Low-energy nuclear spin excitations in NdMg₃ and NdCo₂

We investigated the low-energy excitations in NdMg₃ and NdCo₂ in the μeV range by a backscattering neutron spectrometer. The energy scans on polycrystalline NdMg₃ and NdCo₂ samples revealed inelastic peaks at E=1.70±0.01 μeV and 3.37±0.01 μeV, respectively, at T=2 K on both energy gain and loss sides. The inelastic peaks move gradually toward lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature T_N≈6 K and T_C≈100 K, respectively. We interpret the inelastic peaks to be due to the transition between hyperfine-split nuclear levels of the ¹⁴³Nd and ¹⁴⁵Nd isotopes with spin I=7/2.