E. Navas

Circular dichroism in 4f photoemission from magnetically ordered rare-earth materials

Abstract4f Photoemission (PE) spectra from magnetically ordered rare-earth materials using circularly polarized X-rays exhibit strong Magnetic Circular Dichroism (MCD), i.e., the intensities of the individual multiplet lines depend on the relative orientation of sample magnetization and photon spin. On the example of the wellresolved Tb 4f PE multiplet, it is shown that in relevant cases 4f PE lines are essentially only observed for one magnetization direction, either parallel or antiparallel to the photon spin. These large MCD effects in 4f PE open new perspectives in the analysis of surface and thin-film magnetism and provide a sensor for the degree of circular polarization of soft X-rays over a wide photon-energy range. To demonstrate the potential of MCD in 4f PE as a magnetometer, we studied Gd(0001) and Tb(0001), where the magnetization of the topmost atomic (0001) layer can be easily separated from the bulk magnetization via the surface core-level shift. In multicomponent magnetic thin films containing different rare-earth elements 4f PE allows to monitor magnetization in an element-specilic way, e.g., in case of the hetero-magnetic interface 1 ML En/Gd(0001).

Electronic and magnetic structure of rare-earth materials studied by high-resolution photoemission

Abstract Recent experimental results concerning the surface and bulk electronic and magnetic structure of rare-earth (RE) metals are presented. Surface core-level shifts in photoemission (PE) and inverse photoemission (IPE) experiments are discussed to summarize the phenomena that occur due to the energy lowering of 4f states at the surface. In Ce metal, a highly correlated electronic system, surface effects are influenced by the strong interaction of the 4f electron with the valence band, leading to essentially no observable shift in PE. Resonant PE experiments with varying degree of surface sensitivity have been performed to investigate the surface electronic structure in these systems. With recent progress in the growth of well-ordered epitaxial films of RE metals, detailed band-structure studies have become possible. The existence of a localized d-state at the surface is demonstrated and consequences for the surface properties are discussed. Single-crystalline RE surfaces open up the possibility to study their magnetic properties as shown for the case of Gd, where a strong magnetic circular dichroism has recently been observed.

Application of MCD in 4f photoemission as an element-specific probe for near-surface magnetization in lanthanide materials

4f photoemission (PE) spectra from magnetically ordered lanthanide materials exhibit large magnetic circular dichroism (MCD), i. e. the intensities of individual PE multiplet lines depend strongly on the relative orientation between sample magnetization and photon momentum. We apply MCD-PE as a probe for near-surface magnetization with atomic-layer resolution to the (0001) surfaces of Gd and Tb, where the magnetization of the topmost atomic layer - by help of the surface core-level shift - can be separated spectroscopically from the sub-surface magnetization. In multicomponent magnetic thin films containing different lanthanide elements, MCD-PE allows to monitor the magnetization in an element-specific way. This is demonstrated for the example of the heteromagnetic interface Eu/Gd(0001).

Magnetic circular dichroism in photoemission from rare earth materials: a new technique in surface and thin-film magnetism

Using circularly polarized X-rays to excite photoelectrons from magnetically ordered rare-earth materials gives rise to large magnetic circular dichroism in photoemission (MCD-PE), i.e. the intensity of 4f-photoemission lines strongly depends on the orientation of sample magnetization with respect to the photon spin. To explore its potential for application as near-surface magnetometer with atomic-layer resolution, we studied MCD-PE at the (0001)-surfaces of Gd and Tb where the magnetization of the topmost atomic layer-by help of the surface core-level shift-is spectroscopically separated from the bulk magnetization. In thin films containing different rare earths MCD-PE allows to monitor the magnetization in an element specific way. This is demonstrated using the ordered heteromagnetic interface Eu/Gd as an example.

Resonant photoemission study ofK-derived valence-band states in K X C60

The electronic structure of K-doped C60 was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K3C60 was obtained by resonant PE at the K-2p1/2 threshold. The experimental observations support a complete charge transfer from K to C60 and we clearly observe a finite density of states atEF. From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.

Magnetic Circular Dichroism in Photoemission from Rare-Earth Materials: Basic Concepts and Applications

Magneto-optical Kerr effect and Faraday effect provide the basis of established methods for studying the magnetic properties of matter by polarized light in the visible spectral range. It was only quite recently that an analogous effect in the x-ray region, magnetic circular dichroism in x-ray absorption, was first observed by Gisela Schutz et al. for the near-edge fine structure at the K edge of ferromagnetic iron.1 Later on, magnetic circular x-ray dichroism (MCXD) was also observed at the LII,III thresholds of rare-earths2 and 3d transition metals,3 opening up the possibility for element-specific analyses of magnetic moments in compound magnets and multilayers. Today MCXD is mainly used as a tool at the LII,III x-ray absorption thresholds of 3d transition metals, where relatively large MCD asymmetries in the white lines upon reversal of either sample magnetization or circular polarization (photon spin) of the absorbed light are observed. MCXD can be understood in the simplest way in a one-electron picture by taking the spin polarization of the excited electron due to the inner-shell spin-orbit coupling (Fano effect4) into account as well as the spin-split density of final states at and above the Fermi level.5 More rigorous theoretical treatments have been given,6,7,8 which allow to recognize the three important ingredients for magnetic circular dichroism: (i) Exchange interaction as the driving force for long-range spin order; (ii) use of circularly polarized light with preferential propagation along the magnetic quantization axis; (iii) spin-orbit interaction providing the mechanism for an effective coupling between the angular momentum of the circularly polarized photon and the magnetically ordered electron spins.

Investigation of the Electronic Structure of High-Temperature Superconductors by Polarized X-Ray Absorption Spectroscopy

The electronic structure of high-temperature superconductors near the Fermi level which is of primary interest for the understanding of the mechanism of superconductivity, is not yet well understood. Most theoretical investigations deal with Cu3dx−y and O2px,y orbitals at EF, but whether or not O2pz and Cu3d3z−r orbitals contribute to states at EF, is still under discussion. The contribution of states with out-of-plane orbital character has been investigated by polarized x-ray absorption spectroscopy (XAS) on HTSC single crystals. For La2−xSrxCuO4 a small contribution from O2pz orbitals (8% of holes in O2p valence band states) was observed which increased to about 13% on 30% Sr doping. In n-type doped HTSC there is no clear evidence for O2pz orbitals at EF, but states are observed slightly above EF with O2pz character depending on the doping and annealing conditions. An observed admixture of Cu 3d3z−r states to the predominant 3dx hole states at EF of a few percent did not significantly exceed the expermental error limits.

Strong Magnetic Circular Dichroism in 4F Photoemission

We report on strong magnetic circular dichroism (MCD) in 4f photoemission (PE) from Magnetized Gd(0001)/W(110) films. The shape of the 4f 6–7 F J final-state PE Multiplet depends on the relative orientation between photon spin and sample magnetization and can be described within an atomic Model. The spectra rule out antiferromagnetic alignment of the (0001) surface layer and the bulk of Gd. This MCD in 4f-PE from rare-earth materials opens new perspectives in the analysis of surface and thin-film magnetism and as a sensor for circular polarization of soft x-rays.