No abstract

A fully relativistic theoretical description of magnetic EXAFS (MEXAFS) is presented that has been set up in the framework of relativistic multiple-scattering theory on the basis of the Dirac equation for spin-polarised, magnetic solids. This approach is applied to supply a detailed interpretation of new and very accurate experimental data for the K-and L2,3-edges of bcc-Fe that have been recorded for photon energies up to 600 eV above the Fermi level. In particular it is demonstrated that the MEXAFS spectra can be interpreted on the basis of the so-called sum rules.

The spin-dependent extended x-ray absorption fine-structure (EXAFS), i.e., the magnetic counterpart to the conventional EXAFS when using circularly polarized light in a ferro(i)magnet, was measured at the L2,3 edges of Eu in Eu3Fe5O12 and EuO and at the K edge of CrO2. Complementary near-edge x-ray magnetic circular dichroism measurements at the O K edge were made to directly access the oxygen magnetization. It is shown that there is an influence of the nearly nonmagnetic oxygen atoms on the magnetic EXAFS depending on the local symmetry of the crystal as well as on the relative direction of the magnetic moments of neighboring atoms. The observed influences at distances corresponding to the oxygen neighbors in EuO are interpreted as the harmonic focusing effect, i.e., over-next magnetic neighbors behind the oxygen are responsible for the observed contributions.

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 or spin-dependent extended x-ray-absorption fine-structure ͑EXAFS͒ studies have been performed at the K edges of pure Co and Ni and at the L 1,2,3 edges of Gd metal. Using a two-step model for the absorption process and introducing an additive-exchange contribution to the complex Coulomb scattering amplitude and the electron mean free path, an ansatz for the description of the magnetic EXAFS is derived. A parametrization of these quantities is proposed for an extraction of the spin-dependent values from the experimental data. The results for the elastic exchange scattering cross section in the energy range of 100-600 eV are in excellent agreement with theoretical predictions from a Born-Ockhur approximation. The inelastic exchange scattering effects are found to be negligible. ͓S0163-1829͑98͒01906-7͔