B. Dorner, D. Petitgrand and H. U. Güdel
Minute splitting of magnetic excitations due to dipolar interactions in CsFeCl₃ and RbFeCl₃ observed with polarised neutrons
Physica B 267-268, 263-265 (1999)                 

Abstract: The splitting due to dipolar interactions was investigated for the hexagonal [1 1 0] direction (y-direction). The S^x and S^y fluctuations were exclusively visible in the non-spin-flip and the spin-flip channels, respectively. The splitting creates minima in the dispersion curves away from the commensurate K-point (1/3 1/3 0). In CsFeCl₃, which does not exhibit long-range order, the excitations are temperature independent at low temperature. In RbFeCl₃ a soft mode behaviour is observed and the new minima in the dispersion curves correspond to the observed incommensurate structures. In the commensurate phase of RbFeCl₃ below 1.95 K an acoustic and an optic mode were observed.