Optical spectroscopy in CoO: Phononic, electric, and magnetic excitation spectrum within the charge-transfer gap

Abstract

The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to $28\text{ }000\text{ }{\text{cm}}^{\ensuremath{-}1}$ and for temperatures $8<T<325\text{ }\text{K}$. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at ${T}_{N}=289\text{ }\text{K}$ has been observed. At low temperatures the splitting amounts to $15.0\text{ }{\text{cm}}^{\ensuremath{-}1}$. In addition, we studied the splitting of the cubic crystal-field ground state of the ${\text{Co}}^{2+}$ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below ${T}_{N}$, magnetic-dipole transitions between the exchange-split levels are identified, and the energy-level scheme can be well described with a spin-orbit coupling $\ensuremath{\lambda}=151.1\text{ }{\text{cm}}^{\ensuremath{-}1}$, an exchange constant $J=17.5\text{ }{\text{cm}}^{\ensuremath{-}1}$, and a tetragonal crystal-field parameter $D=\ensuremath{-}47.8\text{ }{\text{cm}}^{\ensuremath{-}1}$. Already in the paramagnetic state electric-quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split $d$ manifold were identified at $8\text{ }000$ and $18\text{ }500\text{ }{\text{cm}}^{\ensuremath{-}1}$.