Magnetic excitations in CoO below TN = 289 K were investigated on single crystals and a powder specimen by using inelastic neutron-scattering spectrometers installed at a steady state neutron-source and a pulsed neutron source. In single-crystal data, dispersion relations of the two magnetic excitation modes below 70 meV were determined along the symmetric directions. The magnetic excitations around 120 meV were also observed. Energy spectra on a powder specimen suggest the existence of higher order magnetic excitations spreading up to around 200 meV. The magnetic modes were analyzed in connection with the freedom of an unquenched orbital angular-momentum L of a Co 2+ ion. The magnitude of the spin-orbit interaction and that of the exchange interaction obtained from the analyses are comparable, indicating that those interactions must be simultaneously considered without an approximation of the total-angular-momentum. The lowest two dispersion curves are interpreted as mixing and splitting between a dispersive excitation of Sz (S-mode) and an individual excitation of Lz (L-mode). The present experiments are the first observation of such hybridization of the S-mode and the L-mode.
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