Magnetic Extended X-ray Absorption Research Articles

Effects of spin-orbit interaction in photoelectron scattering process for magnetic EXAFS

We present an effect of the spin-orbit interaction (SOI) at surrounding atoms on K-edge magnetic extended x-ray absorption fine structure (MEXAFS). A contribution of the SOI to a K-edge MEXAFS spectrum is described by a perturbative way within a multiple scattering (MS) theory. A numerical calculation shows that a photoelectron single scattering by the SOI at surrounding atomic sites has a negligibly small contribution to MEXAFS. Although a negligible contribution of the SOI at surrounding atoms has been attributed to a cancellation of various MS contributions, this small contribution can be understood by the small number of its effective partial wave components, at least within the single scattering approximation. For further detailed analyses of MS with a path expansion method, only the small number of the partial wave components for the SOI is necessary. This leads to faster computations for MEXAFS including the photoelectron scattering by the SOI.

Noncollinear Spin Structure in Fe–Ni Invar Alloy Probed by Magnetic EXAFS at High Pressure

To examine theoretical models of the Invar effect, X-ray magnetic circular dichroism and magnetic extended X-ray absorption fine structure (MEXAFS) measurements are performed under high pressures at the Fe and Ni K -edges in 35.4 at. % Ni–Fe alloy. An oscillatory MEXAFS signal is observed up to 6 GPa. Its amplitude significantly decreases with increasing pressure. The magnetic component of the radial distribution function, obtained by taking the Fourier transform, shows a different reduction in the ferromagnetic correlations of Fe and Ni absorbing atoms. The present results are favorable to the noncollinear spin structure picture rather than the Fe 2γ-state model.

Scattering-path analysis and magnetic scattering properties ofFe∕Ag(100)films: A temperature-dependent magnetic EXAFS study

Magnetic extended x-ray absorption fine structure (MEXAFS) is the spin-dependent counterpart of the well established extended x-ray absorption fine structure (EXAFS) technique. By means of MEXAFS, it is not only possible to analyze the local magnetic structure but also to probe magnetic fluctuations. For a fundamental understanding of this technique, we studied bulklike Fe films on a Ag(100) single crystal substrate by comparing the experimental data to ab initio calculations. The measurements were carried out at different temperatures. To model the temperature dependence, originating from lattice vibrations and spin fluctuations, the rigid band model was applied. The present study enables us to understand the differences in the fine structure of spin-averaged and magnetic EXAFS, to identify the scattering paths contributing to the EXAFS signal, and to reproduce the experimental spectra even with a simple rigid band model separating spin fluctuations from lattice vibrations.

Spin fluctuation in single-crystalline terbium probed by temperature-dependent magnetic EXAFS

Magnetic extended x-ray absorption fine structure (MEXAFS) is a helicity-dependent counterpart of the well-established EXAFS technique. By means of MEXAFS it is possible not only to analyze the local magnetic structure but also to learn about magnetic fluctuations. Here temperature-dependent MEXAFS was employed to study the spin fluctuation of a terbium single crystal in a wide temperature range from 10 to 250 K covering two magnetic transitions. As compared with EXAFS, the MEXAFS signal intensity decreases more dramatically with increasing temperature, especially when spin orientation changes from ferromagnetic order to helical structure and paramagnetic order. The strong temperature dependence of MEXAFS is related to the atomic thermal vibrations (probed by EXAFS) and to the reduction of the magnetization at increasing temperatures. To analyze the individual scattering contributions to the MEXAFS and the EXAFS, ab initio calculations have been performed. The strongly enhanced magnetic multiple scattering was observed at low temperature and interpreted on the basis of the exchange interaction model.

Fully relativistic theory for magnetic EXAFS: Formalism and applications

A fully relativistic theory for magnetic extended x-ray absorption fine structure is presented that is based on the Dirac equation for spin-polarized systems and relativistic multiple scattering theory. This scheme not only allows one to treat spin polarization and all relativistic effects on the same level but can also be applied to go beyond the standard muffin-tin construction by using nonspherical potentials. To illustrate the application of the presented fully relativistic scheme, results for the K-edge spectra of Ni in fcc Ni and the ${L}_{2,3}$ edges of Pt in fcc ${\mathrm{Fe}}_{3}\mathrm{Pt}$ are presented. A very simple and convincing interpretation of the corresponding dichroic spectra is given on the basis of the differential form of the so-called sum rules.

A fully relativistic theory for magnetic EXAFS - formalism and applications.

A fully relativistic theory for magnetic Extended X-ray Absorption Fine Structure (MEXAFS) is presented that is based on the Dirac equation tbr spin-polarized systems and relativistic multiple scattering theory. This scheme not only allows one to treat spinpolarization and all relativistic effects on the same level but can also be applied to go beyond the standard muffin-tin construction by using non-spherical potentials. Results of applications to Fe3Pt are presented and discussed on the basis of the so-called sum-rules.

Short Range Order of Spin Moments in NdCo2 Studied by Magnetic EXAFS

Magnetic Extended X-ray Absorption Fine Structure (MEXAFS) is a dichroic contribution to an x-ray absorption. The MEXAFS has been measured at Co K-edge in NdCo2 and at Fe K-edge in iron thin film. The Fourier analysis was carried out to obtain information about local magnetic structures in these systems. The positions and the sign of peaks in the imaginary part of Fourier transformed MEXAFS show a good agreement with those of conventional EXAFS for the iron thin film. In the case of NdCo2, the peak position for the nearest Nd shell was 2.89 Å in the imaginary part of Fourier transformed MEXAFS, which is the same sign with corresponding peak in EXAFS. The fact that the sign of the peak was positive for the spin moments of Nd, which was not the absorption element, can be explained that the spin polarization of photoelectrons outgoing from the absorbing atoms is antiparallel to that of an absorbing Co atom in NdCo2.

