Magnetic EXAFS Research Articles

Magnetic EXAFS

Magnetic EXAFS

Enhanced Multiple Path Lines in Magnetic EXAFS

In systematic magnetic EXAFS studies it is observed that in the Fourier transform (FT) of the dichroic signal multiple path peaks are strongly enhanced, while the corresponding maxima at higher r-values in the spin-averaged EXAFS FT are much smaller or invisible. This behavior is reflected by the occurrence of additional harmonic frequencies in the magnetic raw spectra and explains their sharper structure. These observations can be interpreted by the different strengths of the elastic scattering cross sections for parallel and anti-parallel orientation of the outgoing photo-electron wave and the spins of the magnetic electrons. This phenomenon might also play an important role in the near edge dichroism of oxides, which show characteristic oscillations superimposed on the dichroic effect.

Embedded Atom Contributions to the Magnetic EXAFS

Measurements at the L 2/3 -edges of Rare-Earth (RE) metallic systems show a smooth low-frequent background on which the magnetic EXAFS are superimposed. While this background is rather strong at the L 2/3 -edges it is vanishing at the L 1 -edge. It is also not observed at the K-edge of Transition Metals and only very weak in RE oxides. It is shown that the oscillating contribution to the magnetic EXAFS does only depend on the spin-polarization of the neighboring atoms. But the ratio of this low-frequent background at the L2- and L 3 -edges reflect the existence of an orbital momentum similar as it is found in the corresponding near-edge X-MCD.

MXD - Model Calculations and a Fully Relativistic Theory for Magnetic EXAFS

New theoretical developments based on a fully relativistic description of magnetic dichroism (MXD) in X-ray absorption spectroscopy are presented. It is demonstrated that it is possible to achieve within this sophisticated framework an unambiguous identification of the role of exchange splitting and spin-orbit coupling for the MXD by means of model calculations. A description of this approach that consists in the separate manipulation of both sources of the MXD is presented together with several applications to K-and L 2,3 -edge-spectra of transition metals. All fully relativistic calculations on the MXD have been done so far in the XANES region using the standard relativistic (κ,μ)-representation. In dealing with magnetic EXAFS (MEXAFS) it seems to be more advantageous to treat the multiple scattering for the final states in real-space adopting the (l, m l ,m s )-representation. A corresponding generalization for the matrix inversion as well as the expansion techniques will be presented.

Strong Magnetic Multi-Electron Excitations at the L-Edges in Rare Earths

In the magnetic or spin-dependent EXAFS (SPEXAFS), i.e. the magnetic counterpart to the conventional EXAFS, additional strong dispersive structures appear at the Gd, Tb and Ho L-edges within 200eV above the absorption threshold. These features are interpreted as magnetic multi-electron excitations. While similar effects in the EXAFS-profiles are seldom seen they are clearly visible in the dichroic signal due to their strong magnetic character. By the presented high accuracy of our SPEXAFS studies it is possible to separate these MMEE structures from the overlapping strong magnetic EXAFS for the first time and therefore obtain new insight into the physical origin of these effects.

Multiple-Scattering Theory of Magnetic EXAFS

This paper discusses various issues of the multiple-scattering theory of magnetic circular dichroism (MCD) in the x-ray range, especially at the Kedge of 3d transition metals. Using a semirelativistic expansion of the Dirac Green function, we show how MCD can be calculated and related to orbital momentum. Moreover, the rigid band model will be used to relate the high-energy behaviour of MCD to the exchange energy. Finally, we generalise our approach to the case of orbital polarisation, which was found to be relevant to MCD.

Magnetic Circular X-Ray Dichroism at the Ce LII,IIIEdges in Ferromagnetic Iron Based Intei metallic

Measurements of magnetic circular dichroism (MCXD) at the Ce L II and L III edges have been performed from CeFe 11 Ti and CeFe 5.5 Ga 6.5 both ferromagnetic compounds. The recorded MCXD signal is presented and compared to earlier reported results [1] on other intermetallic. A significant magnetic moment of 5d origin has been observed, the magnitude of which being very sensitive to the change in the Ce environment. An estimation of the 5d magnetic moment is derived from the measurements. The observed MCXD signal is discussed at the light of macroscopic magnetic properties that are reminded. A magnetic EXAFS of large amplitude is observed on all the studied compound. The comparison of the MCXD spectra at the L II and L III edges shows that the signal is typical of a mixed valence state of Ce.

