The transition temperature $T_C\approx 26\mathrm{K}$ of the recently discovered superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ is extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long-ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment $1.55(4){\mu }_B/\mathrm{Nd}$ and $0.9(1){\mu }_B/\mathrm{Fe}$ at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample ${\mathrm{NdFeAsO}}_{0.80}{\mathrm{F}}_{0.20}$ at any temperatures down to 1.5 K.

• Received 8 July 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.257002