Far Infrared Antiferromagnetic Resonance in MnO and NiO

The uniaxial dipolar anisotropy in antiferromagnetic MnO and NiO aligns the two opposing sublattice moments in an easy (111) plane. The two normal modes for small oscillations of the sublattice magnetizations about a preferred direction within this plane are both nondegenerate and linearly polarized. The high-frequency mode has been observed at low temperatures (2\ifmmode^\circ\else\textdegree\fi{}K) at 27.5 and 36.6 ${\mathrm{cm}}^{\ensuremath{-}1}$ in MnO and NiO, respectively. In agreement with the unobservably small second-order Zeeman shift expected for these non-degenerate modes, a 10.7-kOe field was found to have no effect on the observed resonance frequencies. Increasing the sample temperature decreases the frequency of the mode in each case. The experimental temperature dependence of the antiferromagnetic resonance frequency can be accounted for by present theories only if an anomalous temperature dependence, previously found from neutron-diffraction data, is assumed for the temperature dependence of the sublattice magnetization.