Physical Review Letters 101,16 ( 2008 )
We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe2As2 (TN=200–220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,[h-bar]omega) consists of a Bragg peak at the elastic position ([h-bar]omega=0 meV), a spin gap (Delta<=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 TN, 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.
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