Anomalous spin waves in CsFeCl$_{3}$ and RbFeCl$_{3}$

TL;DR

This study reveals anomalous spin-wave behaviors in CsFeCl₃ and RbFeCl₃, showing that traditional spin-wave theory cannot fully explain the observed anisotropic intensity distributions, likely due to long-range dipolar interactions.

Contribution

The paper demonstrates that standard spin-wave theory fails to describe the magnetic excitations in these materials, highlighting the importance of long-range dipolar interactions.

Findings

Anisotropic azimuthal intensity distribution in spin waves

Model inadequacy in describing measured dispersions

Long-range dipolar interactions cause observed anomalies

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.