George Daniel Waddill

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

Abstract The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe 2 O 3 –60P 2 O 5 (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mossbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mossbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mossbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron–oxygen coordination is in the 4–5 range indicating that iron ions have mixed tetrahedral–octahedral coordination.

X-ray photoelectron and Mössbauer spectroscopic studies of iron phosphate glasses containing U, Cs and Bi

Abstract The atomic structure of iron phosphate glasses and those containing common high level nuclear waste components such as UO2, Cs2O and Bi2O3 has been investigated using X-ray photoelectron spectroscopy (XPS) and Mossbauer spectroscopy. Oxygen 1s spectra indicate that only 16–26% of the oxygen ions are bridging oxygens (BOs). The addition of waste components such as UO2, Cs2O and Bi2O3 does not change the BO/(BOxa0+xa0NBO) ratio appreciably. Some structural similarities in the short range order was found between the glass and crystalline Fe3(P2O7)2 which crystallizes from the glass upon heat treatment. The Mossbauer hyperfine parameters, isomer shift and quadrupole splitting, showed that the environment around iron ions was independent of the composition. In general, the addition of waste elements does not alter the basic features of the parent glass. The measured isomer shifts and quadrupole splittings indicate that the Fe3+ and Fe2+ ions in these glasses are in octahedral or distorted octahedral coordination. The fraction of Fe2+ in the glass increases with the melting temperature. However, most physical properties of these glasses do not depend appreciably on the valence state of iron ions.

Spin-resolved photoelectron spectroscopy of Fe3O4 ☆

Abstract Spin-polarized photoelectron spectroscopy has been used to study the candidate half metal Fe 3 O 4 . By using higher photon energies we can study polarization in “as received” samples, essentially “looking through” the disrupted surface. Our data agree with theoretical calculations for Fe 3 O 4 but exhibit a lowered polarization.

High resolution photoabsorption and circular polarization measurements on the University of California/National Laboratory spherical grating monochromator beamline

Abstract We have measured the N, C, and O 1s photoabsorption of O2,N2 and CO using the University of California/National Laboratory bend magnet beamline located at the Stanford Synchrotron Radiation Laboratory. The photon-energy resolving power at the N 1s edge ranges from 8–12.5 × 103 based upon the vibrationally resolved 1s-π∗ transition, indicating that this spherical grating monochromator is one of the highest-resolution soft-X-ray/ultraviolet photon sources yet reported. Magnetic circular dichroism was also measured with this beamline on highly magnetized thin films. Left and right elliptically polarized soft X-rays were produced by collecting the synchrotron radiation above or below the plane of the electron ring.

A cerium M-edge X-ray absorption study of the CeM2 compounds, where M is Mg, Al, Fe, Co, Ni, Ru and Rh

Abstract The cerium valence in the CeM2 compounds, where M is magnesium, aluminum, iron, cobalt, nickel, ruthenium and rhodium, has been measured at room temperature by X-ray absorption at the cerium M-edge. The observed valencies, which agree with previous more limited measurements made at the cerium LIII-edge, extend the study to additional compounds and lead to the conclusion that the relationship between the cerium volume and its valence in the CeM2 compounds is not straightforward. The results show a clear linear increase in the cerium valence with the atomic number of the metal M and indicate that the increase in the cerium valence from 3.05 in CeMg2 to 3.24 in CeRh2 results from an increase in the hybridization between the cerium 4f orbitals and the highest occupied M orbitals.

Dependence of x‐ray absorption magnetic circular dichroism on layer periodicity in iron–platinum multilayers

Magnetic circular dichroism in x‐ray absorption is observed in Fe/Pt multilayers. The magnetization and helicity are both in the plane of the Fe/Pt multilayers which are prepared by planar magnetron sputter deposition. Lattice and layer pair spacings are measured using grazing incidence and high‐angle x‐ray scattering. The film composition and lattice growth are confirmed using Auger depth profiling and cross‐section transmission electron microscopy, respectively. The state of the film magnetization is found to be dependent on the layer pair spacing as well as the relative thickness of the Fe to Pt layer. The maximum magnetic moment is measured for a (Fe 9.5 A/Pt 9.5 A)92 multilayer.

