AVS1996 Session MI-TuA: Magnetic Dichroism

Tuesday, October 15, 1996 2:00 PM in Room 106A/B

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MI-TuA-1 Core Level Spectroscopy and Dichroism J. Dresselhaus, A. Fanelsa, F. Hillebrecht, E. Kisker, R. Schellenberg, D. Spanke (Heinrich-Heine-Universit\um a\t, Germany) Core level excitation strongly influences several spectroscopic quantities including the photoemission spectrum and the optical properties of the solid. We will focus here on magnetic dichroism (MD) which manifests itself in these quantities in a dependence of the cross section on the direction of the long-range magnetization. It stems from the coexistence of exchange interaction and spin-orbit interactions. In the visible part of the electromagnetic spectrum the MD in the optical properties is very small. It is resonantly enhanced by orders of magnitude at core level thresholds. We will give an overview on recent results from transition metal ultrathin films, obtained by measurements of the core level spin polarization, the spin-averaged photoemission intensity, the photoabsorption cross section, and the optical reflectivity. We will show that photoelectron diffraction leads to an enhancement of the dichroism along specific emission directions with respect to the crystal lattice. The applicability of MD using synchrotron radiation for static and time resolved spatial imaging of magnetic properties will be discussed and demonstrated.
2:40 PM		MI-TuA-3 Magnetic Linear and Circular Dichroism Photoelectron Angular Distributions from Ultrathin Films K. Subramanian, G. Mankey, R. Stockbauer, R. Kurtz (Louisiana State University) Interesting phenomena involving magnetism in thin film systems occur due to the complex interplay between spin-orbit coupling and the exchange interaction. To probe the nature of these interactions, one requires a technique that combines elemental specificity with a high degree of magnetic sensitivity. Magnetic Linear Dichroism (MLD) and Magnetic Circular Dichroism (MCD) have been shown to be very successful in achieving this. We have investigated MLD and MCD in angle resolved photoemission from ultrathin films of Fe, Ni and Co grown in situ on Cu(001). The Plane Grating Monochromator beamline at the Center for Advanced Microstructures and Devices (CAMD) synchrotron has been used to produce both linearly and circularly polarized UV and soft X-ray radiation. We have combined this with the imaging capability of the Ellipsoidal Mirror Analyzer (EMA) to observe the variations in both MLD and MCD in large volumes of the Brillouin zone (BZ). In particular, we observe a 12% primary MLD signal for 21 eV photons at a binding energy of ~ 0.5 eV from 8 ML Co/Cu(001). Due to photoemission selection rules, the observed dichroic signal has a distinct two-fold symmetry in the BZ. The largest asymmetries are observed along the [11] direction in the surface BZ. Results from MCD of Fe and Ni films on Cu(001) are also presented. The results are explained using group theoretical analysis and are consistent with semi-relativistic band structure calculations.
3:00 PM		MI-TuA-4 Direct Comparison of Magnetic X-ray Circular and Linear Dichroisms in High Resolution Photelectron Spectroscopy J. Tobin, K. Goodman (Lawrence Livermore National Laboratory); F. Schumman, R. Willis (The Pennsylvania State University); W. Gammon, D. Pappas (Virginia Commonwealth University); J. Kortright, J. Denlinger, E. Rotenberg, A. Warwick, N. Smith (Lawrence Berkeley National Laboratory) A direct comparison of Magnetic X-Ray Circular Dichroism (MXCD) and Magnetic X-Ray Linear Dichroism (MXLD) in high resolution, angle-resolved photoelectron spectroscopy has been performed. Crucial to the experiments was the development and implementation of a novel transmission multilayer that serves as a soft x-ray phase retarder (quarter wave plate). Measurements were performed upon the Fe3p core levels in NiFe/Cu(001) and the Gd 4f core levels in Gd grown on Y. Strong circular dichroic effects were observed in the normal emission spectra of both the Fe3p and Gd4f states. Comparison between these MXCD results and the normal emission MXLD spectra collected from the same samples will be made. In the case of the fourfold symmetric NiFe/Cu(001), the experimental geometries are essentially identical and provide for an exact comparison of the two techniques. These measurements were made at the Spectromicroscopy Facility (Beamline 7) of the Advanced Light Source at LBNL. This work was done under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. The Spectromicroscopy Facility and the Advanced Light Source were constructed and are operated under the support of the U.S. Department of Energy.
