Edges Of Rare Earths Research Articles

Nonresonant inelastic x-ray scattering from actinides and rare earths

Calculated multiplet spectra are presented for the electric-multipole transitions in nonresonant inelastic x-ray scattering (NIXS) at the ${O}_{4,5}$ edges of actinides and the ${N}_{4,5}$ edges of rare earths. The multiplet structure is characteristic for the $f$ count as well as for the angular momentum coupling. As a remarkable peculiarity, the calculations for the actinide ${O}_{4,5}$ edge show that the higher multipole spectra resemble $jj$ coupling, whereas the dipole spectrum is close to $LS$ coupling. The exchange integral ${G}^{1}(5d,5f)$ is responsible for a shift of $\ensuremath{\sim}$15 eV of the dipole spectrum to higher energy. This high energy is conserved by a sum rule for the average energy of the accessible final states. In the higher multipole spectra, where the allowed final states are different, the splitting by the $5d$ spin-orbit interaction exceeds that of the electrostatic interaction. This leads to a distinct spin-orbit-split doublet structure, with a broad first peak and a narrower second peak. According to the spin-orbit sum rule, the $5f$ spin-orbit interaction per hole is linearly related to the core-level branching ratio, where the proportionality factor is opposite in sign for the $k=3$ and 5 spectra. Compared to the $k=3$ spectrum, the $k=5$ spectrum has overall a lower intensity in the low-energy region but a higher intensity in the high-energy region, especially for the heavier elements in the series. The NIXS at the rare-earth ${N}_{4,5}$ edge is quite different since the electrostatic and core spin-orbit splitting in the $k=3$ and 5 spectra are of similar size. The pre-edge region of the rare-earth spectra, where the linewidth is narrow, shows a rich multiplet structure that depends strongly on the initial-state $J$ value.

LUCIA, a microfocus soft XAS beamline

The beamline “LUCIA” (line for ultimate characterization by imaging and absorption) is a “tender” (0.8–8keV) X-ray microprobe with capabilities for chemical speciation by micro-X-ray absorption spectroscopy (μ-XAS) and for elemental mapping by X-ray micro-fluorescence (μ-XRF). It allows the possibility to study heterogeneous samples at a micrometer scale and to combine these two element-specific and non-destructive techniques. A monochromatic beam of a few micrometer in size is incident on a sample which is mounted on a scanning x–y–z stage. μ-XRF shows the location of the elements, their relative abundances, and their association with other elements. One can take advantage of the monochromatic beam which allows separating out different elements by their absorption edges. After mapping the fluorescence, spots of interest can be analysed by XAS to determine the speciation (local chemistry, quantitative determination of the local geometric structure around the absorbing atom) of the elements and how they depend on the different components.Installed at first at the SLS of the Paul Scherrer Institute (Switzerland), the LUCIA beamline will be transferred to SOLEIL by the beginning of 2008. The energy range offered by the beamline corresponds to the best performances of SLS and SOLEIL in terms of brightness. It allows XAS experiments at the K edge of elements ranging from Na to Fe, L edges from Ni to Gd, and M edges of rare earths and actinides.

Incident photon energy dependence of the resonant x-ray magnetic scattering cross section of a samarium film

Resonant x-ray magnetic scattering is widely used as an element selective probe of magnetism in solids. The present work deals with a different, less frequently addressed aspect: the electronic shell selectivity. Due to the complexity of the atomic effects inherent to the resonant process and at the origin of the electronic shell selectivity, the data are generally considered on a qualitative basis. Here, we try to extend the arguments to a semiquantitative level. We show, through a detailed spectroscopic study of the resonance at the ${L}_{2}$ and ${L}_{3}$ edges of samarium in a single-crystal epitaxial film, how the exploitation of the atomic effects can lead to a deeper understanding of long-range magnetic order in this material. At the ${L}_{2,3}$ edges of rare earths, dipole resonances carry information on the polarization of the $5d$ band, whereas quadrupole resonances reflect the polarization of the $4f$ shell. The narrow width of the $4f$ band permits the interpretation of the quadrupole resonance below the ${L}_{3}$ edge using atomic considerations. A systematic study of the dependence of the $4f$ quadrupole resonance on wave-vector transfer shows that, within our resolution, the magnitude of the $4f$ moments in samarium is independent of the local environment (cubic or hexagonal). On the other hand, the energy dependence of the dipole resonance at and above the ${L}_{2}$ and ${L}_{3}$ absorption edges shows two maxima and is interpreted in the framework of an extended density of $5d$ states. Finally, the relative temperature dependences of the dipole and quadrupole resonances sheds light upon the respective contributions of the $4f$ and $5d$ levels to the long-range magnetic order in samarium.

XMCD at L II,III edges of rare-earths: electric quadrupolar and dipolar effects

X-ray Magnetic Circular Dichroism (XMCD) at LII,III edges of rare earths in rare earth–3d transition metal compounds is still a subject of study. We present experimental evidence of electric quadrupolar transitions at LIII and LII edges of Er in the ErFe2 Laves phase. These experimental results are compared with atomic calculation taking into account crystal electric field effects on the Er site.

