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.
Categories: cond-mat.supr-con