Anomalous spin waves in CsFeCl3 and RbFeCl3

We investigate magnetic excitations in the $S=1$ easy-plane-type triangular antiferromagnets ${\mathrm{CsFeCl}}₃$ and ${\mathrm{RbFeCl}}₃$ through a combination of inelastic neutron scattering measurements and spin-wave theory calculations based on an established exchange-coupling Hamiltonian. We show that in either material the model falls short of providing an adequate description of the measured intensities and for ${\mathrm{RbFeCl}}₃$ even fails to reproduce the measured dispersion relation. The most striking discrepancy is a very anisotropic azimuthal intensity distribution in the acoustic spin-wave branches in the long-wavelength limit, which is incompatible with spin-wave theory on a fundamental level. The observed anomalies are attributed to long-range dipolar interactions.