Nuclear resonance of ytterbium local moments in gold

Abstract

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 ${\ensuremath{\omega}}_{e}{T}_{2}^{e}\ensuremath{\gg}1$ 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 $\ensuremath{\Delta}({\ensuremath{\Gamma}}_{8}\ensuremath{-}{\ensuremath{\Gamma}}_{7})=83.7(\ifmmode\pm\else\textpm\fi{}2.5)$ K and a hyperfine constant $\frac{A}{h}=\ensuremath{-}241.5(\ifmmode\pm\else\textpm\fi{}1.0)$ MHz. These values agree well with those obtained in electron-spin-resonance, M\ossbauer, and magnetic-susceptibility experiments. The transverse nuclear relaxation time varies as ${T}_{2}\ensuremath{\propto}{T}^{\ensuremath{-}1}$ and ${T}_{2}\ensuremath{\propto}{H}_{0}^{2}$. The measured rates agree with predictions for the \textonehalf{} \ensuremath{\leftrightarrow} -\textonehalf{} nuclear transition based on a model involving local-moment fluctuations within the ${\ensuremath{\Gamma}}_{7}$ ground-state doublet using a conduction-electron exchange constant $〈{\mathcal{I}}^{2}〉{\ensuremath{\rho}}^{2}=0.018$ derived from the reported thermal broadening of the ytterbium ESR. The absence of significant contributions to the observed high-field relaxation rates from ${\ensuremath{\Gamma}}_{7}\ensuremath{-}{\ensuremath{\Gamma}}_{8}$ fluctuations provides strong evidence that the local-moment---conduction-electron interaction in $\mathrm{Au}\mathrm{Yb}$ is principally of an anisotropic form.