L. Baumgarten

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.

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.

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.

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.

Magnetic x‐ray dichroism in core level photoemission from ferromagnets (abstract)

We report on the novel effect of magnetic dichroism in core level photoemission from ferromagnets with circularly polarized synchrotron radiation in the soft x‐ray range. Depending on the relative orientation of photon spin and sample magnetization (parallel or antiparallel), a single core level emission line may be resolved into two lines of different intensities. The mechanism is shown to be due to spin‐selective dipole transitions from exchange‐split core levels in the presence of spin‐orbit interaction. Experiments on the 2p1/2 and 2p3/2levels of Fe are compared to fully relativistic photoemission calculations. Rather good qualitative and quantitative agreement is found. The effect does not rely on the spin‐split empty density of states near the Fermi energy, but probes the exchange splitting of core levels directly. Because of its surface sensitivity, x‐ray dichroism in photoemission may serve as a new powerful tool in surface and interface magnetism.