3:20 PM		MI-TuA-5 Magnetic Dichroism in Core-level X-ray Photoemission with Unpolarized Excitation A. Fanelsa, R. Schellenberg, F. Hillebrecht, E. Kisker (Heinrich-Heine-University, Germany); J. Menchero, C. Fadley (Lawrence Berkeley National Laboratory) Magnetic dichroism has been observed in the angular distributions of Fe 2p photoemission spectra excited from Fe(001) and from a metglas containing iron by unpolarized 1.25 keV x-rays. For Fe(001), the dichroism varies strongly around each low-index forward-scattering direction in the crystal, exhibiting an antisymmetry with emission direction and binding energy that is centered on each forward scattering peak. Photoelectron diffraction theory provides a quantitative explanation for these unpolarized dichroism effects, which should be generally useful in surface magnetism studies. For the metglas, a much smaller magnetic dichroism is observed that is correctly predicted within a purely free-atom model.
3:40 PM		MI-TuA-6 Final-state Magnetic Dichroism in Fe 3s Photoemission W. Lademan, L. Klebanoff (Lehigh University) Magnetic dichroism measurements using unpolarized MgK\alpha\ radiation are reported for Fe 3s and 2s x-ray photoemission from ferromagnetic iron. The Fe 3s level displays a small magnetic dichroism whereas any dichroism present in the Fe 2s level is too small to be reliably detected. Since the Fe 3s level does not experience an initial-state spin-orbit interaction, these results reveal the existence of a final-state magnetic dichroism in photoemission. Two possible sources of the dichroism are discussed. The first considers a combination of the Fano Effect and core-valence exchange to introduce magnetic dichroism into the Fe 3s photoemission. An alternative explanation explores the possibility that configuration interaction (CI) in the 3s hole state introduces 3p\super 4\ hole character into the 3s photoemission, with the Fe 3p spin-orbit coupling providing the dichroic character.
4:00 PM	Invited	MI-TuA-7 Application of X-ray Magnetic Circular Dichroism Spectroscopy to Scientific and Technological Problems in Magnetism J. St\um o\hr (IBM Almaden Research Center) This talk focusses on unique contributions the technique of x-ray magnetic circular dichroism (XMCD) spectroscopy has made, and is capable of making, to the solution of scientific and technological problems in magnetism. After outlining some of these problems in the context of magnetic data storage technology the concepts and experimental procedures of XMCD spectroscopy will be discussed. Examples will then be given how XMCD spectroscopy, through quantitative determination of magnetic moments and their anisotropy, has contributed to our understanding of induced magnetic moments in "non-magnetic" metals, the existence of magnetic dead layers, and the microscopic origin of the magnetic anisotropy
4:40 PM		MI-TuA-9 Probing Structure-Property Relationships in Perpendicularly Magnetized Fe/Cu(001) using MXLD and XPD G. Waddill (University of Missouri, Rolla); K. Goodman, J. Tobin (Lawrence Livermore National Laboratory); J. Denlinger, E. Rotenberg, A. Warwick (Lawrence Berkeley National Laboratory) Magnetic x-ray linear dichroism (MXLD) in photoelectron spectroscopy and x-ray photoelectron diffraction (XPD) of the Fe3p core levels have been used to probe the magnetic structure-property relationships of perpendicularly magnetized Fe/Cu(001), in an element-specific fashion. A strong MXLD effect was observed in the high-resolution photoelectron spectroscopy of the Fe3p at "normal" emission and was used to follow the loss of perpendicular ferromagnetic ordering as the temperature was raised toward room temperature. In parallel with this, "forward focussing" in XPD was used as a direct measure of geometric structure in the overlayer. These results and the implications of their correlation will be discussed. Additionally, an investigation of the effect of Mn doping of the Fe/Cu(001) will be described. These measurements were performed at the Spectromicroscopy Facility (Beamline 7) of the Advanced Light Source. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. The Spectromicroscopy Facility and the Advanced Light Source were constructed and are operated under support of the U.S. Department of Energy.