Magnetic extended x-ray absorption fine structure at the L3,2 edges of Fe and Co on Cu(001)

We report magnetic extended x-ray absorption fine structure (EXAFS) measurements on 30 monolayer Fe and Co films on Cu(001) substrates at L3,2 edges. For 3d transition metals, the magnetic EXAFS at the L edges is of great interest since it probes final d states which mainly contribute to magnetism. Magnetic EXAFS oscillations were detected up to 500 eV above the edge corresponding to 11.5 Å−1 in k space. Over such a large range, the long wavelength wiggles from the Fe and Co nearest neighbor backscattering and previously observed fast oscillations are clearly seen. It is shown that without using any deconvolution procedure a meaningful analysis can be performed despite an interference of L3 and L2 edges. In the case of Fe, experimental data are compared with multiple-scattering calculations.

Probing local magnetic disorder by investigating spin dependent photoelectron scattering

We present here systematic temperature dependent magnetic extended x-ray absorption spectroscopy (MEXAFS) measurements. This fine structure can be explained by the spin dependent scattering of the photoelectron and yields information about the local magnetic surroundings of the absorbing atom. We find a strong temperature dependence at temperatures far below the Curie temperature. This shows that the intensity of the MEXAFS signal is not only determined by the value of the magnetic moment but also by the relative vibrations of the backscattering atom to the absorbing atom.

Fully relativistic theory for the magnetic extended x-ray absorption fine structure

A fully relativistic theoretical description of the magnetic extended x-ray absorption fine structure (MEXAFS) is presented that has been set up within the framework of relativistic multiple scattering theory on the basis of the Dirac equation for spin-polarized, magnetic solids. The multiple scattering processes in the final state are treated using the scattering path expansion technique for finite atomic clusters. Results of the approach for the Fe K and the Gd L edges are presented and are compared with experiment. An analysis of the theoretical spectra clearly shows that the MEXAFS spectra can be interpreted on the basis of the so-called sum rules in a way completely analogous to the case of near-edge x-ray magnetic circular dichroism. This means that the K- and L1-edge spectra directly reflect the orbital polarization of the final states with p character. The L2,3-edge spectra, on the other hand, give, after suitable superposition, access to the spin and orbital polarization of the d states.

Magnetic extended x-ray absorption fine structure at the edges of 3d elements

We report normal and magnetic extended x-ray absorption fine-structure (EXAFS) measurements made on 30 ML Fe films on Cu(001) substrates at the edges. The magnetic EXAFS at the L edges of 3d metals is particularly important as it can be used to probe the magnetism of the d states. Magnetic EXAFS oscillations were detected up to 500 eV above the edge, corresponding to in k-space. Over such a large range, we were able to see long-wavelength wiggles and separate them from Fe nearest-neighbour backscattering and the fast oscillations that had been previously seen. It is shown that without using any deconvolution procedure, a meaningful analysis can be performed despite the interference of the and edges. The experimental data were compared with the results of multiple-scattering calculations, and this enabled us to assign all of the features in both the normal and the magnetic EXAFS Fourier transforms to various single- and multiple-scattering paths. Also, there was found to be less temperature-dependent damping for magnetic EXAFS as than for the normal EXAFS between 75 K and 300 K.

Magnetic extended x-ray absorption fine structure of transition metal films and alloys (abstract)

A very large helicity dependence of the extended x-ray absorption fine structure (EXAFS) above the L2,3 edges of the ferromagnetic transition metals Fe, Co, and Ni are observed. Transmission measurements of circularly polarized soft x rays through thin films of bcc Fe, hcp Co, fcc Ni, and FexNi1−x alloys deposited on ultrathin, semitransparent substrates show L-edge magnetic EXAFS (MEXAFS) oscillations nearly two orders of magnitude larger than the respective K-edge MEXAFS oscillations. Surprisingly, comparing the EXAFS and MEXAFS oscillations, after separation of the combined L2 and L3 spectra by an iterative deconvolution, show a linear dependence of the MEXAFS intensity on the spin moment of the scattering atom. Furthermore, the resulting fourier transforms show that, for the MEXAFS, the scattering paths traditionally associated with multiple scattering paths are enhanced compared to the L-edge EXAFS, suggesting the increased importance of multiple scattering in MEXAFS.

Spin-dependent part of the electron scattering amplitude Magnetic extended X-ray absorption fine structure suitable approach

On the basis of spin-dependent exchange, we consider scattering of an electron on a polarized rare earth ion (REI). Assuming a spin-dependent extended X-ray absorption fine structure (SPEXAFS) application we calculate the coherent backscattering matrices for electrons with energies ranging from 3.4 to 340 eV (wavevector ranging from 0.5 to 5.0 reciprocal Bohr radii). Only the case of perfectly polarized REI in Hund's ground state is considered. Phase shifts are assumed to be spin-independent. The complete ion potential is represented as a sum of core and magnetic 4f shell potentials. The former is constructed in accordance with a standard muffin-tin scheme while the latter is used in explicit form. We find that the exchange-induced spin-dependent terms of the scattering amplitude for scattering of electrons with angular momentum l = f = 3 are of the same order of magnitude as the amplitude itself. Examples are given.