  Structure and magnetic properties of high-quality polycrystlline CeMnAsO, a
parent compound of the "1111"-type oxypnictides, have been investigated using
neutron powder diffraction and magnetization measurements. We find that CeMnAsO
undergoes a C-type antiferromagnetic order with Mn$^{2+}$ ($S=5/2$) moments
pointing along the \textit{c}-axis below a relatively high N\'{e}el temperature
of $T_{\rm N} = 345$ K. Below $T_{\rm SR} = 35$ K, two instantaneous
transitions occur where the Mn moments reorient from the $c$-axis to the
\textit{ab}-plane preserving the C-type magnetic order, and Ce moments undergo
long-range AFM ordering with antiparallel moments pointing in the
\textit{ab}-plane. Another transition to a noncollinear magnetic structure
occurs below 7 K. The ordered moments of Mn and Ce at 2 K are 3.32(4) $\mu_{B}$
and 0.81(4)$\mu_{B}$, respectively. We find that CeMnAsO primarily falls into
the category of a local-moment antiferromagnetic insulator in which the
nearest-neighbor interaction ($J_{1}$) is dominant with $J_{2}<J_{1}/2$ in the
context of $J_{1}-J_{2}-J_{c}$ model. The spin reorientation transition driven
by the coupling between rare earth Ce and transition metal seems to be common
to Mn, Fe and Cr ions, but not to Co and Ni ions in the iso-structural
oxypnictides. A schematic illustration of magnetic structures in Mn and Ce
sublattices in CeMnAsO is proposed.
