Neutron diffraction on antiferromagnetic ordering in single-crystal BaFe2As2

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe2As2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Neel temperature T_N observed in the experiment.