Low energy spin waves and magnetic interactions in SrFe$_2$As$_2$

TL;DR

This study investigates magnetic excitations in SrFe$_2$As$_2$ using inelastic neutron scattering, revealing low energy spin waves, a spin gap, and insights into magnetic interactions and phase transition nature.

Contribution

It provides the first detailed characterization of magnetic excitations and exchange interactions in SrFe$_2$As$_2$, highlighting its first order antiferromagnetic transition.

Findings

Observation of a spin gap up to 6.5 meV

Identification of sharp spin wave excitations

Evidence for a first order magnetic phase transition

Abstract

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe$_{2}$As$_{2}$ ($T_{N}=220$ K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the spectrum of magnetic excitations $S(Q,\hbar \omega)$ consists of a Bragg peak at the elastic position ($\hbar \omega =0$ meV), a spin gap ($ \Delta \leq 6.5$ meV), and sharp spin wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across $T_{N} $, the low temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe$_{2}$As$_{2}$ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.