AVS1997 Session MI-MoA: Magnetic Spectroscopies

Monday, October 20, 1997 2:00 PM in Room J3

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MI-MoA-1 Spin-Resolved Resonant Photoemission: Probe of Electron Correlation Effects in Magnetic Solids B. Sinkovic (New York University); L.H. Tjeng (University of Groningen, The Netherlands); N.B. Brookes (ESRF, France); G.A. Sawatzky (University of Groningen, The Netherlands) Spin resolved photoemission (SRPES) has made valuable contribution to the understanding of the ground state properties of ferromagnetic solids and novel magnetic structures. SRPES measurements performed at the core-level absorption resonances (M_2,3) have furthermore revealed importance of many-body effects in ferromagnetic solids. We have explored the soft x-ray version of such measurements at L₃ resonances which further enhance the capability of this technique to probe electron correlation effects and the influence on the electronic structure of solids. By combining this technique with the magnetic circular dichroism (MCD) we overcome the limitation of traditional SRPES technique to probe only the solids with remanent magnetization (ferromagnets) and were able to study entire new class of highly correlated solids that exhibit antiferromagnetic or paramagnetic behavior. Example studies of Fe, Ni (above and below the Curie temperature), CuO, and preliminary results from other 3d transition metal oxides will be presented. The work is supported by NSF-DMR-9625340. Authors acknowledge valuable contributions to different aspects of this work made by J.B. Goedkoop, R. Hesper, E. Pellegrin, F.M.F. de Groot, S. Altieri, S.L. Hulbert, and E. Shekel.
2:40 PM		MI-MoA-3 Spin-Orbit and Exchange Interactions in Photoelectron Diffraction F.J. García de Abajo (Lawrence Berkeley National Lab & Universidad del Pais Vasco, Spain); Y. Chen (Lawrence Berkeley National Laboratory); A. Chassé (Martin Luther University, Germany); J. Morais, R. Denecke (Lawrence Berkeley National Laboratory); E.D. Tober (University of California, Davis); M.A. Van Hove (Lawrence Berkeley National Laboratory); C.S. Fadley (Lawrence Berkeley National Lab & Univ. of California, Davis) A fully relativistic description of photoelectron diffraction is presented within the cluster model using a new fast algorithm designed to operate with an exact treatment of multiple scattering up to high orders. No approximation is made in treating the scattering matrix beyond the usual muffin-tin model. Spin-orbit and exchange splitting are included in the initial states, spin-orbit splitting in the final states, and both spin-flip and exchange interactions are considered in the electron scattering. The interplay among these various effects is investigated in connection with spin-polarized photoelectron diffraction, magnetic circular dichroism in angular distributions, and the angular dependence of spin-orbit branching ratios. In particular, spin polarized photoelectron diffraction and magnetic circular dichroism in photoelectron angular distributions obtained for Gd 4s, 5s, 4d, and 4f core-level emission from Gd(0001) are analyzed. The angular distribution of each spin species is discussed. The relative roles played by spin-flip and exchange processes during multiple scattering of electrons coming from different layers is considered. Finally, the influence of partial loss of local magnetic order near the Gd surface is studied. Work supported by the Basque Government, ONR (Contract N00014-94-1-0162), DOE, BES, and Mat. Sci. Div. (Contract DE-AC03-76SF00098).
