Electron-Spin Resonance of Rare-Earth Ions in the Actinide Cubic Metal Th

The electron-spin resonance of Gd, Er, and Dy of nominal concentrations between 40 and 3250 ppm in the actinide metal Th is reported. The temperature dependence of the linewidth yields values for the exchange coupling Jfs between the rare-earth localized moment and the conduction electrons. We find Jfs to be almost independent of the $4f$ occupation number, in agreement with the values of Jfs obtained previously by others from superconducting-transition-temperature measurements in the same alloys.