Impurity g shift and host exchange enhancement in a nearly ferromagnetic compound

We generalize the Mills-Lederer model for locally enhanced spin fluctuations to evaluate the g shift, 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 U0 by a positive amount U=Ui-U0. The coefficient of proportionality between g and the enhanced host susceptibility is shown to vary with temperature and host enhancement (hence, with U0). 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 Ux Th1-xPd3:Gd. Previous measurements are supplemented by new evidence to show that the host Ux Th1-xPd3 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.