Internal oscillations of kink-type solitons in one-dimensional antiferromagnets like TMMC

The problem of internal oscillations of kink-type solitons in a one-dimensi onal easy-plane antiferromagnet is studied by analytical methods. Apart fro m the Goldstone mode a second local mode which is due to the coupling betwe en in-plane and out-of-plane spin components has been derived. For typical experimental conditions in a model system like (CH3)(4)NMnCl3 (TMMC) the fr equency of this mode is very close to the bottom of the magnon band. Consid ering both a twofold and a weak sixfold in-plane crystal field anisotropy w e found that close to the spin-flop transition the separation of this local mode from the magnon band is increased and dramatically affected by the si xfold anisotropy. We show that quantum properties of this mode are not impo rtant.