Nuclear Spin Relaxation in Metals: Rhodium, Palladium, and Silver

The nuclear spin-lattice (T₁) and spin-spin (T₂) relaxation times have been studied by means of transient nuclear magnetic resonance techniques in metallic rhodium, palladium, and silver in the temperature range 1≤T≤4^∘K. The spin-lattice relaxation rates are dominated by conduction-electron processes, as evidenced by the observation of temperature-independent rates R≡(T₁T)^-1 in all three metals. The measured values, in (sec^∘K)^-1, are R(¹⁰³Rh)=0.10±0.02, R(¹⁰⁵Pd)=9±2, R(¹⁰⁷Ag)=0.083±0.008, and R(¹⁰⁹Ag)=0.111±0.010. The relaxation rate in silver is approximately one-half of the rate predicted by the Korringa relation on the basis of the known Knight shift. This discrepancy is attributed to an enhancement of the conduction-electron susceptibility due to collective-electron effects. Since the contact Knight shift is proportional to the product of the s conduction-electron susceptibility and the contact hyperfine field, this result implies that the average $5s$ hyperfine field in metallic silver may be substantially smaller than has been assumed generally. In rhodium the principal contributions to the spin-lattice relaxation are shown to arise from both $5s$ contact and $4d$ orbital interactions. The magnitude of the measured ¹⁰³Rh rate is in satisfactory agreement with estimates based on the results of a published Knight-shift study. In palladium, on the other hand, the measured rate exceeds the estimated rate by a factor of about 10. Possible reasons for this large difference in palladium are discussed. The spin-spin relaxation processes in rhodium and silver, as measured by the decay of the spin-echo envelope, exhibit Gaussian time dependences with characteristic times, in msec, T₂(¹⁰³Rh)=10.5±1.0, T₂(¹⁰⁷Ag)=10.4±0.2, and T₂(¹⁰⁹Ag)=9.1±0.2. The palladium spin-spin relaxation process has an exponential time dependence with T₂(¹⁰⁵Pd)=5.6±0.5 msec. These transverse relaxation rates give evidence for weak conduction-electron-coupled spin-spin interactions in these metals.