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Low energy spin waves and magnetic interactions in SrFe2As2.

Research paper by Jun J Zhao, Dao-Xin DX Yao, Shiliang S Li, Tao T Hong, Y Y Chen, S S Chang, W W Ratcliff, J W JW Lynn, H A HA Mook, G F GF Chen, J L JL Luo, N L NL Wang, E W EW Carlson, Jiangping J Hu, Pengcheng P Dai

Indexed on: 13 Nov '08Published on: 13 Nov '08Published in: Physical review letters



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.