  Neutron powder diffraction experiments were carried out on polycrystalline
SrFe$_2$As$_2$ in order to determine the magnetic structure and its
relationship with the crystallographic one. Below $T_0$=205 K, magnetic
reflections appear simultaneously with the onset of the orthorhombic
distortion. From a detailed Rietveld analysis, the magnetic propagation vector
of SrFe$_2$As$_2$ is determined to be {\textit{\textbf{q}}}=(1 0 1); the
coupling of Fe moments is antiferromagnetic along the longer $a$ direction
within the Fe-As layer, and the interlayer coupling is antiferromagnetic as
well. The size of the Fe magnetic moment is deduced to be 1.01(3) ${\mu}_{\rm
B}$ with an orientation parallel to the a-axis. The temperature dependence of
the magnetic moment shows excellent agreement with not only that of the muon
precession frequency but also with that of the structural distortion, revealing
the strong coupling of the columnar magnetic order and the structural
distortion in SrFe$_2$As$_2$.
