A Brillouin scattering investigation of NiO

Abstract

Brillouin scattering from single crystals of NiO show that, apart from the expected phonon peaks, there is a broad central peak. The phonon peaks have Fano-like lineshapes, indicating a strong coupling between them and the broad feature, and lead to elastic constants that differ considerably from ultrasonic values. The broad feature is probably ‘magnetic’ in origin since it disappears at the Néel temperature. We speculate that it may be the lower of the two AFMR modes predicted for easy-plane antiferromagnets but which has previously been observed only in EuTe and MnO. The interaction between the phonons and the ‘magnetic’ excitation provides a phenomenological explanation for the differences in the elastic constants determined by different techniques.