  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 $T_{\textrm c}$ $= 118(2)$ 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.
