Collinear antiferromagnetism in trigonal SrMn$_{2}$As$_{2}$ revealed by single-crystal neutron diffraction

Fe 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 ($Pbar{3}m1$) and undergoes an antiferromagnetic (AFM) transition at $T_{textrm c}$ $= 118(2)$ K. The magnetic susceptibility remains nearly constant at temperatures $T le T_{textrm N}$ with $textbf{H}parallel textbf{c}$ whereas it decreases significantly with $textbf{H}parallel textbf{ab}$. This shows that the ordered Mn moments lie in the $textbf{ab}$-plane instead of aligning along the $textbf{c}$-axis as in tetragonal BaMn$_{2}$As$_{2}$. Single-crystal neutron diffraction measurements on SrMn$_{2}$As$_{2}$ demonstrate that the Mn moments are ordered in a collinear N{e}el AFM phase with $180^circ$ 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 SrMn$_{2}$As$_{2}$ as evident from the temperature dependence of the order parameter.