Abstract
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 T0=205 K, magnetic reflections appear simultaneously with the onset of the orthorhombic distortion. From a detailed Rietveld analysis, 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 determined to be 1.01(3) µB 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 columnar magnetic order and the structural distortion in SrFe2As2.
We now turn to the discussion of the relation between the structural distortion and the magnetic structure. As mentioned before, the magnetic order in the FeAs system occurs either after the structural distortion or simultaneously. A simple approach would be to consider a superexchange path between the nearest Fe neighbors. Since the positional parameter of As and the lattice constant c does not exhibit significant change, both the packing of the Fe-As layer and the Fe-As distance stay constant on passing through T0. Thus, the distortion within the Fe-As layer at T0 mainly results in a slight change in the Fe-Fe distance and the Fe-As-Fe bond angle. The longer a lattice constant leads to a slightly smaller bond angle along a, 6 (Fe-As-Fe)a=71.2◦ as compared to that along b,6 (Fe-As-Fe)b=72.1◦ , in other words, the difference is less than 1◦ . It is unlikely that such minor distortions lead to significant differences in exchange parameters. In contrast, it should be noted that the observed orthorhombic distortion and the columnar magnetic structure in SrFe2As2 are consistent with the results of a band structure calculation[14]. The calculation gives both a comparable distortion and the correct magnetic structure in which the AFM arrangement is along the long a-axis. These calculation predict the distortion to be stable only for the columnar state, not for other magnetic structures or the non-magnetic state, indicating the strong coupling of magnetic order and orthorhombic distortion. Similar results were also obtained for LaFeAsO.[17, 18] For this compound, T. Yildirim discusses the lattice distortion to be related to a lifting of the double degeneracy of the columnar state within a localized spin model for a frustrated square lattice[17]. However, I. I. Mazin et al. questioned the applicability of such a model for these layered FeAs systems[19]. A detailed neutron scattering study on single crystals is expected to give further insights into this fascinating coupling.
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