AVS1996 Session MI-TuM: Surface Magnetism and Spectroscopy

Tuesday, October 15, 1996 8:20 AM in Room 106A/B

Tuesday Morning

Start	Invited?	Item
8:20 AM		MI-TuM-1 Magnetic Dichroism Effect of Binary Alloys using Circularly Polarized X-ray S. Wu, F. Schumann, R. Willis (Pennsylvania State University); K. Goodman, J. Tobin (Lawrence Livermore National Laboratory); R. Carr (Stanford University) We have studied the magnetic properties of CoNi binary alloy films with various chemical compositions using soft x-ray magnetic circular dichroism (XMCD). The alloy films were deposited on a single Cu(100) crystal in situ using our well established epitaxial growth technique to achieve a layer-by-layer growth and a metastable fcc structure, with all the films exhibiting an in-plane magnetic anisotropy. The high density, circularly polarized x-ray beam was supplied by the Elliptically Polarizing Undulator at the Stanford Synchrotron Radiation Laboratory. Utilizing the element-specific ability and nanostructure magnetization sensitivity of this technique, we have been able to perform the absorption measurements at L\sub 2\ and L\sub 3\ edges of Co and Ni atoms and observed a large dichroism signal. The spin moment and orbital moment for each element has been determined separately using a pair of magneto-optical sum rules. A comparison with the x-ray magnetic linear dichroism (XMLD) measurements which derive the asymmetry of the 3p core level photoemission spectra from two mirror-image configurations was made. These results have been found to be complementary to our earlier study of this system using magneto-optic Kerr effect.
8:40 AM	Invited	MI-TuM-2 Magnetic Properties of Metastable Structures W. O'Brien (Synchrotron Radiation Center); B. Tonner (University of Wisconsin, Milwaukee) Metastable films of bcc Ni, bcc Co, fcc Co and fcc Fe were grown by molecular beam epitaxy on single crystal substrates. The metastable phases of bcc Ni, bcc Co and fcc Co are found to be ferromagnetically ordered while the metastable phase of fcc Fe is anitferromagnetically ordered. In order to learn more about the magnetic properties of these metastable films we have used x-ray magnetic circular dichroism which gives element specific information on orbital moments and magnetic ordering. Large changes in the coercive field, Hc, are found for growth of fcc Co/Cu(001), Ni/Cu(001), and Fe/fcc Co(001). These large changes in Hc are correlated to the formation of misfit dislocations which occur at the phase transition between coherent and noncoherent growth. By concentrating on the regions of coherent growth we investigate changes in the orbital moments vs. film thickness for fcc Co and fcc Fe. A 40% enhancement of the orbital moment is found for fcc Co surface atoms. Changes in the orbital moment at the surface for fcc Fe are compared to that of bcc Fe for growth on Ag(001) and Pd(001). In a final set of experiments we investigate the magnetic ordering of monolayers of Mn and Cr on bcc Fe, Co and Ni and fcc Co. In a number of cases, Mn/fcc Co and Mn/bcc Co, the type of magnetic ordering which is found would not be predicted by the bulk magnetic phase diagram.
9:20 AM		MI-TuM-4 Spin Polarized Photoelectron Diffraction from Gd(0001) Films E. Tober (Lawrence Berkeley National Laboratory and University of California, Davis); F. Palomares (Lawrence Berkeley National Laboratory); Z. Wang, R. Ynzunza (Lawrence Berkeley National Laboratory and University of California, Davis); Z. Hussain (Lawrence Berkeley National Laboratory); C. Fadley (Lawrence Berkeley National Laboratory and University of California, Davis) Utilizing an advanced photoelectron spectrometer/diffractometer on the high- resolution beamline 9.3.2 of the Advanced Light Source, we have made the first temperature-dependent spin polarized photoelectron diffraction (SPPD) measurements on a ferromagnetic system. These measurements were performed on thick (~100 ML) Gd(0001) films grown on W(110) substrates, and made use of the inherently spin-polarized Gd 4s and 5s multiplets, each of which splits into \super 7\S and \super 9\S peaks. The SPPD technique is ideally suited for probing short-range and surface magnetic ordering due to the nature of the diffraction process and the low photoelectron kinetic energies of ~100 eV. Examining the asymmetry of the \super 7\S to \super 9\S peak ratios from both 4s and 5s as a function of temperature from 200 - 520 K, we have observed two distinct breaks in these asymmetries. One such break occurs near 295 K, the bulk Curie temperature of Gd. The second occurs near 355 K, very close to what has previously been proposed to be the Curie temperature of the Gd(0001) surface. These results, coupled with spin-dependent full multiple scattering calculations, lend further support to the existence of surface magnetic ordering beyond the bulk transition temperature for this ferromagnet. The potential of SPPD for future studies of ferromagnetic systems will also be discussed. **Work supported by ONR (Contracts N00014-90-5-1457 and N00014-94-1-0162) and DOE, BES, Mat. Sci. Div. (Contract DOE-AC03-76SF00098).
