M5 Edge Research Articles

Praseodymium valence determination in Lu2SiO5, Y2SiO5, and Lu3Al5O12 scintillators by x-ray absorption spectroscopy

Until now, determination of both Pr3+ and Pr4+ at the low concentration levels commonly used in single crystal scintillators has proven to be difficult. We have found that it is possible to use synchrotron radiation and superconducting tunnel junction detectors to measure the X-ray absorption on the M4 and M5 edges of Pr to directly determine Pr3+ and Pr4+ in Lu2SiO5, Y2SiO5, and Lu3Al5O12. The spectra were measured at room temperature and compared to model samples of trivalent and tetravalent praseodymium, which provided clear signatures of the two charge states. The results show predominant Pr(III) in most samples.

Intrinsic ferromagnetism and magnetic anisotropy in Gd-doped ZnO thin films synthesized by pulsed spray pyrolysis method

This paper reports an in-depth study on nanocrystalline Gd-doped ZnO thin films synthesized using versatile pulsed spray pyrolysis method and exhibit room temperature ferromagnetism. The detailed structural and microstructural studies confirm that the doped Gd ions occupy Zn sites and the peak shift can be elucidated by charge neutrality. Optical investigation shows that the Gd doping in ZnO lattice leads to a decrease in the near band edge position due to the introduction of new unoccupied states by Gd 4f electrons. The electronic structure of the Zn1−xGdxO at the O K edge shows the evolution of pre-edge spectral features similar to cuprates and manganites, and also confirms the strong hybridization of O 2p–Gd 4f/5d states. Furthermore, the Gd M5 edge provides evidence that Gd ions are in the trivalent state. Hysteresis measurements demonstrate that the Gd-doped ZnO films are magnetically anisotropic and exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe values is observed for a field applied perpendicular to the sample surface compared to the in-plane direction.

The effect of core–valence intra-atomic quadrupolar interaction in resonant x-ray scattering at theDy M4,5 edgesin DyB2C2

The dependences on energy of the resonant soft x-ray Bragg diffraction intensities inDyB2C2 for the reflection at the Dy M4,5 edges have been calculated with an atomic multiplet Hamiltonian includingthe effect of the crystal field and introducing an intra-atomic quadrupolarinteraction between the 3d core and 4f valence shell. These calculations arecompared with experimental results (Mulders et al 2006 J. Phys.: Condens.Matter 18 11195) for the antiferroquadrupolar and antiferromagnetic phases ofDyB2C2. We reproduce all the features appearing in the reflection energy profile in the antiferroquadrupolar ordered phase, and we reproduce thebehaviour of the resonant x-ray scattering intensity at different energies in the vicinity of the Dy M5 edge when the temperature is lowered within the antiferromagnetic phase. Thesecalculations show that a detailed description of the energy dependences of resonant x-rayscattering signals at the M edges in 4f and 5f systems with multipolar orderingmay require the inclusion of an aspherical intra-atomic Coulomb interaction.

Investigations on the structural, optical and electronic properties of Nd doped ZnO thin films

We report the synthesis and characterization of Nd doped ZnO thin films grown on Si (1 0 0) substrates by the spray pyrolysis method. The surface morphology of these thin films was investigated by scanning electron microscopy and shows the presence of randomly distributed structures of nanorods. Grazing angle x-ray diffraction studies confirm that the doped Nd ions occupied Zn sites and these samples exhibited a wurtzite hexagonal-like crystal structure similar to that of the parent compound, ZnO. The micro-photoluminescence measurement shows a decrease in the near band edge position with Nd doping in the ZnO matrix due to the impurity levels. The near-edge x-ray absorption fine structure (NEXAFS) measurements at the O K edge clearly exhibit a pre-edge spectral feature which evolves with Nd doping, suggesting incorporation of more charge carriers in the ZnO system and the presence of strong hybridization between O 2p–Nd 5d orbitals. The Nd M5 edge NEXAFS spectra reveal that the Nd ions are in the trivalent state.

Magnetic characterization of ultrathin EuO films with XMCD

We present work done on EuO films with thicknesses varying from 10 to 60 Å grown as a stepped wedge on Si/SiO2/Cr(20 Å)/Cu(90 Å) and capped with Y(20 Å)/Al(80 Å). The films were characterized by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the europium M5 and copper L3 edges. The films' high quality and consistent magnetic properties were confirmed by superconducting quantum interference device magnetometry, which revealed a constant saturation moment independent of film thickness. XAS at the Cu L3 edge showed that the bottom Cu electrode is metallic (oxidation free). We report an XMCD intensity of 52% (±4.3), in excellent agreement with theoretical calculations.

