Collinear antiferromagnetism in trigonal SrMnAs 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 SrMnAs that crystallizes in a trigonal structure ([Formula: see text]) and undergoes an antiferromagnetic (AFM) transition at [Formula: see text] K. The magnetic susceptibility remains nearly constant at temperatures [Formula: see text] with [Formula: see text] whereas it decreases significantly with [Formula: see text]. This shows that the ordered Mn moments lie in the [Formula: see text] plane instead of aligning along the [Formula: see text]-axis as in tetragonal BaMnAs. Single-crystal neutron diffraction measurements on SrMnAs demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with [Formula: see text] 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 SrMnAs as evident from the temperature dependence of the order parameter.