VOLUME 53, NUMBER 7 PHYSICAL REVIEW LETTERS 13 AUGUsT 1984 Spin-Flip Stoner Excitations in a Ferromagnet Observed by Inelastic Spin-Polarized Electron Scattering H. Hopster, R. Raue, '~ and R. Clauberg Institut fur Festk'orperforschung, Kernforschungsanlage 1'ulich, D 51 7-0 Julich, West Germany, and Il PhysIkalisches Institut, Uni versitat K'oln, D-5000 Koln 41, 8'est Germany (Received 12 September 1983) It is shown by electron-energy-loss spectroscopy with spin analysis that electrons inelasti- cally scattered from a ferromagnet (Fe82Bt2si6) can have a high spin polarization as a result of exchange scattering. A maximum of the spin polarization occurs around 2.2-eV energy loss corresponding to the ferromagnetic exchange splitting. This experiment offers new pos- sibilities for studying spin-flip Stoner excitations in ferromagnets. PACS numbers: 75.30.Et, 71.70.6m, 75.50.Kj, 79.20.Kz Over the last decade electron-energy-loss spec- troscopy (EELS) on metal surfaces (in the low- energy-loss regime) has mainly concentrated on vi- brational spectroscopy of adsorbates. ' Little atten- tion has been paid to the electronic excitations of the substrate which are present as a continuous background. Only very recently have there been some studies on low-energy excitations on clean metal surfaces. In electron scattering from a fer- romagnetic sample the excitation of especially mag- nons and electron-hole pairs is expected to lead to spin-polarization effects because of the imbalance of the number of spin-up and spin-down electrons in the target. Bringer et al. and Yin and Tosatti5 have recently suggested that important spin- polarization effects should occur for electron-hole pair excitations. Glazer and Tosatti have further pointed out that spin-flip excitations across the Stoner gap can be generated and can be observed both in spin-polarized and in ordinary EELS, and they should be characterized by a strong primary- electron energy dependence. Stoner excitations are fundamental in the theory of itinerant-electron fer- romagnetism, although they have never been ob- served directly. In this Letter we show that Stoner excitations are readily observed by inelastic electron scattering with spin analysis. A ferromagnetic glass, Fe828&2Si6, was studied, since it is easily magnetized and has a large ferromagnetic exchange splitting between spin-up and spin-down states. The experiments were performed with the ap- paratus previously described. The sample, a rib- bon of 80x9&0.5 mm, was mechanically clamped together to form a circular loop with the ends over- lapping a few millimeters (see Fig. 1). In this way it forms a closed magnetic circuit and it can readily be magnetized by a current pulse through a coil wrapped around it. This sample geometry has also been used successfully in a recent experiment in- volving scattering of primary spin-polarized elec- Magnetic Ribbon to Energy Anatyze and Mott Detector Coil FIG. 1. Schematic of the scattering geometry. trons. The sample was cleaned by 1.5-keV Ne-ion bombardment, initially for several hours, until a sharp Fermi edge developed in the He r photoemis- sion spectrum and no contamination was detected. The source of the primary electrons is a commercial LEED/Auger electron gun. The overall energy resolution (source and energy analyzer) is 650 meV (full width at half maximum). The electrons im- pinged on the surface at 30' off normal. The scat- tered electrons were analyzed for normal exit from the sample. The scattering angle is thus 150 . The spin polarization of the scattered electrons is due to magnetic effects only, because spin-orbit-induced polarization effects were averaged out by reversing the magnetization of the sample. Figure 2 (lower panel) shows the typical, rather smooth intensity distribution curve in the range of energy loss from 0 to 12 eV for 180 eV primary beam energy. No significant structure is observed in this energy range. The two upper panels of Fig. 2 show the measured spin-polarization spectra for two different primary energies. For the elastically scat- 1984 The American Physical Society 695