Optical Spectroscopy in CoO: Phonons, Electric, and Magnetic Excitations

TL;DR

This study uses optical spectroscopy to analyze phonons, electric, and magnetic excitations in CoO across a wide temperature range, revealing phase transition effects, crystal field splittings, and electronic level structures.

Contribution

It provides a detailed spectroscopic characterization of CoO, including phonon splitting, magnetic excitations, and electronic levels, with new insights into the spin-orbit and crystal field interactions.

Findings

Phonon mode splits below the antiferromagnetic transition at 289 K.

Magnetic dipole transitions identified below T_N.

Electronic levels of the crystal-field-split d-manifold observed at 8,000 and 18,500 cm⁻¹.

Abstract

The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to 28,000\wn and for temperatures 8 $< T <$ 325 K\@. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at $T_N$ = 289 K has been observed. At low temperatures the splitting amounts to 15.0\wn. In addition, we studied the splitting of the cubic crystal field ground state of the 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 $\lambda = 151.1\wn$, an exchange constant $J = 17.5\wn$, and a tetragonal crystal-field parameter $D = -47.8\wn$. 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\wn.