9:40 AM		MI-TuM-5 Spin-Resolved Resonant Photoemission at Fe 2p Core: Probe of Electron Correlation Effects B. Sinkovic, E. Shekel (New York University); S. Hulbert (Brookhaven National Laboratory) We report on spin resolved resonant Fe3p and Fe 3s photoemission measurements at Fe L\sub 3\ absorption edge. The experiments were performed with NYU's spin-resolving electron spectrometer and linearly polarized soft X-ray radiation from the U13UA beam line at NSLS. Samples were thick Fe films grown epitaxially on Ag(001) substrate. A giant enhancement (~ two orders of magnitude) at Fe L\sub 3\ edge makes the resonant photoemission channel dominate the spectra which in turn makes the interference effects with the direct photoemission negligible, eliminating complications encountered at M\sub 2,3\ resonance. We use this strong resonant enhancement to study small satellite structures that are usually very weak or not seen in core-level spectra. The Fe 3p spectra on- and off- L\sub 3\ resonance (using hv=707 eV and 260 eV photons respectively) are compared. In order to account for the dependence of the resonant photoemission process on the Auger matrix elements, resonant Fe 3p photoemission spectra are also compared to spin resolved Fe 2p3p3d Auger spectra. These comparisons indicates presence of high binding energy Fe 3p satellite not observed in the off-resonance Fe 3p spectra. The spin analysis indicates that this feature carries strong positive polarization. Implication of such satellite feature for explanation of negative Fe 3p net spin polarization is discussed. Similar analysis is performed for Fe 3s emission as well as for Fe 2p3p3p resonant photoemission.
10:00 AM		MI-TuM-6 Resonant Enhancement of Magnetic Linear Dichroism Photoemission from Gd on Y(0001) W. Gammon (Virginia Commonwealth University); K. Goodman (Lawrence Livermore National Laboratory); F. Schumann (Pennsylvania State University); J. Tobin (Lawrence Livermore National Laboratory); N. Smith (Lawrence Berkeley National Laboratory); D. Pappas (Virginia Commonwealth University) The enhancement of the magnetic linear dichroism in resonant 4f photoemission (MLDRPE) is studied from a 50 monolayer film of Gd/Y(0001). The ALS at beamline 7 provided the source of linearly polarized x-rays used in this study. The polarized light was incident at an angle of 30\sub o\ relative to the film plane, and the sample magnetization was parallel or antiparallel to the photon polarization. The linear dichroism of the 4f core levels is measured as the photon energy is tuned through the 4d-4f resonance. We find that the MLDRPE asymmetry is strongest at the resonance. Near the threshold the asymmetry has several features which are out of phase with the fine structure of the total yield. We would like to thank R. Willis for his support. This work is based on work supported by the National Science Foundation under grant No. DDMR-9458004. Additional support from Research Corporation under grant No. CC3778 and the Jeffress Trust, No. J338 is also acknowledged.
