Strong K-edge magnetic circular dichroism observed in photon-in/photon-out spectroscopy

The effect of magnetic dichroism was foreseen for X-ray absorption spectra (XAS) almost 25 years ago [2] and the first experimental observations of linear and circular X-ray magnetic dichroism (XMCD) spectra were subsequently reported [3]. XMCD has now become a routine probe of element specific magnetisation in anti- and ferro(ferri)- magnetic systems. The3d valence orbitals are conveniently studied by dipole-allowed transitions from the 2p shell, i.e. L-edge absorption spectroscopy. Hard X-rays are used at the K-edge but the very weak XMCD signal and the absence of spin-orbit split edges do not allow a detailed quantitative interpretation. Most soft X-ray XMCD measurements employ total electron yield because significant self-absorption effects are observed when using total fluorescence yield detection. Thus, L-edge XMCD is mainly sensitive to the sample surface and, in addition, is not compatible with demanding sample environments such as high-pressure cells, which limits its scope of applications. There is therefore a need for a magnetic spectroscopic method in the hard X-ray range that can provide information on the ordering and the value of magnetic moments. We show that this goal can be achieved by coupling XMCD with 1s2p RIXS at the K pre-edge.

RIXS is a second-order optical process where first a hole in an inner electron shell is created (intermediate state) that is then replaced by a hole in an outer shell (final state) (Figure 1d). This results in sharper spectral features and often in a rich multiplet structure that reveals electron-electron and spin-orbit interactions [4]. The 1s2p RIXS probes the evolution of the Kα emission (2p → 1s) following resonant excitation of a 1s electron. The K-edge absorption spectra of most 3d transition metal compounds show weak pre-edge features that are sensitive to the valence orbitals and dominated by the 3d density of unoccupied states. The 2p⁵3dⁿ⁺¹ final state electron configuration in 1s2p RIXS is identical to the 2p spin-orbit split L_2,3 absorption edges. The idea of this work was to combine a bulk sensitive hard X-ray probe with the magnetic circular dichroism (MCD) sensitivity of a 2p (L-edge) core hole, i.e. to perform 1s2p RIXS at the K absorption pre-edge in 3d transition metals using circular polarised light.

Figure 1. The 1s2p RIXS-MCD in magnetite. a) The experimental setup uses an external magnetic field and circularly polarised incident X-rays (left and right, pictured as red and blue). The emitted fluorescence is analysed by a set of four spherically bent crystals and then focused on an avalanche photodiode (APD). b) Experimental 1s2p RIXS plane measured at the Fe K–edge in magnetite and averaged over the two circular polarisations. The energy transfer is the difference between incident and emitted energy. c) Experimental RIXS-MCD plane of magnetite, plotted as the difference between the RIXS planes measured for opposite helicities of circularly polarised light. d) Theoretical model used in the crystal field multiplet calculations, involving only a tetrahedral FeIII ion of magnetite (yellow atom in inset). The 1s2p RIXS involves quadrupole excitation from the ground state (GS) into the intermediate state (IS) and dipole decay to the final states (FS). A schematic representation of the electronic transitions for each helicity of circularly polarised light is given (red and blue arrows). e) The theoretical RIXS-MCD plane calculated for tetrahedral FeIII compares well with the experimental one since its contribution dominates over those of the octahedral sites. f) The theoretical RIXS-MCD plotted for the 2p3/253d6 final state with a reduced broadening reveals the origin of the energy shifts that yield the dichroism. The arrows indicate to which multiplet interactions the splittings are proportional.

1s2p RIXS-MCD experiments were performed at room temperature on magnetite [Fe^III]_tetra[Fe^IIFe^III]_octaO₄ (Figure 1a) at beamline ID26. The experimental RIXS-MCD plane, plotted as the difference between left and right circularly polarised light, reveals a strong dichroism (Figure 1c). The comparison to the RIXS plane averaged over the two photon helicities (Figure 1b) shows that only the resonant features give rise to the XMCD. The experimental data were compared with the theoretical RIXS-MCD calculated within the crystal field multiplet approach and good agreement between theory and experiment is achieved. We found a RIXS-MCD amplitude as large as 16%, comparable to L-edge MCD. The crystal field multiplet calculations show that the enhancement of the K-edge XMCD signal in RIXS is a result of increased splitting of the XMCD spectral features in the 2p⁵3dⁿ⁺¹ final state and the smaller lifetime broadening of the 2p when compared to the 1s core hole.

The large penetration depth of the X-rays in 1s2p RIXS-MCD makes this technique an attractive tool whenever bulk sensitivity is required, e.g. in high pressure experiments, buried interfaces and active sites in metalloenzymes. Although the applicability of sum rules as they are used for L-edge XMCD has to be confirmed, the significant progress achieved in synchrotron radiation sources and beamlines equipped with high resolution fluorescence spectrometers will allow the technique to become widely accessible.

References
[1] B.T. Thole et al., Phys. Rev. Lett. 68, 1943 (1992); P. Carra et al., Phys. Rev. Lett. 70, 694 (1993).
[2] B.T. Thole et al. Phys. Rev. Lett. 55, 2086 (1985).
[3] G. Schütz et al. Phys. Rev. Lett. 58, 737 (1987); G. van der Laan, Phys. Rev. B, 34, 6529 (1985).
[4] F. de Groot, and A. Kotani, Core Level Spectroscopy of Solids (CRC Press, Boca Raton, 2008); P. Glatzel and U. Bergmann, Coord. Chem. Rev. 249, 65 (2005).

Principal publication and authors
M. Sikora (a), A. Juhin (b), T. Weng (c), Ph. Sainctavit (d), C. Detlefs (c), F.M. F. de Groot (b), and P. Glatzel (c), Physical Review Letters 105, 037202 (2010).
(a) AGH University of Science and Technology, Kraków (Poland)
(b) Utrecht University (The Netherlands)
(c) ESRF
(d) Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, Paris  (France)

Top image: Magnetic circular dichroism (MCD) combined with resonant inelastic X-ray scattering (RIXS).