Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high-resolution inelastic neutron scattering

We investigated the low-energy excitations in the parent compound NdFeAsO of the Fe-pnictide superconductor in the mu eV range by a back-scattering neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed inelastic peaks at E = 1.600 +/- 0.003 mu eV at T = 0.055 K on both energy-gain and energy-loss sides. The inelastic peaks move gradually toward lower energy with increasing temperature and finally merge with the elastic peak at about 6 K. We interpret the inelastic peaks to be due to the transition between the hyperfine-split nuclear levels of the Nd-143 and Nd-145 isotopes with spin I = 7/2. The hyperfine field is produced by the ordering of the electronic magnetic moment of Nd at low temperature, and thus the present investigation gives direct evidence for the ordering of the Nd magnetic sublattice of NdFeAsO at low temperature.