  A variable temperature neutron and synchrotron diffraction study have been
performed on the giant magnetoresistant oxypnictides LnMnAsO (Ln = La, Nd). The
low temperature magnetic structures have been studied and results show a spin
reorientation of the Mn2+ spins below TN (Nd) for NdMnAsO. The Mn2+ spins
rotate from alignment along c to alignment into the basal plane and the Mn2+
and Nd3+ moments refine to 3.54(4) \mu B and 1.93(4) \mu B respectively at 2 K.
In contrast there is no change in magnetic structure with temperature for
LaMnAsO. There is no evidence of a structural transition down to 2 K, however
discontinuities in the cell volume, Ln-O and Mn-As bond lengths are detected at
\sim 150 K for both materials. This temperature coincides with the electronic
transition previously reported and suggests a coupling between electronic and
lattice degrees of freedom.
