No abstract

The recent discovery of superconductivity in (Ba,K)Fe2As2, which crystallizes in the ThCr2Si2 (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-T{C} superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of q=(101) antiferromagnetic order in BaFe2As2, with a saturation moment 0.87(3)micro {B} per Fe that is orientated along the longer a axis of the ab planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.

Antiferromagnetism is relevant to high temperature (high-T c ) superconductivity because copper oxide and iron arsenide high-T c superconductors arise from electron-or hole-doping of their antiferromagnetic (AF) ordered parent compounds 1-6 . There are two broad classes of explanation for the phenomenon of antiferromagnetism: in the "local moment" picture, appropriate for the insulating copper oxides 1 , AF interactions are well described by a Heisenberg Hamiltonian 7,8 ; while in the "itinerant model", suitable for metallic chromium, AF order arises from quasiparticle excitations of a nested Fermi surface 9,10 . There has been contradictory evidence regarding the microscopic origin of the AF order in iron arsenide materials 5,6 , with some favoring a localized picture 11-15 while others supporting an