G.D. Waddill

Evidence of dynamical spin shielding in Ce from spin-resolved photoelectron spectroscopy

Using Fano effect measurements upon polycrystalline Ce, we have observed a phase reversal between the spectral structure at the Fermi edge and the other 4f derived feature near a binding energy of 2 eV. The Fano effect is the observation of spin polarized photoelectron emission from nonmagnetic materials, under chirally selective excitation, such as circularly polarized photons. The observation of phase reversal between the two peaks is a direct experimental proof of Kondo shielding in Ce, confirming the predictions of Gunnarsson and Shoenhammer, albeit with a small modification.

Narrowing the range of possible solutions to the Pu electronic structure problem: Developing a new Bremstrahlung Isochromat Spectroscopy capability

Bremstrahlung Isochromat Spectroscopy (BIS) has a proven record for the probing of unoccupied electronic structure, including feasibility studies of actinides such as U and Th. A description of the BIS or Inverse Photoelectron Spectroscopy (IPES) process and of our reasoning concerning the potential importance of extending the application of BIS related techniques to the actinides, will be provided. To put this into the proper context, our past successes, using soft x-ray techniques to interrogate the electronic structure of Pu, will be briefly reviewed. Finally, the initial results of the calibration of the BIS device, including preliminary spectra from of the surrogate system CeOxide, will be presented.

Spin-polarized electron energy loss spectroscopy on Fe(100) thin films grown on Ag(100)

We report sharp spin-dependent energy loss features in electron scattering from bcc Fe(100) thin films grown on Ag(100). Majority spin features are observed at ~1.8 and 2.5 eV energy loss, and a minority spin feature is observed at ~2.0 eV energy loss. The majority spin peaks are attributed to spin-flip exchange scattering from the Fe films. The minority spin peak is attributed to non-flip exchange scattering with an energy corresponding to the separation between occupied and unoccupied minority spin bands. The observed peak energies compare favourably with electronic structure calculations for Fe and with spin-resolved photoemission and inverse photoemission results.

Observation of x-ray absorption magnetic circular dichroism in well-characterized iron-cobalt-platinum multilayers

Magnetic circular dichroism in the Fe 2p x-ray absorption is observed in multilayers of(Fe9.5{Angstrom}/Pt9.5{Angstrom}){sub 92}. The magnetization and helicity are both in the plane of this multilayer which is prepared by magnetron sputter deposition. This sample is part of a study to examine magnetization in the ternary multilayer system of FeCo/Pt. Lattice and layer pair spacings are measured using x-ray scattering. The atomic concentration profiles of the multilayer films are characterized using Auger electron spectroscopy coupled with depth profiling. Conventional and high resolution transmission electron microscopy are used to examine the thin film, growth morphology and atomic structure.

The Utilization of Spin Polarized Photoelectron Spectroscopy as a Probe of Electron Correlation with an Ultimate Goal of Pu

Spring 2008 Materials Research Society Meeting Actinides IV, Symposium NN San Francisco, CA, USA March 24-28, 2008 The Utilization of Spin Polarized Photoelectron Spectroscopy as a Probe of Electron Correlation with an Ultimate Goal of Pu J.G. Tobin 1 , S.W. Yu 1 , B.W. Chung 1 , S.A. Morton 1,2 , T. Komesu 3 ,* and G.D. Waddill 3 1.Lawrence Livermore National Laboratory, LLNS-LLC, Livermore, CA, USA 2.Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3.University of Missouri-Rolla, Department of Physics, Rolla, MO, USA We are developing the technique of spin-polarized photoelectron spectroscopy as a probe of electron correlation with the ultimate goal of resolving the Pu electronic structure controversy. Over the last several years, we have demonstrated the utility of spin polarized photoelectron spectroscopy for determining the fine details of the electronic structure in complex systems such as those shown below. The proposed half-metallic ferro-magnet Fe3O4 [Figure 1, Ref 2] Figure 1 a) Spin resolved photoemission spectra from a) “as received” sample hν=160eV. b) Spin resolved photoemission spectra from a sample after Ne+ sputtering, hν=160eV. Note the large effect observed in the as received sample in (a) and the reduction of the effect in (b) Tobin Page 1

Magnetic coupling in spin-valves from magnetic circular dichroism

The authors have applied magnetic x-ray circular dichroism (MXCD) to extract elemental magnetic moments for real, thin-film spin valve devices and demonstrated the correlation between MXCD and R-H measurements on those devices. The spin valves consisted of NiFe/Co/Cu/Co/NiFe sandwiches with variable Cu spacer-layer thickness, grown on a thick NiO film, as described previously. One permalloy layer is therefore magnetically pinned to the substrate which it contacts. Spin-polarized, X-ray absorption spectra were collected using beamline 8-2 at the Stanford Synchrotron Radiation Laboratory. The Poynting vector of the incident beam was aligned at grazing incidence to the films, either parallel or antiparallel to their remanent magnetization. For each spin valve, four absorption spectra were collected, With the remanent magnetization initially parallel to the incident beam direction, a pair of spectra were recorded, one with the helicity of the incident photons parallel to the remanent magnetization of the device, and one with their helicity antiparallel to the remanent magnetization. Next, the samples were rotated in order to align their remanent magnetization antiparallel to the incident beam direction, and a pair of absorption spectra was similarly obtained from beams polarized with opposite helicity. Finally, each absorption spectrum was normalized to the incident photon flux.

Magnetic x-ray circular dichroism in nickel-gold multilayers

Magnetic circular dichroism in x-ray absorption is used to investigate the in-plane, remnant magnetization of well-characterized Ni{sub 0.48}/Au{sub 0.52} multilayers. Large superlattice strains are found in this multilayer system for samples with a 2nm layer pair spacing. A larger dichroism is found in the Ni 2p absorption edge for a 1.8 nm than for a 4.4 nm layer pair sample. The larger dichroism is consistent with a larger magnitude of in-plane strain for the Ni layers and a larger total magnetic anisotropy energy as previously shown from magnetization curves.