Impuritygshift and host exchange enhancement in a nearly ferromagnetic compound

Abstract

We generalize the Mills-Lederer model for locally enhanced spin fluctuations to evaluate the $g$ shift, $\Delta g$, of the magnetic resonance of an impurity dissolved in a nearly ferromagnetic metal. The impurity is assumed to increase locally the strong host electron-electron Coulomb interaction $U_0$ by a positive amount $\Delta U=U_i-U_0$. The coefficient of proportionality between $\Delta g$ and the enhanced host susceptibility $\chi$ is shown to vary with temperature and host enhancement (hence, with $U_0$). The latter can be altered through, for example, changes in host composition. The model is applied to the interpretation of the spin resonance properties of ${\mathrm{U}}_x{\mathrm{Th}}_{1-x}{\mathrm{Pd}}_3:\mathrm{Gd}$. Previous measurements are supplemented by new evidence to show that the host ${\mathrm{U}}_x{\mathrm{Th}}_{1-x}{\mathrm{Pd}}_3$ can be considered as a nearly magnetic metal, the more nearly magnetic the larger $x$. We examine the implications of our theory for the resonance $g$ shift and linewidth of dilute Gd impurities in such a host.