Collinear antiferromagnetism in trigonal SrMn2As2 revealed by single-crystal neutron diffraction

Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn(2)AS(2) that crystallizes in a trigonal structure (P (3) over bar m1) and undergoes an antiferromagnetic (AFM) transition at T-N = 118(2) K. The magnetic susceptibility remains nearly constant at temperatures T <= T-N with H parallel to c whereas it decreases significantly with H parallel to ab. This shows that the ordered Mn moments lie in the ab plane instead of aligning along the c-axis as in tetragonal BaMn2As2. Single-crystal neutron diffraction measurements on SrMn2As2 demonstrate that the Mn moments are ordered in a collinear Neel AFM phase with 180 degrees AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn2As2 as evident from the temperature dependence of the order parameter.