Experimental and theoretical comparison between absorption, total electron yield, and fluorescence spectra of rare-earthM5edges

Besides the now well-known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state may occur which render x-ray fluorescence different from the true absorption in $3d$ transition metals at the $L$ edge and at the ${M}_{4,5}$ edges of rare earths. Special selection rules of the radiative de-excitation process play an important role there. We have measured the absorption coefficient of thin films of lanthanum, samarium, and thulium deposited on an aluminum foil, at room temperature, through the simultaneous detection of the transmission, total electron yield, and 150-eV bandwidth fluorescence yield. The latter result shows differences as compared to the other two, and exhibits polarization effects depending upon the angle between incident and outgoing photons. The resonant x-ray fluorescence spectrum is calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. Very good agreement is obtained between the theory and experiment.

Total electron yield method in x-ray absorption spectroscopy: A closer look at the saturation/self-absorption effects (abstract)

Total electron yield (TEY) is a common method by which photoabsorption cross sections are measured in x-ray absorption spectroscopy (XAS). Since various material parameters are extracted from these XAS spectra, how well TEY spectra mimic the absorption cross section is a major concern. In cases in which the photoabsorption cross sections are large (e.g., at the absorption edge white lines), saturation and self-absorption effects can play a detrimental role, significantly altering the spectral weights in the near-edge region. In this presentation, we show that at the L edges of transition metals (as well as M edges of rare earths), the XAS spectra measured via TEY do not represent the true absorption cross sections. As a case in point, we discuss the ramifications of this saturation/self-absorption effects on the magnetic circular dichroism measurements, which often rely on TEY data. We evaluate the changes in the spectral weights of the near-edge features and determine the resulting errors in the extracted material quantities, i.e., orbital and spin moments, as a function of film thickness and photon incidence angle.

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.

Hard-XMCD and Magnetic EXAFS in the Multiple Scattering Approach

We simulated X-ray Magnetic Circular Dichroism and magnetic EXAFS at the L III edges of Rare Earths (RE) in some REFe 2 intermetallic alloys within the full multiple-scattering approach. The up and down potentials were obtained by means of a linear-muffin-tin orbital method. To test the influence of 4f electrons, whose very large role at the L edges of REis now well known, we chose Lu which has no 4f magnetic moment, Gd which has no 4f orbital momentum and Er which has a strong 4f orbital momentum. The edge and EXAFS regions are investigated. We compare the calculations with measurements.

Elliptically polarizing undulator beamlines at the Advanced Light Source

Circular polarization insertion devices and beamlines at the Advanced Light Source are described. The facility will consist of multiple undulators feeding two independent beamlines, one optimized for microscopy and the other for spectroscopy. The energy range of the beamlines will go from below 100 eV to 1800 eV, enabling studies of the magnetically important L2,3 edges of transition metals and the M4,5 edges of rare earths.

Absorption cross sections at the M4,5 edges of rare earths: a soft X-ray transmission experiment

We have determined the absorption coefficients of several rare earths (Gd, Dy, Ho and Er) at the M 4,5 edges (3d → 4f excitations) by measuring the soft X-ray transmission through layers of different thicknesses between 2 and 400 Å. Our results are in good agreement with atomic calculations. We also compare transmission-derived absorption curves with spectra collected simultaneously in the standard total electron yield mode. Advantages and limitations of the transmission mode for soft X-rays and also its range of application are discussed.

Study of 5d magnetism in rare-earth-transition-metal (Fe, Co) intermetallic compounds by magnetic circular X-ray dichroism

Magnetic circular X-ray dichroism (MCXD) experiments have been performed at the L 2,3 of rare-earth (RE) elements in several Re-transition-metal (Fe, Co) compounds. Experiments at the L 2,3 edges give, in principle, direct information on the 5d magnetic properties of the RE atom which results from the hybridization between the 3d and the 5d electrons. We show in this paper that the use of MCXD experiments at the L 2,3 absorption edges seems to be really complicated when localized 4f electrons are present in the ground state. This unexpected situation is clearly demonstrated by comparing the results obtained on a cerium mixed-valent compound (CeFe 2), in which the 4f electrons are highly hybridized with the conduction electrons, and those obtained on a normal trivalent cerium (4f 1) compound (CeRu 2Ge 2). Our results suggest that the 4f electrons play an essential role in the description of MCXD at the edges of rare earths.

Multielectron excitations in rare-earth compounds revealed by magnetic circular x-ray dichroism.

This paper reports experimental evidence of multielectron excitations in magnetic circular x-ray dichroism (MCXD) spectra, measured at the ${\mathit{L}}_{\mathrm{I},\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edges of rare earths (RE) in several RE\char21{}transition-metal (Fe, Co) compounds. These multielectron features involve transitions from the 2p and 4d levels and exhibit a very strong magnetic enhancement compared to normal MCXD.

X‐Ray Absorption Spectroscopic Study of Rare Earth–manganese Intermetallic Compounds

AbstractChemical shifts of the K‐absorption edge of Mn and the L3‐absorption edges of rare earths (R = Gd, Tb, Dy, Ho, Er, Tm, and Lu) in RMn2 intermetallic compounds are measured and the effective charges on the Mn and R ions are estimated. Extended X‐ray absorption edge fine structures (EXAFS) associated with the absorption edges are investigated and R‐Mn and Mn‐Mn bond lengths in R–Mn2 compounds are determined. The results are interpreted in the light of the available data on these intermetallic compounds.