Direct and Temperature-Modulated Reflectance Spectra of MnO, CoO, and NiO

Abstract

Direct near-normal-incidence reflectance spectra were measured for MnO, CoO, and NiO between 3 and 27 eV at room temperature and below, and the corresponding dielectric response functions, $\ensuremath{\epsilon}={\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2}$, were obtained by Kramers-Kronig inversion. Thermoreflectance (TR) spectra between 4 and 8 eV at various temperatures were also measured. Spectra for all three materials support a model containing both localized and one-electron band states. In MnO peaks with temperature coefficients of \ensuremath{\sim} ${10}^{\ensuremath{-}3}$ eV/K were observed at 5.7 and 6.9 eV, temperature-independent structure occurred at 5.4, 6.3, and 7.2 eV, and spectral features with indeterminate temperature dependence were seen at 4.6 and 5.5 eV. The temperature-dependent structure was assigned to one-electron interband transitions associated with anion $2p$ and cation $4s$ states. The temperature-independent structure was assigned to crystal-field-split localized interionic transitions between the $3d$ states of neighboring Mn ions. The crystal field splitting of the $3d$ states was determined to be 0.9 eV. TR spectra for CoO exhibited temperature-dependent structure (9.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ eV/K) at 5.0, 6.0, and 7.2 eV and additional structure at 2.8, 3.2, 3.8, 4.2, 5.35, and 6.25 eV. Interpretations of the optical spectra of CoO and NiO analogous to those of MnO are proposed. The interband gap between the anion $2p$ and cation $4s$ bands was determined to be 5.7 eV in MnO, 6.0 eV in CoO, and 6.2 eV in NiO.