10:20 AM	Invited	MI-TuM-7 Vector Magnetometry in Ultrathin Magnetic Structures with Atomic Layer Resolution by Polarized Neutron Reflection J. Bland (University of Cambridge, United Kingdom) The way in which the magnetic structure at interfaces in thin and ultrathin films can be affected by the presence of atomic scale features such as atomic steps, defects and chemical intermixing is now recognised as central to understanding phenomena such as interlayer exchange coupling, giant magnetoresistance (GMR) and interface anisotropies. The interface magnetic moments differ from those of the film interior due to the modified electronic structure associated with the interface, reduced atomic coordination, chemical intermixing and strain effects for example. In this talk the capabilities of polarized neutron reflection (PNR) for directly determining the magnetic and non-magnetic structure with atomic layer resolution are reviewed, including measurements with polarisation analysis. The combination of structural and magnetometric information therefore makes PNR an ideal probe of the magnetic structure of ultrathin films. Estimates of the interface roughness and layer thickness can be accurately made and diffuse scattering measurements can be used to probe spin and structural disorder at interfaces. The magnetisation profile across chemically homogeneous ultrathin films can be measured with atomic layer precision in favourable cases and examples of recent measurements of the interface magnetisation in ultrathin Fe films are discussed. The magnetometric information provided by PNR is therefore complementary to that provided for example by X-ray circular dichroism (XMCD). Measurements of the specular and diffuse reflectivity are shown yield both the interface dependent roughness amplitude and lateral correlation length. Vector magnetometry measurements of the layer dependent magnetic moment orientations are shown to provide a powerful approach to studying spin orientations in ferromagnetic (FM)/non-magnetic (NM)/FM trilayer structures as function of magnetic anisotropies and interlayer exchange coupling. For weakly coupled epitaxial FeNi/Cu/Co(001) spin valve structures with uniaxial anisotropy it is demonstrated that the layer dependent moment orientation and amplitude can be determined with percent precision as a function of applied field and the conditions for coherent rotation of the full FeNi moment described. The interface magnetic moments obtained for the Co and FeNi films are shown to agree with SQUID magnetometry measurements of the average moment. Finally the thickness dependent (Ni thickness up to ~200 angstroms) magnetic moment of epitaxial Cu/fct Ni/Cu(001) structures with competing strain-induced perpendicular and in-plane shape anisotropies is determined as a function of applied field and compared with the results of conventional magnetometry measurements. A strong variation in average moment is found which is reflected in the variation of the ratio of orbital and spin moments with thickness determined from XMCD measurements. A decreasing magnetic moment occurs with decreasing Ni thickness from a near bulk value close to 150 angstroms Ni thickness. Reflection high energy electron diffraction measurements of the in-plane lattice parameter show a strain variation which is strongly correlated with that of the magnetic moment. Finally, the form of the fitted magnetisation profile across the Ni layer is discussed.
11:20 AM		MI-TuM-10 Spin-Resolved and Circularly-Polarized Resonant Photoemission: Study of Ni Above and Below the Curie Temperature B. Sinkovic (New York University); L. Tjeng, G. Sawatzky (University of Groningen,The Netherlands); N. Brookes (ESRF, France) We report on the first spin- and circular-resolved resonant (L\sub 3\) photoemission measurements. In such measurement the role of sample magnetization is replaced by the photon spin polarization (through magnetic circular dichroism) making this new technique applicable to study of local spin polarization of any magnetic material, including antiferromagnets and paramagnets. The experiments were performed at ESRF, Grenoble employing circularly polarized soft X-ray radiation (at Ni 2p\sub 3/2\->3d\super *\resonance) from ID12-BL26 beam line and NYU's spin-resolving electron spectrometer. The first measurements were performed on thick Ni(001) films (>50\Ao\ grown on Cu(001). Below the Curie temperature (at room temperature, 0.5T\sub c\) measurements performed on demagnetized Ni films show large positive spin polarization demonstrating the feasibility of such experiments. The spectra also reveal the local spin character of Ni single-particle excitation spectrum (d\super 8\ final states: \super 1\G, \super 3\F ) demonstrating the importance of \super 3\F triplet and its mixing into the ground state wavefunction. Measurements performed above the Curie temperature (1.04T\sub c\) show no appreciable change in the local spin polarization suggesting no change in the magnitude of Ni moment above the Curie temperature, supporting the fluctuating band model.