  The magnetic properties of the layered oxypnictide LaMnAsO have been
revisited using neutron scattering and magnetization measurements. The present
measurements identify the N\'{e}el temperature $T_N$ = 360(1) K. Below $T_N$
the critical exponent describing the magnetic order parameter is $\beta$ =
0.33$-$0.35, consistent with a three dimensional Heisenberg model. Above this
temperature, diffuse magnetic scattering indicative of short-range magnetic
order is observed, and this scattering persists up to $T_{SRO}$ = 650(10) K.
The magnetic susceptibility shows a weak anomaly at $T_{SRO}$ and no anomaly at
$T_N$. Analysis of the diffuse scattering data using a reverse Monte Carlo
algorithm indicates that above $T_N$ nearly two- dimensional, short-range
magnetic order is present with a correlation length of 9.3(3) {\AA} within the
Mn layers at 400 K. The inelastic scattering data reveal a spin-gap of 3.5 meV
in the long-range ordered state, and strong, low-energy (quasi-elastic)
magnetic excitations emerging in the short-range ordered state. Comparison with
other related compounds correlates the distortion of the Mn coordination
tetrahedra to the sign of the magnetic exchange along the layer-stacking
direction, and suggests that short-range order above $T_N$ is a common feature
in the magnetic behavior of layered Mn-based pnictides and oxypnictides.
