Nuclear resonance of ytterbium local moments in gold

We have observed the ${}_{}{}^{173}{}_{}{}^{}\mathrm{Yb}$ nuclear resonance in dilute $\mathrm{Au}\mathrm{Yb}$ alloys using a phase-coherent spin-echo spectrometer in the frequency range 450-530 MHz, with external fields of 67-119 kOe and temperatures of 1-2 K. At these field strengths and temperatures, the electronic moment is strongly polarized and the local-moment relaxation regime ${\omega }_e{T}_2^e\gg 1$ applies. The resonance has been studied in both powders and single crystals. The field dependence of the resonance frequencies can be fit with a crystal-field splitting $\Delta ({\Gamma }_8-{\Gamma }_7)=83.7(\pm 2.5)$ K and a hyperfine constant $\frac{A}{h}=-241.5(\pm 1.0)$ MHz. These values agree well with those obtained in electron-spin-resonance, Mössbauer, and magnetic-susceptibility experiments. The transverse nuclear relaxation time varies as $T_2\propto T^{-1\mathrm{}}$ and $T_2\propto H_0^2$. The measured rates agree with predictions for the ½ ↔ -½ nuclear transition based on a model involving local-moment fluctuations within the ${\Gamma }_7$ ground-state doublet using a conduction-electron exchange constant $〈{\mathcal{I}}^2〉{\rho }^2=0.018\mathrm{}$ derived from the reported thermal broadening of the ytterbium ESR. The absence of significant contributions to the observed high-field relaxation rates from ${\Gamma }_7-{\Gamma }_8$ fluctuations provides strong evidence that the local-moment—conduction-electron interaction in $\mathrm{Au}\mathrm{Yb}$ is principally of an anisotropic form.