Crystal Dynamics and Magnetic Excitations in Cobaltous Oxide

The low-lying phonon and magnetic excitations of CoO have been studied experimentally and theoretically, both above and below the antiferromagnetic ordering temperature. The experiments were performed using the technique of inelastic scattering of slow neutrons with the aid of a triple-axis crystal spectrometer. These experiments were difficult because the phonon and magnetic excitations have very similar frequencies, and also because the specimen has a domain structure in the antiferromagnetic phase. Nevertheless, models were obtained for the phonon spectra. The magnetic spectra of the paramagnetic phase can be understood if the spin-orbit coupling parameter of ${\mathrm{Co}}^{2+}$ of CoO is reduced to 40% of its free-ion value. With this value it is then possible to understand the excitations in the antiferromagnetic phase in at least a qualitative way by introducing nearest- and next-nearest-neighbor exchange parameters. A detailed evaluation of this theory of the magnetic excitations is not possible, because of the difficulties encountered in the experimental part of the study; nevertheless, it is believed that the description given here is essentially correct.