La substitution effect and hyperfine-enhanced ¹⁴¹Pr nuclear spin dynamics in PrPb₃: ¹³⁹La NMR study in Pr _0.97La_0.03Pb₃

L 139 a NMR studies have been performed on PrPb₃ substituted with 3% La. The temperature dependence of the local magnetic susceptibility around La ions has been extracted from the L 139 a Knight shift. These data show that the nonmagnetic Γ3 crystalline-electric-field (CEF) ground state (GS) is preserved even at the nearest neighboring Pr ions, although their CEF level scheme is slightly modified due to the La substitution. On the other hand, the temperature dependence of the nuclear spin-lattice relaxation rate 1/ T1 is found to be well reproduced by assuming the same CEF level scheme over a wide temperature range. However, 1/ T1 shows a strong upturn below 10 K, which is not expected from the nonmagnetic CEF GS. We show that the low-temperature anomaly can be quantitatively understood in terms of a cross-relaxation process between P 141 r and L 139 a nuclear spins. Analysis of the cross-relaxation process reveals strong nuclear spin-spin coupling among the P 141 r nuclei, which we suggest is enhanced by a hyperfine mechanism of Pr ions and then mediated by relatively large indirect magnetic coupling between them.