Localizedd-dexcitations in NiO(100) and CoO(100)

The localized character of the 3d electrons in the antiferromagnetic oxides NiO(100) and CoO(100) has been studied with electron energy-loss spectroscopy at primary energies between 20 and 1200 eV. The spectra of both charge-transfer compounds exhibit weak but sharp loss structures within the insulating gap region due to crystal-field excitations of the 3${\mathit{d}}^{\mathit{n}}$ configuration (${\mathit{d}}^{\mathit{n}}$→${\mathit{d}}^{\mathit{n}\mathrm{*}}$). These parity forbidden d-d excitations are strongly enhanced at low primary energies due to exchange scattering. Their observed loss intensity depends on the momentum transfer at the scattering process as is shown by angle-dependent measurements. At NiO(100) a surface d state is found that is due to the lower symmetry of the ligand field at the surface. The measurements demonstrate the existence of spin-forbidden exchange excitations ${}_{}{}^3{}_{}{}^{}$ ${\mathit{A}}_{2\mathit{g}}$→${(}^1$ ${\mathit{E}}_{\mathit{g}}$ ${,}^1$ ${\mathit{T}}_{1\mathit{g}}$) in NiO(100) and reveal their excitation energies. A resonant enhancement of all intra-atomic d-d transitions in NiO(100) is found at the Ni 3s excitation threshold. Temperature-dependent measurements up to the Néel point show only little influence of the antiferromagnetic ordering on the EELS spectra of NiO(100). The different shapes of the absorption edges due to interatomic d-d transitions across the insulating gaps of NiO(100) and CoO(100) are discussed.