Comparative study of ESR spectra in incommensurate antiferromagnets

The electron spin resonance is studied for noncollinear low-dimensional antiferromagnets RbMnBr$_3$ and RbFe(MoO$_4$)$_2$ in a wide range of frequencies and fields. Both compounds have incommensurate spin structures appearing due to a low-symmetry distortion of an ideal hexagonal crystal lattice. Magnetic field applied in the spin plane induces a first order transition into the commensurate phase. The low-energy resonance branch corresponding to a uniform oscillation of the spin system in the easy-plane is observed in the two compounds both in incommensurate and commensurate phases, with a dramatic change of the spectra taking place near the transition field. The resonance spectrum of a nearly commensurate spin structure with long-wave modulations is analyzed in clean and dirty limits in the framework of a hydrodynamic approach. The resonance branch with steep field dependence in the incommensurate state is attributed to the acoustic mode with the gap resulted from pinning of local domain walls (discommensurations) on defects of the crystal structure.