at Fe L₃ peak on the dependence of applied field suggested that the majority magnetic moment of L1₀ FePt films resulted from Fe.     

Phase composition at surface of Fe-3%Si alloy

The structure, phase and chemical compositions of surface layers in different depths of Fe-3%Si alloy were investigated. According to the X-ray Photoelectron Spectroscopy (XPS) spectrum (penetration depth of up to ∼ 1nm) of the as-prepared sample, a layer of SiO₂ was present on the top. After the subsequent Ar⁺ sputtering (removing the SiO₂ layer), a segregation of Si atoms and two other phases were observed. The phases were described as the cubic c-FeSi and Fe₃Si. The emission⁵⁷Fe M?ssbauer spectra confirmed a presence of these phases. The α-Fe and solid solution of α-Fe + 1wt.%Si were recognized in the Conversion Electron M?ssbauer spectra (penetration depth ∼ 300nm) while the M?ssbauer spectra taken in scattering geometry with detection of 14.4 keV gamma radiation (scanning depth of ∼ 30 μm) indicate Fe-3wt.%Si solid solution as a main phase. Presented at International Colloquium “M?ssbauer Spectroscopy in Materials Science”, Všemina, Czech Republic, June 1–4, 2004. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Contract No. IAA1041404).     

Epitaxy and magnetic properties of surfactant-mediated growth of bcc cobalt

High resolution core level photoemission spectroscopy, photoelectron diffraction, and x-ray magnetic circular dicroism (XMCD) have been used to characterize the structural and magnetic properties of bcc-cobalt films grown on GaAs(110) substrates by using Sb as a surfactant. We have unambiguously disentangled the surfactant role played by the Sb which improves the crystallinity and reduces the lattice distortion of the metallic films as well as changes the interdiffusion process at the interface compared to the Co/GaAs(110) system. As a consequence of these combined effects, an improvement on the magnetic response of the grown Co thin films has been observed by XMCD measurements.