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

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 $S(Q,\hslash \omega )$ consists of a Bragg peak at the elastic position ($\hslash \omega =0$ meV), a spin gap ($\Delta \le 6.5$ 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.

4 pages, 4 figures

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