Static and dynamic magnetic properties of a localized moment: NMR ofCo59in diluteMoCo andWCo alloys

Because of unusually small cobalt hyperfine fields it has been possible to study the static and dynamic properties of the Kondo alloys MoCo and WCo in considerable detail by means of impurity NMR techniques. Measurements are reported for the temperature range 1- ∼ 500 K in magnetic fields to 130 kOe. From the Curie-Weiss behavior of the weak-field resonance shifts Kondo temperatures ${\Theta }_K=45\mathrm{}\left(3\right)\mathrm{}\mathrm{K}\mathrm{and}1.8\left(2\right)\mathrm{}\mathrm{K}$ are obtained for the two alloys, respectively. The corresponding hyperfine fields are $H_{\mathrm{hf}}=-26\mathrm{}\mathrm{and}-1.5\mathrm{}$ kOe/${\mathrm{\mu }}_{\mathit{B}}$. The $g$ values are $g=1.3\mathrm{}\mathrm{and}1.4$. A consistent interpretation of our data is achieved by means of an ionic model with a ${\Gamma }_2$ orbital singlet ground state arising from the ${\mathrm{Co}}^{2+}3d^7\left({}_{}{}^4{}_{}{}^{}F\right)$ term in the presence of crystal-field, spin-orbit, and local-moment-conduction-electron exchange perturbations. The small hyperfine fields are attributed to spin-orbit induced orbital contributions, and the negative $g$ shifts to the exchange coupling. Measurements of the ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ relaxation rates $T_2^{-1\mathrm{}}$ in $W\mathrm{Co}$ over a wide range of temperatures and magnetic field strengths give evidence for competing local-moment fluctuation and direct conduction-electron mechanisms. The former dominates at temperatures below ∼ 50 K; in applied fields above ∼ 100 kOe this relaxation process can be explained satisfactorily by means of standard perturbation theory. With decreasing field strength Kondo anomalies become increasingly important and the nuclear relaxation rate appears to saturate as predicted by simple scaling arguments.