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 ($P\bar{3}m1$) and undergoes an antiferromagnetic (AFM) transition at $T_{\text{c}}$ $=118(2)$ K. The magnetic susceptibility remains nearly constant at temperatures $T \le T_{\textrm N}$with$$\mathbf{\text{H}}$\parallel $\mathbf{\text{c}}$$ whereas it decreases significantly with $\mathbf{\text{H}}\parallel \mathbf{\text{ab}}$. This shows that the ordered Mn moments lie in the $\mathbf{\text{ab}}$-plane instead of aligning along the $\mathbf{\text{c}}$-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.