Antiferromagnet with two coupled antiferromagnetic sublattices in a magnetic field

A. V. Sizanov, A. V. Syromyatnikov

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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.
Original languageUndefined
Pages (from-to)146002_1-8
JournalJournal of Physics: Condensed Matter
Issue number14
StatePublished - 2011
Externally publishedYes

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