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 µeV range by a back-scattering neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed inelastic peaks at E = 1.600 ± 0.003 µeV 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 143Nd and 145Nd isotopes with spin I = 7/2. The hypertine 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. [ABSTRACT FROM AUTHOR]