The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to $28000{\text{cm}}^{-1}$ and for temperatures $8<T<325\text{K}$. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at $T_N=289\text{K}$ has been observed. At low temperatures the splitting amounts to $15.0{\text{cm}}^{-1}$. 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 $\lambda =151.1{\text{cm}}^{-1}$, an exchange constant $J=17.5{\text{cm}}^{-1}$, and a tetragonal crystal-field parameter $D=-47.8{\text{cm}}^{-1}$. 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 $8000$ and $18500{\text{cm}}^{-1}$.

• Received 5 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.245103