Magnetic order of the iron spins in NdFeAsO

Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below [4](6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c axis are antiparallel like LaFeAsO. The ordered moment is 0.25(7) mu(B), which is the smallest moment found so far in these systems.