Spin-flip low-energy electron-exchange scattering in NiO(100)

Spin-polarized electron-energy-loss spectroscopy was applied to study electron-exchange processes in insulating NiO(100). Dipole-forbidden d-d excitations of the ${\mathrm{Ni}}^{2+}$ ion give rise to sharp gap structures in the energy-loss spectrum. Our measurements show strong spin-flip electron-exchange scattering in the excitations with 1.1- and 1.6-eV energy loss up to high primary electron energies of more than 100 eV. No spin-flip exchange scattering is found in the excitations leading to the loss structure around 2.8 eV.