Electron exchange in the f-f excitations of EuO -: art. no. 125114

We investigated the dipole-forbidden, multiplicity-changing f-f excitations of Eu3+ ions by spin polarized electron energy-loss spectroscopy, using a polarized primary electron beam as well as polarization analysis of the scattered electrons. This is the only experimental technique to prove electron-exchange excitations directly. The f-f excitations give rise to a variety of intense sharp structures in the energy-loss range between approximate to2 and 6 eV. The energy-loss spectra do not change significantly in the primary-energy range between 20 and 100 eV and it could be proved by spin-resolved scattering-geometry dependent measurements that electron exchange is the only excitation mechanism in this energy range. For higher primary energies, the f-f excitation peaks begin to vanish, as expected for excitations by electron exchange, but they reappear at the 4d-4f threshold because of a resonant enhancement, caused by simultaneous 4f-4f and 4d-4f excitations.