Hepatocellular transport and gastrointestinal absorption of lanthanum in chronic renal failure

Lanthanum carbonate is a new phosphate binder that is poorly absorbed from the gastrointestinal tract and eliminated largely by the liver. After oral treatment, we and others had noticed 2-3 fold higher lanthanum levels in the livers of rats with chronic renal failure compared to rats with normal renal function. Here we studied the kinetics and tissue distribution, absorption, and subcellular localization of lanthanum in the liver using transmission electron microscopy, electron energy loss spectrometry, and X-ray fluorescence. We found that in the liver lanthanum was located in lysosomes and in the biliary canal but not in any other cellular organelles. This suggests that lanthanum is transported and eliminated by the liver via a transcellular, endosomal-lysosomal-biliary canicular transport route. Feeding rats with chronic renal failure orally with lanthanum resulted in a doubling of the liver levels compared to rats with normal renal function, but the serum levels were similar in both animal groups. These levels plateaued after 6 weeks at a concentration below 3 microg/g in both groups. When lanthanum was administered intravenously, thereby bypassing the gastrointestinal tract-portal vein pathway, no difference in liver levels was found between rats with and without renal failure. This suggests that there is an increased gastrointestinal permeability or absorption of oral lanthanum in uremia. Lanthanum levels in the brain and heart fluctuated near its detection limit with long-term treatment (20 weeks) having no effect on organ weight, liver enzyme activities, or liver histology. We suggest that the kinetics of lanthanum in the liver are consistent with a transcellular transport pathway, with higher levels in the liver of uremic rats due to higher intestinal absorption.

Nanoscale Imaging of Buried Structures with Elemental Specificity Using Resonant X-Ray Diffraction Microscopy

We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of approximately 15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M5 edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.

Multi-kmagnetic structures inUSb0.9Te0.1andUAs0.8Se0.2observed via resonant x-ray scattering at the UM4edge

Experiments with resonant photons at the U M4 edge have been performed on a sample of USb_{0.9}Te_{0.1}, which has an incommensurate magnetic structure with k = 0.596(2) reciprocal lattice units. The reflections of the form <kkk>, as observed previously in a commensurate k = 1/2 system [N. Bernhoeft et al., Phys. Rev. B 69 174415 (2004)] are observed, removing any doubt that these occur because of multiple scattering or high-order contamination of the incident photon beam. They are clearly connected with the presence of a 3k configuration. Measurements of the <kkk> reflections from the sample UAs_{0.8}Se_{0.2} in a magnetic field show that the transition at T* ~ 50 K is between a low-temperature 2k and high-temperature 3k state and that this transition is sensitive to an applied magnetic field. These experiments stress the need for quantitative theory to explain the intensities of these <kkk> reflections.

Electronic structure of the charge-density-wave compoundEr5Ir4Si10

Photoemission and x-ray absorption measurements have been performed onEr5Ir4Si10, a compound that exhibits an incommensurate charge-density-wave formation below 155 K.The resonant photoemission and x-ray absorption measurements performed across the ErN54 andEr M5 edge identify the Er 4f multiplet lines, the 4f occupancy and the character of the statesclose to the Fermi edge.

Magnetic Compton scattering study of UCoAl

We have performed a magnetic Compton scattering experiment across the critical magnetic field of the metamagnetic transition in UCoAl. The sign of the total spin moment is negative at 2.0 T. This indicates that the orbital moment is dominant in the field-induced ferromagnetic state. The ratio of the orbital moment to the spin moment obtained is much larger than that in a magnetic circular dichroism experiment at U M4 and M5 edges. This leads to a conclusion that Co atoms have a significant spin moment with positive sign.

Properties of thin film europium oxide by x-ray magnetic circular dichroism

Magnetic circular dichroism was measured as function of temperature for the Eu M5 and M4 absorption edges in single crystal EuO thin films. The M5 edge is resolved into two major features, one with the same helicity dependence as the M4 edge, and one with opposite helicity dependence. While the ratio of the absorption peaks with similar helicity dependence remains constant as a function of temperature, the ratio of those with opposite dependence is found to change linearly with temperature below 75 K. The physical mechanism for this linear behavior in the absorption peak intensity is not well understood.

Imaging material components of an integrated circuit interconnect

Two regions of interest on a copper/tungsten integrated circuit interconnect were imaged using two techniques: (a) the absorption spectrum was measured at 15 x-ray energies between 1687 and 1897 eV, and (b) the x-ray fluorescence spectrum was recorded with incident photon energies of 1822, 1797, and 1722 eV. The energies were chosen to optionally excite tungsten and tantalum above their M5 edges yet stay below the silicon K edge. Four materials in the circuits, tantalum, tungsten, silica, and copper were mapped using both techniques. The two sets of images agree in their main features, but differ for finer features.

Mass Absorption Coefficient of Tungsten and Tantalum, 1450 eV to 2350 eV: Experiment, Theory, and Application.

