Columnar magnetic structure coupled with orthorhombic distortion in the antiferromagnetic iron arsenide SrFe2As2

Neutron powder-diffraction experiments were carried out on polycrystalline SrFe2As2 in order to determine the magnetic structure and its relationship with the crystallographic one. Below T-0=205 K, magnetic reflections appear simultaneously with the onset of the orthorhombic distortion. The magnetic propagation vector of SrFe2As2 is determined to be q=(1 0 1); the coupling of Fe moments is antiferromagnetic along the longer a direction within the Fe-As layer, and the interlayer coupling is antiferromagnetic as well. The size of the Fe magnetic moment is deduced to be 1.01(3) mu(B) with an orientation parallel to the a axis. The temperature dependence of the magnetic moment shows excellent agreement with not only that of the muon precession frequency but also with that of the structural distortion, revealing the strong coupling of the magnetic order and the structural distortion in SrFe2As2. The discrepancy in the deduced size of the ordered moment between neutron and Mossbauer and mu SR may imply an unusual magnetism in SrFe2As2.