Nuclear Magnetic Resonance in Exchange-Enhanced Metals:Rh103Knight Shifts and Relaxation Rates in Palladium-Rhodium and Nickel-Rhodium Alloys

Low-temperature ($T\approx 1-4$°K) measurements of the resonance shift $K$, spin-lattice relaxation time $T_1$, and inhomogeneous linewidth $W$ of ${}_{}{}^{103}{}_{}{}^{}\mathrm{Rh}$ in the two alloy systems ${\mathrm{Pd}}_{1-x}{\mathrm{Rh}}_x \left(0.02\mathrm{}\le x\le 1\right)$ and ${\mathrm{Ni}}_{1-x}{\mathrm{Rh}}_x \left(0.\mathrm{}38\le x\le 1\right)$ have been carried out using transient NMR techniques. In ${\mathrm{Pd}}_{1-x}{\mathrm{Rh}}_x$ both $T_{1}T$ and $K$ vary monotonically with $K$ decreasing from +0.43 to -15%, and $T_{1}T$ from 9.2 to 0.7 sec °K. Similar results are found in ${\mathrm{Ni}}_{1-x}{\mathrm{Rh}}_x$, with $K$ decreasing from +0.43 to -8.1%, and $T_{1}T$ from 9.2 to 1.2 sec °K. The ${}_{}{}^{103}{}_{}{}^{}\mathrm{Rh}$ NMR in both alloy systems is characterized by severe inhomogeneous broadening, with $W$ approaching $K$ in magnitude in some cases. This result demonstrates that the local susceptibility is very sensitive to statistical variations in the near-neighbor alloy composition. The minimum $K$ value in Pd-Rh agrees with the infinite-dilution value $K=-14.\mathrm{}7\%$ obtained by Rao, Matthias, and Shirley using perturbed-angular-correlation techniques, and indicates that $K$ is essentially independent of $x$ for Rh concentrations up to ∼ 5 at.%. A comparison of this result with the ${}_{}{}^{105}{}_{}{}^{}\mathrm{Pd}$ Knight shift in palladium metal shows that the local susceptibility at the rhodium sites is approximately three times greater than the palladium host susceptibility. This deviation from rigid-band behavior develops gradully with decreasing rhodium concentration for $x<\mathrm{}0.\mathrm{}4$. In contrast, the Ni-Rh NMR data suggest that the rhodium susceptibility is significantly smaller than the nickel susceptibility at all rhodium concentrations. An analysis of the ${}_{}{}^{103}{}_{}{}^{}\mathrm{Rh}$ shift and relaxation data for Pd-Rh and Ni-Rh demonstrates that both quantities are dominated by an exchange-enhanced $d$-spin (core-polarization) hyperfine mechanism for sufficiently low rhodium concentrations. Moreover, the observed relationship between