Magnetic transitions in triangular antiferromagnets with distorted exchange structure

Finite-field properties of easy-plane antiferromagnets with deformed stacked triangular lattices are considered. Different distortions of the ideal 120° spin structure at H=0 lead to different types of behavior in a magnetic field applied in the basal plane. For several of them, additional orientational phase transitions are predicted. The theory explains unusual properties of ${\mathrm{RbMnBr}}_3$ including the low-field phase transition between incommensurate and commensurate spin structures. We report also the results of antiferromagnetic resonance measurements in ${\mathrm{KNiCl}}_3$, which detect unambiguously two nonequivalent interchain exchange constants in this compound.