The mass absorption coefficients of tungsten and tantalum were measured with soft x-ray photons from 1450 eV to 2350 eV using an undulator source. This region includes the M3, M4, and M5 absorption edges. X-ray absorption fine structure was calculated within a real-space multiple scattering formalism; the predicted structure was observed for tungsten and to a lesser degree tantalum as well. Separately, the effects of dynamic screening were observed as shown by an atomic calculation within the relativistic time-dependent local-density approximation. Dynamic screening effects influence the spectra at the 25 % level and are observed for both tungsten and tantalum. We applied these results to characterize spatially-resolved spectra of a tungsten integrated circuit interconnect obtained using a scanning transmission x-ray microscope. The results indicate tungsten fiducial markers were deposited into silica trenches with a depths of 50 % and 60 % of the markers’ heights.

Strong anisotropy ofresonant inelastic x-ray scattering from charge-transferexcitations in UO3

Resonant x-ray scattering spectra of UO3 were measured at theU M5 edge using monochromatic photon excitation. Largedifferences in inelastic scattering profiles were observedbetween two 90°-scattering geometries with thepolarization vector of incident synchrotron radiation parallel and perpendicular to the scattering plane,respectively. For the latter case, a ~9.5 eV energy-lossstructure, associated with the O 2p→U 5fcharge-transfer satellite, shows a significant intensityenhancement. The observed resonance behaviour allows anaccurate estimation of the energy of this satellite, which isobscured in the other experimental geometry, and indicates thecharge-transfer origin of the U M5 absorption structures atthe respective excitation energies.

Charge-transfer excitations in lanthanum compounds measured by resonant inelastic x-ray scattering at the M5 edge

We present resonant inelastic x-ray scattering spectra from La metal ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{LaF}}_{3}.$ Monochromatic photon excitation with energy across the La ${3d}_{5/2}$ ${(M}_{5})$ threshold was used and the valence scattering was monitored at selected incoming energies. The spectra vs the outgoing photon energy show the elastic peak, the $5p\ensuremath{-}{3d}_{5/2}$ decay peak and, in between, a feature due to charge transfer excitation. This brings an electron from the ligand to the lanthanum $4f$ states (in the case of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{LaF}}_{3})$ and results from a charge fluctuation from ${4f}^{0}$ to ${4f}^{1}$ occupation in the case of La metal. The energy transferred to the solid when this process occurs is remarkably different in the three cases and correlates with the difference in the electronegativity with the lanthanum neighbors. The present approach allows us to reveal features that are completely hidden in the x-ray absorption spectra.

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the UxLa1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U4+ or a U3+ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M5 edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x c .

5d and 4f Magnetic Profiles in Ce/Fe Multilayers Probed by XRMS

The element and electronic shell specificities of x-ray resonant magnetic scattering have been used to investigate the induced magnetization of the 5d and 4f electronic states of Ce in a [Ce(10Å)/Fe(30Å)]×100 multilayer. We show that the measurement of the magnetic contribution to the intensities diffracted at low angles at the L2 and M4 edge of the rare-earth allows to investigate the profile of the 5d and 4f magnetic polarisation across the rare-earth layer. The Ce 5d polarization is found to be oscillating across the Ce layer with a slow decrease from the interfaces with Fe towards the center of the layer. Preliminary results on the Ce 4f polarisation indicate a constant profile.

Experimental and Theoretical Comparison between Absorption, Total Electron Yield and Fluorescence Spectra of the Thulium M5 Edge

Beside the now well known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state the occur which will render the X-ray fluorescence different from the true absorption result on 3d transition metals at the L edge and on the M4,5 edges of rare earths. Special selection rules of the radiative de-excitation process may play an important role there. We have measured the absorption coefficient of a thin film of thulium deposited onto an aluminum foil, at room temperature, through the simultaneous detection of the transmission, 160 eV bandwidth fluorescence yield and total electron yield. While transmission and electron yield results are very similar, as expected for a very thin sample, the fluorescence yield spectrum is definitely different. Theoretically, the resonant X-ray fluorescence spectrum was calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. A very good agreement is obtained between the theory and experiment.

The M4,5spectrum of58Ce and the γ-α phase transition

Absolute absorption coefficients in the region of the M4 and M5 edges are presented for cerium in CeF3, CeO2 and in the metal. Strong line absorptions (3d to 4f) occur to the low energy side of weak M4 and M5 'edges'. At each 'edge' the absorption passes through a weak broad peak attributed to transitions into a band of higher f states. The spectra of CeF3 and the metal at room temperature are structurally similar but differ clearly from the quadrivalent oxide. A shift to higher energy is found for both absorption lines and edges in the sequence fluoride-oxide-metal at room temperature. On cooling the evaporated metal absorption foil ( gamma phase) to 77 K both M4 and M5 line profiles change into forms accurately resolvable into a combination of trivalent (CeF3 like) and quadrivalent (CeO2 like) curves, suitably displaced in energy, in the proportion 59% to 43% respectively.