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 cm-1 and for temperatures 8<T<325 K. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at TN =289 K has been observed. At low temperatures the splitting amounts to 15.0 cm-1. In addition, we studied the splitting of the cubic crystal-field ground state of the Co2+ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below TN, magnetic-dipole transitions between the exchange-split levels are identified, and the energy-level scheme can be well described with a spin-orbit coupling λ=151.1 cm-1, an exchange constant J=17.5 cm-1, and a tetragonal crystal-field parameter D=-47.8 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 8 000 and 18 500 cm-1. © 2008 The American Physical Society.