Spiral antiferromagnet

### Intermediate phase in the spiral antiferromagnet Ba2CuGe2O7

J. Chovan and N. Papanicolaou
Department of Physics, University of Crete, and Research Center of Crete, Heraklion, Greece

S. Komineas
Physikalisches Institut, Universitaet Bayreuth D-95440 Bayreuth, Germany

Abstract

The magnetic compound Ba$_{2}$CuGe$_{2}$O$_{7}$ has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the $c$-axis exceeds a certain critical value $H_c$. The $T=0$ dynamics is described here in terms of a continuum field theory in the form of a nonlinear $\sigma$ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields $H_1$ and $H_2$ such that \$H_1