J. Chovan, M. Marder, N. Papanicolaou
We present a theoretical study of magnetic properties of the two-dimensional spiral antiferromagnet Ba2CuGe2O7 in the presence of an external magnetic field. Some time ago we predicted that when a field is applied parallel to the c axis, a previously anticipated Dzyaloshinskii-type incommensurate-to-commensurate phase transition is actually mediated by an intermediate phase. This phase was recently observed in experiments where magnetic fields were nearly parallel to the c axis and referred to as a double-k structure. Here we calculate the low-energy magnon spectrum throughout the phase transition, verifying stability of the intermediate phase, and providing the opportunity for comparison with inelastic neutron scattering. In addition, we calculate the T=0 phase diagram for arbitrary canted magnetic fields, in general agreement with experiment. In particular we show that a weakly broken U(1) symmetry of our model accounts for sudden pi/2 rotations of the magnetic structures observed in experiment. Finally, our analysis suggests a nonzero weak-ferromagnetic component in the underlying Dzyaloshinskii-Moriya anisotropy, which is not crucial for the appearance of the double-k structure, but is important to restore quantitative agreement with experiment.
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http://arxiv.org/abs/1211.5782
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