Minute splitting of magnetic excitations due to dipolar interactions in CsFeCl3 and RbFeCl3 observed with polarised neutrons

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. (C) 1999 Elsevier Science B.V. All rights reserved.