Magnetic EXAFS 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.

Determination of spin moments from magnetic EXAFS

Determination of spin moments from magnetic EXAFS

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.

Retraction: Spin fluctuation in single-crystalline terbium probed by temperature-dependent magnetic EXAFS [Phys. Rev. B68, 134406 (2003)

Retraction: Spin fluctuation in single-crystalline terbium probed by temperature-dependent magnetic EXAFS [Phys. Rev. B<b>68</b>, 134406 (2003)

Information in magnetic EXAFS

To understand the information one can obtain from MEXAFS, we develop a theory of MEXAFS within the spherical wave approximation. We show that the dominant signal is directly proportional to the magnetic moment per atom and smaller than regular EXAFS by an order of magnitude. We show that this explains the observed temperature dependence of MEXAFS. The higher order correlation functions can, in principle, be determined from double scattering paths, but they give contributions which are smaller by another two orders of magnitudes compared to that from the spin moment. Thus MEXAFS can serve as an alternative method for the element specific extraction of total spin magnetic moments. This can be achieved by modification of the regular EXAFS fitting routines and the output of the spin-dependent FEFF8 code.

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.

Temperature-dependent magnetic EXAFS investigation of Gd

Magnetic EXAFS (MEXAFS) is the 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 we present the MEXAFS of a Gd single crystal at the L 3,2-edges in the temperature range of 10–250 K. For the first time MEXAFS was probed over a large range in reduced temperature of 0.04⩽ T/ T C⩽0.85 with T C=293 K. We show that the vibrational damping described by means of a Debye temperature of θ D=160 K must be taken into account for the spin-dependent MEXAFS before analyzing magnetic fluctuations. For a detailed analysis of the MEXAFS and the EXAFS, the experimental data are compared to ab initio calculations. This enables us to separate the individual single- from the multiple-scattering contributions. The MEXAFS data have been recorded at the ID 12A beamline of the European Synchrotron Radiation Facility (ESRF). To ensure that the degree of circular polarization is kept constant, the undulator gap was scanned so that the maximum photon flux at every energy point was used.

Magnetic EXAFS at Gd L-edges: the spin-pair-distribution function of Gd neighbors.

The purpose of the experiment is to study the normal and the Magnetic EXAFS (MEXAFS) since EXAFS is the method of choice to investigate the local pair- and spin-pair-distribution function. We present MEXAFS and EXAFS measurements at the L-edges of a Gd single crystal in the temperature range of 10 K to 250 K. Therefore we are able to investigate the MEXAFS in a wide range of the reduced temperature t=T/Tc of 0.04 < or = t < or = 0.85 with Tc=293 K. We find a strong decrease of the nearest neighbor EXAFS which retains only about 35% of its 10 K value already at 250 K. This highlights the importance of lattice vibrations. To analyze the individual scattering contributions to the MEXAFS and the EXAFS, ab initio calculations (FEFF code) have been carried out. The comparison of the temperature-dependent damping of the normal EXAFS with the spin-dependent MEXAFS allows us to separate the influence of lattice vibrations (Debye temperature 160 K) from the magnetic ordering (Curie temperature) on the MEXAFS.

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.

Magnetic L-edge EXAFS of 3d elements: multiple-scattering analysis and spin dynamics

Magnetic EXAFS (MEXAFS) adds magnetic selectivity to the well established EXAFS technique opening the door to the exploration of local magnetic structure and disorder. Of particular interest is the ...

Magnetic EXAFS calculations for disordered alloys.

Magnetic EXAFS calculations for disordered alloys.

Magnetic X-ray absorption spectroscopy

Basic principles of the magnetisation dependent effects of the absorption of circularly polarised X-rays are discussed. At the inner core absorption edges the effect is manifested in a difference of absorption coefficients for the right and left circularly polarised quanta in ferromagnetic media and is called X-ray magnetic circular dichroism (XMCD). It originates from the interplay of spin orbit interaction in the initial state and angular momentum conservation in the absorption process and reflects the local spin and orbital polarisation of the final states. An outstanding feature of XMCD is the applicability of the so-called ‘sum rules’ which can be used to determine spin and orbital contributions to the local magnetic moments separately. Its inherent element selectivity and a high sensitivity makes it especially useful in a study of technologically important multicomponent materials. Magnetic contributions occur also in the extended X-ray absorption fine structure (EXAFS) range and are called magnetic EXAFS (MEXAFS). They originate from an increased (decreased) absorption coefficient due to the constructive (destructive) interference of the outgoing polarised photoelectron wave with the wave backscattered by the adjacent magnetic atoms. The backscattering amplitude gives a quantitative measure of the local magnetic spin moments of the nearest neighbours. Some experimental data for the rare earth–3d element intermetallics, magnetic thin films, multilayers and oxide compounds in the hard and soft X-ray regime are selected to illustrate the potential of XMCD and MEXAFS. A very promising perspective of application of XMCD as a contrast mechanism in various established techniques using X-rays, e.g. imaging of magnetic domains in the magnetic X-ray transmission microscopy is discussed.

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 EXAFS at the K- and L 2,3-edges of FCC-Ni

Results of a previously developed fully relativistic theory of magnetic EXAFS (MEXAFS), that has been set up in the framework of relativistic multiple scattering theory, are presented. The multiple scattering process in the final states is treated using matrix inversion technique for finite clusters. The K- and L 2,3-edge spectra of FCC-Ni are calculated on the basis of the Dirac equation for spin-polarized, magnetic solids and compared with experiment. As for the magnetic circular X-ray dichroism in the near-edge region, it is shown that the magnetic EXAFS spectra can be interpreted on the basis of the so-called sum rules.

Distribution of Spin Polarization in Ferrimagnetic DyFe 2 by a Novel Analysis of Magnetic EXAFS

Magnetic EXAFS (MEXAFS) at the Fe K-edge in DyFe 2 powder and an Fe thin film were measured. In order to derive a ferrimagnetic coupling of spin moments between Dy and Fe from MEXAFS spectra, novel analysis for MEXAFS has been tried. The σ( r ) profiles, which are defined as the difference between the absolute values of Fourier transformed k 3 χ + ( k ) and k 3 χ - ( k ), were calculated for the measured spectra. χ + ( k ) and χ - ( k ) are EXAFS functions when the correlation between a X-ray beam and a magnetic field applied to a sample is parallel and anti-parallel, respectively. The σ( r ) has well described the distribution of spin polarization with its sign for ferrimagnetic DyFe 2 . A simulated distribution profile, in which only spin moments were considered, has been compared with the σ( r ) in DyFe 2 and a good coincidence between them has been obtained. The σ( r ) profile for an Fe thin film completely reflects the ferromagnetic character and this result supports the validity of the present analysis.

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.

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.