Mössbauer relaxation studies ofYb170in diluteAu:Tm170sources in external magnetic fields up to 55 kG

Relaxation effects of Yb impurities in Au have been studied by Mössbauer-source experiments at both 4.2 K in external magnetic fields ranging from 10 to 55 kG, and at 50 kG in the temperature range 4.2 to 40 K. In order to analyze the results a general microscopic relaxation theory for an effective-field hyperfine interaction is derived, which allows for both a generalized tensor-type $4f$ conduction-electron coupling, and frequency-dependent relaxation processes between the electronic energy levels. It is pointed out that in Mössbauer-source experiments the preceeding nuclear $\beta$ decay may produce population effects in the lowest electronic energy states due to slow relaxation rearrangement in the electronic shell. The experimental Mössbauer spectra of ${}_{}{}^{170}{}_{}{}^{}\mathrm{Yb}$ in dilute $\mathrm{Au}$: ${}_{}{}^{170}{}_{}{}^{}\mathrm{Tm}$ sources are found to be well described by the relaxation theory including the new rearrangement ideas for Mössbauer-source experiments.