title: Low energy spin waves and magnetic interactions in SrFe2As2.
abstract: 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,Planck's omega) consists of a Bragg peak at the elastic position (Planck's omega=0 meV), a spin gap (Delta< or =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.
authors.affiliation: Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996-1200, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhao
authors.forename: Dao-Xin
authors.initials: DX
authors.lastname: Yao
authors.forename: Shiliang
authors.initials: S
authors.lastname: Li
authors.forename: Tao
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authors.lastname: Hong
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authors.lastname: Chen
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authors.lastname: Chang
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authors.lastname: Ratcliff
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authors.lastname: Lynn
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authors.initials: HA
authors.lastname: Mook
authors.forename: G F
authors.initials: GF
authors.lastname: Chen
authors.forename: J L
authors.initials: JL
authors.lastname: Luo
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authors.lastname: Wang
authors.forename: E W
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authors.lastname: Carlson
authors.forename: Jiangping
authors.initials: J
authors.lastname: Hu
authors.forename: Pengcheng
authors.initials: P
authors.lastname: Dai