Low Energy Spin Waves and Magnetic Interactions in SrFe₂As₂

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe₂As₂ (T_N=200–220  K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7  K), the magnetic spectrum S(Q,ℏω) consists of a Bragg peak at the elastic position (ℏω=0  meV), a spin gap (Δ≤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 SrFe₂As₂ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.