Anomalous spin waves in CsFeCl${}_3$ and RbFeCl${}_3$

Abstract

We investigate magnetic excitations in the $S=1$ easy-plan-type triangular antiferromagnets CsFeCl${}_3$ and 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 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.

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