Infrared Antiferromagnetic Resonance in MnO

Antiferromagnetic resonance has been observed in powdered MnO at 364 μ at 1.5°K. The resonance frequency as a function of temperature falls roughly as the square root of the Brillouin function of spin $\frac{5}{2}$ . Fields of the order of ${10}^4$ oe have no observable effect on the line at 1.5°K. All of these results give quantitative confirmation of a model in which the sublattice magnetizations are constrained to a (111) plane by magnetic dipolar interactions and are held to a preferred direction within the plane by a much weaker anisotropy.