A variable temperature neutron and synchrotron diffraction study has been performed on the giant magnetoresistant oxypnictides $L$MnAsO ($L$ = La, Nd). The low-temperature magnetic structures have been studied, and results show a spin reorientation of the Mn${}^{2+}$ spins below $T_N$ (Nd) for NdMnAsO. The Mn${}^{2+}$ spins rotate from alignment along $c$ to alignment into the basal plane, and the Mn${}^{2+}$ and Nd${}^{3+}$ moments refine to 3.54(4) ${\mu }_{\mathrm{B}}$ and 1.93(4) ${\mu }_{\mathrm{B}}$, 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 and $L$-O and Mn-As bond lengths are detected at ∼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.

• Received 13 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.144429