3:00 PM		MI-MoA-4 Magnetic X-ray Linear Dichroism in Gd 5p Core Level Photoemission S.R. Mishra, D.P. Pappas (Virginia Commonwealth University); T.R. Cummins, G.D. Waddill (University of Missouri, Rolla); K.W. Goodman, J.G. Tobin (Lawrence Livermore National Laboratory) Magnetic x-ray Linear Dichroism in the angle-resolved photoelectron spectroscopy (MXLD) of the Gd 5p core levels has been observed. Thick epitaxial films (~50ML) of Gd were grown on Y(0001) and magnetised in-plane. Photoemission measurements recorded below the Curie temperature were performed in a chiral geometry wherein the Poynting vector and linear polarisation vector of the x-rays and the electron emission direction (along the surface normal) were all in the horizontal plane, with the magnetisation vector perpendicular to this plane. MXLD measurements were achieved by sample rotation of 180 degrees. These measurements were carried out at the Spectromicroscopy Facility [1] at beamline 7 of the Advanced Light Source. Strong spin-orbit and exchange interactions split the Gd 5p core level in the photoemission final state giving rise to intense, well resolved fine structure. The resulting Gd 5p_{3/2, 1/2} multiplets exhibit strong dichroisms reflecting the preferential spin states accessed for differing magnetisation directions. These observations are correlated with our previous observations of MXLD in Gd 4f photoemission [2], including resonant photoemission behavior as well as the magnetic x-ray circular dichroism results of Arenholz et al. [3] This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. The Advanced Light Source and Spectromicroscopy Facility were built and supported by U. S. Department of Energy. This work is based on research supported by the National Science Foundation under Grant No. DDMR-9458004. Additional support from from Research Corporation under Grant no. CC3778 and the Jeffress Trust, No. 338 is also acknowledged. [1] J. G. Tobin, K. W. Goodman, G. J. Mankey, R. F. Willis, J. D. Denlinger, E. Rotenberg, and A. Warwick, J. Appl. Phys. 79, 5626 (1996) and J. Vac. Sci. Tech. B 14, 3171 (1996). [2]. W. J. Gammon, S. Mishra, D. P. Pappas, K. W. Goodman, J. G. Tobin, F. O. Schumann, R. F. Willis, J. D. Denlinger, E. Roternberg, A. Warwick, and N. V. Smith, J. Vac. Sci. Tech. A 15, May/June 1997. [3]. E. Arenholz, Ph.D. Thesis, "Magnetic Dichroism in Photoemission from Lanthanide Materials: Basic Concepts and Applications', and G. van der Laan, E. Arenholz, E. Navas, A. Bauer, and G. Kaindl, Phys. Rev. B 53 R5998 (1996) and references therein.
3:20 PM		MI-MoA-5 Spin Polarization of 3s Photoemission Satellites J.C. Glueckstein, C.C. Hsueh, W.J. Lademan, L.E. Klebanoff (Lehigh University) Spin-resolved x-ray photoelectron spectroscopy (SRXPS) studies are reported of a high-binding-energy satellite associated with Fe 3s photoemission. The satellite is found to possess a complex spin polarization with three distinct components. Two majority-spin components are observed with binding energies 111.0 and 123.4 eV. These majority-spin satellites are accompanied by a minority-spin peak located at 118.6 eV binding energy. The spin analysis, combined with electron-energy-loss measurements, indicate that these 3s satellites do not result from plasmon excitation. Comparison of the SRXPS data with theory for the atomic Fe 3s spectrum suggests the majority-spin satellites are caused by final-state configuration interaction between the 3s¹3p⁶3d⁷ hole state and the nearly degenerate 3s²3p⁴3d⁸ configuration. The origin of the minority-spin satellite component is not understood. Preliminary results from the Co 3s satellite will also be shown.
3:40 PM		MI-MoA-6 Magnetic Dichroism and Spin Polarization in Core-Level Photoemission from Itinerant Ferromagnets J.G. Menchero, C.S. Fadley (Lawrence Berkeley National Laboratory) We present a theoretical investigation of angle- and spin-resolved core-level photoemission from ferromagnetic Fe and Ni. Magneto-dichroic effects due to reversal of photon helicity and/or reversal of sample magnetization are also considered. For the case of Ni, we employ a small-cluster model with periodic boundary conditions ¹ to analyze photoemission spectra from the Ni 2p and 3p core levels. The model treats itineracy and many-body interactions on an equal footing. Calculated results are in excellent agreement with experiment. By considering various limiting cases, we are able to disentangle the effects due to core-valence exchange interaction, core-level spin-orbit splitting, and extra-atomic screening. For the case of Fe, we use an oriented-atom approach to investigate the effect of photoelectron diffraction on the magnetic dichroism with unpolarized light. We find that diffraction effects can lead to large modulations in the dichroism about low-index forward-scattering peaks ². A simple description of the effect is provided. Finally, results from a large cluster calculation including multiple-scattering effects are presented, and found to be in excellent agreement with experiment. ¹J. G. Menchero, Phys. Rev. B55, 5505 (1997). ²A. Fanelsa, R. Schellenberg, F. U. Hillebrecht, E. Kisker, J. G. Menchero, A. P. Kaduwela, C. S. Fadley, and M. A. Van Hove, Phys. Rev. B54, 17962 (1996).