Characterization and photoemission dichroism of epitaxially grown Gd(0001)/Y(0001)

Gadolinium thin films approximately 100 A thick have been grown epitaxially on a Y(0001) substrate. A threefold characterization has been performed. The surface structural analyses of the yttrium substrate and the gadolinium films were performed using x-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED). The results of the XPD and LEED studies strongly suggest that gadolinium films have an effective C6v surface symmetry, consistent with earlier studies of other hcp (0001) surfaces. The elemental analysis of the substrate and the film was done with x-ray photoemission using Mg, Al Kα x rays and synchrotron radiation. The magnetic analysis is based upon magnetic x-ray dichroisms observed in angle-resolved photoelectron spectroscopy, using both linearly polarized and circularly polarized synchrotron x-ray radiation as the excitation. Photoemission from the Gd 4f and 5p core-level states were used in this magnetic characterization and will be presented. This includes novel magnetic...

MAGNETIC X-RAY CIRCULAR DICHROISM IN PHOTOELECTRON SPECTROSCOPY AND DIFFRACTION

In this paper, we discuss a series of photoelectron-spectroscopy (PS) and photoelectron-diffraction (PD) studies utilizing magnetic x-ray circular dichroism and fully spin-specific, multiple-scattering calculations for analysis of the experimental results. The test system used was perpendicularly magnetized, ferromagnetic monolayer Fe/Cu(001), prepared and probed at low temperature. Two sets of states were used: the Fe2p at binding energies of over 700 eV (large spin–orbit and small exchange splittings) and the Fe3p at a binding energy of near 53 eV (spin–orbit and exchange splittings approximately equal). The PS and PD measurements made were angularly resolving and compared to calculational results which were similarly angularly dependent. Both “exchange shifts” and intensity variations of spectral structure were observed. The impact of “j-state mixing” was seen in both the Fe2p and 3p spectra. Our results indicate a ferromagnetically ordered FCC Fe with a lattice parameter near that of bulk Cu. Nevertheless, the sensitivity of the multiple-scattering calculations of both the Fe2p and Fe3p simulations to local order has been used to show that the overlayer structure posseses some level of imperfection or disorder as well.

A cerium M-edge X-ray absorption and an iron L-edge magnetic circular dichroism study of the Ce2Fe17−xMx solid solutions, where M is Al and Si

Abstract The cerium M-edge X-ray absorption spectra and the iron L-edge magnetic circular dichroism spectra of the Ce 2 Fe 17− x M x solid solutions, where M is aluminum and silicon, were measured at room temperature. The cerium M-edge spectra was used to determine the variation in the valence of cerium in the solid solutions. The resulting values, which range from 3.20 in Ce 2 Fe 16.8 Si 0.2 to 3.02 in Ce 2 Fe 6 Al 11 , decrease linearly with x in both series, but the decrease is twice as large in Ce 2 Fe 17− x Si x . Interestingly, the resulting valencies are quantitatively different from those determined using cerium L-edge X-ray absorption data, but the qualitative trends in the valence with metalloid substitution is the same using either the cerium L- or M-edge data. The cerium valencies also correlate linearly with the cerium Wigner–Seitz cell volumes, but with a positive slope for the silicon and a negative slope for the aluminum solid solutions. This difference results because of the more extensive cerium–silicon covalent bonding interaction in Ce 2 Fe 17− x Si x as compared to the cerium–aluminum interaction in Ce 2 Fe 17− x Al x . The iron L-edge magnetic circular dichroism spectra indicate that a non-zero iron orbital moment is coupled parallel to the iron spin moment. The orbital moments range from 0.0 to 0.2xa0 μ B and reveal that the orbital moments are probably independent of x and are ca. 0.07xa0 μ B for both series of solid solutions. The cerium M-edge magnetic circular dichroism spectra reveal no dependence upon the relative orientation of the magnetization or the photon helicity, indicating that there is no 4 f magnetic moment associated with cerium in these solid solutions.

Magnetic circular dichroism in x‐ray absorption for well characterized Fe/Pt multilayers

The subtle structure‐property relationship in magnetic multilayers is addressed by an array of techniques. Fe/Pt multilayers prepared by magnetron sputter deposition are characterized using x‐ray diffraction, Auger depth profiling, electron microscopy, magnetometry, and x‐ray magnetic circular dichroism. We concentrate on the (Fe9/Pt9)92 which is [111] textured in the growth direction, but randomly oriented in‐plane, with average grain size of 30 nm. This sample is magnetized in‐plane, as determined by magnetometry as well as magnetic circular dichroism (MCD). The potential of MCD to follow the magnetic axis orientation in these samples as well as provide an element specific determination of the magnetic moment will be discussed.