VOLUME 61, NUMBER 10 PHYSICAL REVIEW LETTERS 5 SEPTEMBER 1988 Stoner Transitions and Spin-Selective Excitations in bcc Cobalt Y. U. Idzerda, D. M. Lind, D. A. Papaconstantopoulos, G. A. Prinz, B. T. Jonker, and J. J. Krebs Naval Research Laboratory, 8'ashington, D. C. 20375 (Received 30 March 1988) Spin-selective excitations (Stoner excitations) have been unambiguously identified by comparisons be- tween spin-polarized-electron energy-loss spectra and calculated transition density of states. Analysis of the specular beam for polarized electrons incident on thin films of bcc Co shows three sharp, highly po- larized loss features (background corrected asymmetry near 100%). Comparison with the theoretical Stoner and spin-nonflip transition density of states sho~s excellent agreement, permitting identification of the associated loss mechanisms. PACS numbers: 75.30.Et, 71. 70.6m, 75.SO. Cc, 79.20. Kz Collective excitations (spin waves) have been experi- mentally verified and characterized by neutron scattering for quite some time. ' Much more recently, experimental techniques to probe single-particle excitations, especial- ly spin-flip Stoner transitions (electronic excitations resulting in an electron in the conduction band and a hole of the opposite spin in the valence band), have been developed. Spin-polarized-electron energy-loss (SPEELS) measurements on single-crystal Ni and poly- crystalline alloys of Fe have yielded evidence for the first observations of Stoner transitions, but the analysis of these measurements has been impaired by a lack of com- parison with theoretical calculations of the Stoner transi- tion density of states (DOS) and by an intrinsic compli- cation arising from the existence of possible spin-nonflip excitations (direct excitations) which can have the same incident polarization dependence and loss character as the Stoner transition. (Stoner DOS have been calculat- ed for spin-wave measurements and very recently for use in SPEELS analysis, but with only limited success. ) This Letter reports on SPEELS measurements on bcc Co which is a nearly ideal ferromagnetic material for the observation of Stoner excitations. For the first time, multiple, sharp, loss features are observed. Comparison of the rich experimental spectra with calculated Stoner DOS derived from calculations of the bcc Co band struc- ture' " show excellent agreement with the SPEELS features allowing for an unambiguous difl'erentiation be- tween Stoner and spin-nonflip transitions. The calculat- ed band structure of bcc Co shows that it is a good ma- terial for Stoner transition measurements; a completely band-saturated ferromagnetic (i. e. , one with completely filled majority states) with a large mean exchange split- ting of =1. 6 eV. Although the bcc structure of Co has not been synthesized in bulk form, atomically clean and well characterized thin films of limited thickness have been epitaxially deposited on GaAs. In contrast, the fer- romagnetic materials previously examined by SPEELS are not ideal — Fe is a band-unsaturated ferromagnet, possessing a significant number of unfilled majority states, and Ni has a small exchange splitting. n [001] i I ]'i M E. VIEW OF SCATTERING PLANE INCIDENT, g--':-'. - a', . ) II its ~~ [110] POLARIZED BEAIN SPECULAR BEAM ANAI YZED FIG. 1. The scattering geometry of SPEELS measurement. Polarized electrons are aligned (spin-up) or antialigned (spin- down) with the magnetization direction. The corresponding view of the scattering plane with the incident and reflected electron directions are also indicated. In these experiments, a =40-A film of bcc Co(110) grown epitaxially on GaAs(110) is used as the target. The film is characterized by Auger-electron spectrosco- py, reflection high-energy electron diffraction, and vibrating-sample magnetometry with similar results as to those previously described. ' ' The film can be com- pletely magnetized along the magnetically easy [001] direction by poling in situ with a solenoid. The electron energy-loss measurements are made with a =30% polar- ized e beam, generated by photoemission from a GaAs source, ' incident at 45' to the sample normal and along the [100] direction as shown in Fig. 1. The specular beam, which is emitted at 90' from the electron-source axis and along the [010] direction, is energy analyzed by a hemispherical detector with a 4' acceptance angle. The experimental loss spectrum for the two incident polarization directions is shown in Fig. 2(a). The elastic peak [with a measured full width at half maximum (FWHM) of 0.6 eV] has been removed. The width of the two lowest-lying loss peaks (centered at 1. 6 and 3. 1 eV) is =0. 6 eV, the instrument resolution. Therefore, the underlying loss features themselves are sharp, in con- trast to the broad Stoner features previously reported for Ni, Fe, and Fe alloys (where only one loss feature was observed). A third, much broader feature peaks at 4.4 1222 1988 The American Physical Society