237Np nuclear relaxation rate in heavy fermion superconductor NpPd5Al2

Al-27 nuclear spin-lattice relaxation rate 1/T-27(1) has been measured in heavy fermion superconductor NpPd5Al2 for a wide range of external fields between 0.7 and 11 T. We found that 1/T-27(1) exhibits a strong, field-dependent behavior at low fields below H similar to 3 T, which is attributed to unlike-spin cross relaxation between Np-237 and Al-27. An analysis based on the cross relaxation process allows us to make the first quantitative estimates of the Np nuclear relaxation rate 1/(237)tau in an itinerant 5f electron system. The estimated values of 1/(237)tau similar to 6 x 10(7) s(-1), with a weak temperature variation, are below the resolution of NMR relaxation times, indicating that a direct observation of the Np-237 NMR signal would be very difficult, at least in the paramagnetic state. It is proposed that a direct observation of Np-237 NMR might be possible in the superconducting state at T similar to 1 K.