X-ray Magnetic Circular Dichroism Sum Research Articles

Quantitative atomic order characterization of a Mn2FeAl Heusler epitaxial thin film

In this work, we investigate the effect of anti-site disorder on the half-metallic properties of a Mn2FeAl Heusler alloy thin film. The film was grown on TiN-buffered MgO 001 substrates via magnetron sputtering. A detailed structural characterization using x-ray diffraction (XRD) and anomalous XRD showed that the film crystallizes in the partially disordered L21 B structure with 33% disorder between the Mn(B) and Al(D) sites. We measure a positive anisotropic magnetoresistance in the film, which is an indication of non-half metallic behaviour. Our x-ray magnetic circular dichroism sum rules analysis shows that Mn carries the magnetic moment in the film, with a positive Fe moment. Experimentally determined moments correspond most closely with those found by density functional calculated for the L21 B structure with Mn(B) and Al(D) site disorder, matching the experimental structural analysis. We thus attribute the deviation from half-metallic behaviour to the formation of the L21 B structure. To realize a half-metallic Mn2FeAl film it is important that the inverse Heusler XA structure is stabilized with minimal anti-site atomic disorder.

Rare-earth orbital moment contributions to the magnetic anisotropy in magnetostrictive Tb0.3Dy0.7Fe2

Soft x-ray Magnetic Circular Dichroism spectroscopy at the Dy and Tb M4,5 and the Fe L2,3 edges was performed on a sputter deposited polycrystalline Terfenol-D (Tb0.3Dy0.7Fe2) film on sapphire substrates at temperatures from 100 to 300 K to evaluate the elementwise contribution to the magnetocrystalline anisotropy and coercive field. The elemental spin and orbital magnetic moments were calculated using the x-ray Magnetic Circular Dichroism sum rules. As temperatures decreased, the Tb and Fe moments plateau at 200 K with values of 7.6 μB/atom and 1.8 μB/atom, respectively, while the Dy moment increases to 8.9 μB/atom at 100 K. Between 300 and 200 K, the change in magnetic anisotropy is dominated by thermally induced magnetoelastic effects while for temperatures below 200 K magnetocrystalline anisotropy (MCA) changes are dominant. The MCA changes below 200 K appear to be due to increases in the Dy orbital moment with decreasing temperature in this temperature regime.

X-ray magnetic circular dichroism sum rule correction for the light transition metals

Distinguishable L2 and L3 edges and a clear separation into j 3/2 and j 1/2 excitations are necessary for the application of L2,3 edge X-ray magnetic circular dichroism (XMCD) sum rules, which provide element-specific information about spin and orbital magnetic moments. This separation is present for the heavy transition metals (TM), like Co and Ni, due to their large L2,3 spin–orbit splitting. However, for the light TM, the 2p spin–orbit splitting is strongly reduced and quantum mechanical mixing of j 3/2 and j 1/2 excitations is present. This mixing reduces the observed XMCD related spin and magnetic dipole term contributions and prevents the direct application of XMCD spin sum rules. A large number of 2p → 3d absorption spectra have been fitted nearly perfectly by a simple and phenomenological model, which takes into account lifetime effects and provides quantitative information about jj-mixing at the light TMs. On the basis of this mixing coefficient, sum rule correction factors have been determined. The proposed model results in renormalized magnetic projected XMCD spin moments, verified for different compounds of V, Cr, and Mn. A comparison with complementary methods gives consistent results. This or a similar fitting procedure and the estimated correction factors can be used in the future as a light element XMCD spin renormalization technique.

Sum rules for resonant inelastic x-ray scattering: Explicit form and angular dependence in perpendicular geometry

Resonant inelastic x-ray scattering (RIXS) and resonant photoemission spectroscopy (RPES) can be used to selectively measure the ground-state properties of atoms in solid materials. For the two types of experiment we compare the sum rules developed in the past years to extract quantitative information from the measured spectra. We show that if the measurements are not sensitive to the emitted photon polarization state (in RIXS) or to the photoelectron spin orientation (in RPES), the two experiments exhibit the same angular dependence of the spectral intensities but differ in some numerical coefficients in the sum rules. In particular we give explicit expressions for the RIXS sum rules in the so-called perpendicular geometry for all the cases of practical interest. These rules can serve, in combination with the well-known x-ray magnetic circular dichroism sum rules in absorption, to evaluate the quadrupole and octupole moments of the scattering atoms.

X-ray magnetic circular dichroism study of SmAl2 using the M4,5 x-ray absorption edges

The application of x-ray magnetic circular dichroism sum rules to the M4,5 edges of rare-earth metals has been developed for Sm3+. The jj mixing of intensities between the spin–orbit split M4,5 edges and the influence of the magnetic dipole term, T, on the sum rules have been evaluated by comparison with results from neutron scattering and magnetometry. This analysis leads to the unusual result that 〈Tz〉 is parallel to the orbital magnetic moment, 〈Lz〉, but antiparallel to the spin magnetic moment, 〈Sz〉.

Application of the x-ray magnetic circular dichroism sum rules to Co/Pt multilayers (abstract)

The presence of perpendicular anisotropy in Co/Pt multilayers is often attributed to an enhanced orbital magnetization at the surface or interfaces of the multilayers. By the application of the 〈Lz/Sz〉 sum rule to magnetic x-ray circular dichroism spectra it is possible to probe directly the orbital to spin ratio. In this study a comparison has been made of this ratio between Co/Pt multilayers with different anisotropies and also with single films of Co with and without Pt. The multilayers were prepared in high vacuum by evaporation on Si substrates with a 2.4 nm Pt buffer and a 1.2 nm Pt capping layers, characterized by x-ray diffraction and magneto-optic Kerr effect. The magnetic circular dichroism experiments were carried out at Daresbury laboratory in 75% polarization in ±1 T perpendicular applied fields as a function of temperature from 25 to 300 K. We find that the 〈Lz/Sz〉 ratio is enhanced for a multilayer with perpendicular anisotropy compared to a similar multilayer with in-plane anisotropy.

First principles investigation of the validity and range of applicability of the x-ray magnetic circular dichroism sum rule.

X-ray magnetic circular dichroism (MCD) spectra and orbital angular momentum, [l angle][ital L][sub [ital z]][r angle], for transition metal bulk and surfaces were studied for both ground state and core hole excitations using a highly precise local density approach. For Fe(001), we predict a double peak structure in both the MCD and total absorption spectra and a strong enhancement of [l angle][ital I][sub [ital z]][r angle]. Surprisingly, the MCD orbital sum rule is found to be valid to within (5--10)%. Finally, the results suggest possible solutions to several problems faced in applying the MCD sum rule to measure [l angle][ital L][sub [ital z]][r angle].