Spin-polarized electron energy-loss spectroscopy of metastable bcc cobalt films (abstract)

Abstract

Recent developments now allow for the study of the spin dependence of electron energy-loss features for scattering from magnetized ferromagnetic materials. Particular interest in such studies has been focused on Stoner (or spin-flip) excitations1 because of their fundamental nature as well as their proposed role in the polarization enhancement for low energy secondary electron emission from ferromagnets.2 We have performed spin-polarized electron energy loss spectroscopy (SPEELS) on thin (∼40-Å) metastable bcc Co(110) films which have been grown by e-beam evaporation on atomically clean GaAs(110) substrates. Surface order and cleanliness of the substrate and grown film are monitored by LEED and AES. Details of the bcc Co deposition, growth, and characterization have been reported elsewhere.3 After growth, the thin ferromagnetic films are magnetized to near-saturation remanence magnetization along the in-plane [001] (magnetically easy) direction3 by placement in the center of a pulsed coil. The spin-polarized primary electron beam is obtained by photoemission from a negative electron affinity GaAs surface excited by circularly polarized light. The direction of polarization of the electron beam is easily reversed without changing the beam intensity or transport characteristics. The degree of polarization of the source is estimated to be 28–30%. The scattering geometry is chosen so that only the energy-loss spectrum of the specularly reflected beam is measured by a hemispherical energy analyzer. We obtain spin dependent loss spectra for specularly reflected electron beams for a range of primary beam energies. Two polarization dependent features are observed in the energy-loss spectra, a shoulder at 1.6 eV and a well-defined peak at 2.8 eV. The data obtained is normalized to the measured elastic peak intensity, which is assumed to be the same for both spin polarization directions. Polarization analysis shows both features to be due almost entirely to inelastic scattering of incident electrons polarized parallel to the spin-down (minority) direction. The FWHM of both features are quite narrow, suggesting a near rigid exchange splitting for the bcc Co bands. The magnetization reversal behavior and the energy dependence of these spin-dependent features are consistant with the behavior of Stoner excitations. We have tentatively identified the feature at 1.6 eV as the primary Stoner excitation which implies the exchange splitting for bcc Co is also 1.6 eV. The higher-energy peak is most likely a second peak in the Stoner density-of-states.4 Additional theoretical calculations are being performed to verify these identifications.5