²⁹ Si nuclear spin-lattice relaxation in CePtSi _1-xGe_x near a magnetic instability

²⁹ Si nuclear spin-lattice relaxation measurements have been performed in the heavy-fermion alloys CePtSi_1-xGe_x, x=0 and 0.1, in order to study spin dynamics near a magnetic instability. The spin-relaxation curves for both x=0 and x=0.1 are found to fit to a stretched exponential slightly better than a single exponential function, which suggests that the relaxation rates are inhomogeneous to some extent, due to structural disorder. Within experimental resolution, the relaxation curve is in agreement with the theoretical curve calculated from the Kondo-disorder model. The temperature-dependent relaxation rate 1/T₁ follows the Korringa relation in CePtSi below 4 K, but not in CePtSi_0.9Ge_0.1. This means that Fermi-liquid and non-Fermi-liquid excitations appear in x=0 and x=0.1 samples, respectively, which agrees with the results from specific heat experiments. The effective moments of Ce³⁺ ions in CePtSi _1-xGe_x for directions parallel and perpendicular to the c axis, calculated from the crystalline electric field (CEF) levels, explain the anisotropy in the observed susceptibilities, hyperfine coupling constants, and spin-lattice relaxation rates. The magnitude of the CEF-corrected Korringa products for the two samples shows the same order as the expected value for a Fermi gas, which indicates that no obvious spin-correlated fluctuations or magnetic order are present. This seems to disagree with the Griffiths phase model, in which magnetic clusters are spatially-extended objects containing many spins.