We discuss magnon spectrum of an antiferromagnet (AF) in magnetic field consisting of two interpenetrating AF sublattices coupled by exchange interaction and much smaller Dzyaloshinsky-Moria (DM) one. One-ion easy-plane anisotropy is also taken into account which is relevant, in particular, to $\rm NiCl_2$-$\rm 4SC(NH_2)_2$ (DTN). We calculate using $1/S$ expansion the gap in the spectrum and the optical mode splitting which are caused by the inter-sublattice interaction. The latter quantity is determined solely by exchange coupling between sublattices whereas DM interaction, despite its smallness, can make the main contribution to the gap value. It is shown that arbitrary small DM interaction leads to a reorientation transition in magnetic field during which mutual orientation changes of AF sublattices. Application of the results obtained to DTN is discussed.
|Journal||Journal of Physics: Condensed Matter|
|State||Published - 2011|