Spin Waves and Spatially Anisotropic Exchange Interactions in the $\mathrm{S=2}$ Stripe Antiferromagnet Rb$_{0.8}$Fe$_{1.5}$S$_2$

TL;DR

This study investigates spin waves in Rb$_{0.8}$Fe$_{1.5}$S$_2$, revealing anisotropic exchange interactions and a localized high-spin state, indicating a Mott-like insulating behavior distinct from iron pnictides.

Contribution

It provides the first detailed inelastic neutron scattering analysis of spin waves in Rb$_{0.8}$Fe$_{1.5}$S$_2$, demonstrating anisotropic exchange and localized electron behavior.

Findings

Spin wave spectra are well described by a Heisenberg model with anisotropic exchange.

The total magnetic moment suggests a high-spin $S \\approx 2$ state.

The material is a Mott-like insulator with localized $3d$ electrons.

Abstract

An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb$_{0.8}$Fe$_{1.5}$S$_2$ throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of $27.6\pm4.2\mu_B^2$/Fe, consistent with $\mathrm{S \approx 2}$. Unlike $X$Fe$_2$As$_2$ ($X=$ Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb$_{0.8}$Fe$_{1.5}$S$_2$ is a Mott-like insulator with fully localized $3d$ electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.