Deconvolution of 3d transition metal L-edge EXAFS and magnetic EXAFS signals

Due to the closely overlapping L-edge EXAFS regions for the ferromagnetic 3d metals, their analyses have not generally been attempted. Similarly, although analysis of L-edge magnetic EXAFS (MEXAFS) is desirable because of the enhanced amplitude of the signal compared to K-edge MEXAFS, such experiments on the ferromagnetic 3d metals have also not been attempted. In order to address this problem, we describe the use of an iterative Van Cittert deconvolution approach to isolate the LIII-edge from the LII-edge EXAFS and MEXAFS signals for Fe, Co and Ni thin films. The deconvoluted LIII-edge EXAFS data are in agreement with theoretically generated LIII-edge data, thus demonstrating the validity of this approach. Results from the deconvolution of the MEXAFS data also qualitatively agree with previously published MEXAFS data on the same elements at the K-edge. The results for the Fe EXAFS and MEXAFS data will be presented here.

Spin dependent EXAFS in pure metals and garnets

Magnetic EXAFS are studied using circularly polarized synchrotron radiation in the energy range of 6000–9000eV at the L-edges of pure Gd metal, the L 2,3-edges of Eu in Eu 3Fe 5O 12 and the K-edges of Fe, Co and Ni metal. On the basis of a two-step model the dichroic absorption effects are described in terms of an additive contribution to the backscattering amplitude caused by the exchange interaction, which is directly correlated to the spin-moment of the neighboring atom. The damped low-frequent background found in the dichroic L 2,3-spectra of Gd and Tb metal is interpreted as an atomic x-ray fine structure of strong magnetic character.

SPEXAFS at the L2,3 edges of Eu3Fe5O12, Gd and Tb metal and the alloy HoGd (abstract)

Systematic studies of the magnetic or spin-dependent EXAFS (SPEXAFS) were performed at the L2/3 edges of Eu3Fe5O12, Gd and Tb metal and the alloy Ho20Gd80. The magnetic absorption exhibits distinct EXAFS structure, more pronounced than the nonmagnetic EXAFS. The Fourier transform of the Eu L2/3-edges SPEXAFS spectra, which are measured with an accuracy of less than 5×10−5, show a complete disappearance of the oxygen peak. This allows an identification of the positions of the magnetic neighbors for a significantly longer range as it is possible by using the conventional EXAFS profile. Studies of the alloy Ho20Gd80 at the L2/3 edges of both components, which show no significant differences in the polarization averaged EXAFS, indicate an essential influence of the spin structure at the absorbing atom on the magnetic EXAFS spectrum. At the L3 edge of Gd and Tb metal (both of hcp structure) we found similar structures in the conventional as well as in the magnetic EXAFS. The data are interpreted in terms of the sensibility of the SPEXAFS on the angular momentum of the absorbing and neighboring atoms.

Hypothesis for Structure and Oxidation State of Manganese Cluster in Oxygen-Evolving Center Deduced from Their Magnetic Properties

Abstract Magnetic Interaction, Manganese C om pounds, Photosystem II, Manganese Clusters Based on the investigations for the magnetic interaction in the synthetic binuclear man­ganese com pounds of (μ-oxo bridge and for results by magnetic measurements and EXA FS studies on the native enzyme, it was concluded that there are two dimeric manganese units in the oxygen-evolving center of the photosystem II. One of them is a di-(μ-oxo-bridged dimer with M n -M n distance of 2.7 Å, and it has a Mn2(III/III) oxidation state in both the S1,-and S2-states. A second binuclear unit, which we will call as X-pair, is assumed to have a Mn2(II/III) oxidation state in the S1-state, and is oxidized in the S,-to S2-state transition. The multiline ESR signal observed for the S2-state has been attributed to be due to a partially oxidized (Mn2-(III/IV) oxidation state) of the 2.7 A-pair, which should be formed through the interac­ tion between the 2.7 A-pair with Mn-,(III/III) state and the X-pair of Mn2(III/III) which is gen­ erated in the S1-to S2-state transition.