4:00 PM	Invited	MI-MoA-7 STM-Based Measurements of Magnetic Multilayers P.N. First, D.K. Guthrie (Georgia Institute of Technology); S.S.P. Parkin (IBM Almaden Research Center) In order to understand spin-dependent electron transmission in magnetic multilayers, one can measure diffusive transport properties (such as resistivity) as a function of applied magnetic field. However, these measurements average over the sample volume, and provide information about electron transmission only at the Fermi energy. We will describe experiments that use ballistic electron emission spectroscopy (BEES) and scanning tunneling spectroscopy (STS) to obtain energy- and spatially-resolved information about the electron transmittance and density of states in magnetic multilayers. The geometry is current-perpendicular-to-the-plane (CPP) for both spectroscopies (though no special lithography is used) and spin-dependent data is acquired by controlling the relative orientation of the magnetic layers using an external field. Energy-resolved BEES measurements will be relatively easy to compare with theoretical calculations of the electron transmittance versus energy. We also expect that spatially-resolved measurements of the electron transmittance or density-of-states should help to identify sources of variability in the giant magnetoresistance.¹ ¹Supported by ONR and NSF
4:40 PM		MI-MoA-9 Voltage Dependence of Magnetic Tunnel Junctions A.C. Marley, S.S.P. Parkin (IBM Almaden Research Center) Magnetic tunnel junctions (MTJ's) display large resistance changes in response to small changes in the applied magnetic field at fields of a few Oersteds. Magnetoresistance (MR) values greater than 25% have been observed at room temperature with peak MR sensitivities greater than 26%/Oe. One puzzle associated with the MTJ's is the voltage dependence of the MR. Typically, the magnitude of the MR peaks near zero bias and decreases as the applied bias increases. We have studied the voltage dependence of the MR for MTJ's with an Al₂O₃ tunneling barrier using various ferromagnetic materials (Ni, Fe, Co alloys) and different tunneling barrier thicknesses. We find the MR rolloff is fairly insensitive to the ferromagnetic materials indicating a simple density of states explanation for the MR decrease with voltage is inadequate. The MR is found to decrease more quickly with bias for junctions where the underlying magnetic layer has been partially oxidized at the barrier/ferromagnetic interface indicating the presence of interfacial spin depolarization of the tunneling electrons. The temperature dependence of the MTJ I-V characteristics for both the parallel and antiparallel configurations will also be discussed.
5:00 PM		MI-MoA-10 Determination of Magnetic Layer Switching of a Multilayer Y.U. Idzerda, J.W. Freeland, V. Chakarian (Naval Research Laboratory); S. Doherty, J.-G. Zhu (Carnegie Mellon University) The order of switching of magnetic layers in a NiFe/Cu/Co and a Co/Cr/Co trilayer magnetic structure is determined directly using circularly polarized resonant and off-resonant scattering. By monitoring changes in the angular dependence of the magnetic contributions to the helicity-dependent specular reflectivity near the Fe and Co L3 absorption edge energies as a function of an applied magnetic field, the order of the magnetic layer switching is directly obtained. For these heterostructure, the order of magnetic layer switching is defined by the material anisotropies, but is modified by the presence of intra- and inter-layer coupling. Comparing single layer and multilayer magnetometry curves can quantitatively compare these coupling strengths. Varying the energy of the incident photon away from resonance still gives elemental selectivity, but with a greatly modified probing depth, allowing for depth characterization. The order of layer switching of larger multilayers consisting of 3-6 magnetic layers can also be determined.