Spin excitations in K$_{2}$Fe$_{4+x}$Se$_{5}$: linear response approach

TL;DR

This study uses ab initio linear response techniques to accurately calculate spin wave spectra in K$_{2}$Fe$_{4+x}$Se$_{5}$, revealing anisotropic exchange interactions and their origins, with implications for understanding magnetic properties.

Contribution

It demonstrates the effectiveness of linear response methods in modeling spin excitations and clarifies the nature of exchange anisotropy in this compound.

Findings

Spin wave spectra agree with experimental data.

Exchange coupling anisotropy has itinerant and structural origins.

Spin spectra depend on Fe vacancy filling.

Abstract

Using \emph{ab initio} linear response techniques we calculate spin wave spectra in K$_{2}$Fe$_{4+x}$Se$_{5}$, and find it to be in excellent agreement with a recent experiment. The spectrum can be alternatively described rather well by localized spin Hamiltonian restricted to first and second nearest neighbor couplings. We confirm that exchange coupling between nearest neighbor Fe magnetic moments is strongly anisotropic, and show directly that in the ideal system this anisotropy has itinerant nature which can be imitated by introducing higher order terms in effective localized spin Hamiltonian (biquadratic coupling). In the real system, structural relaxation provides an additional source of the exchange anisotropy of approximately the same magnitude. The dependence of spin wave spectra on filling of Fe vacancy sites is also discussed.