Magnetization measurements of giant negative magnetoresistive amorphous ${\mathrm{Gd}}_x{\mathrm{Si}}_{1-x}$ ( $0.04<x<\mathrm{}0.19\mathrm{}$) show strong mixed ferromagnetic and antiferromagnetic interactions and spin-glass freezing. The interactions appear to be an RKKY-like indirect exchange mediated by the conduction electrons, but are strikingly independent of their localization at the metal-insulator (MI) transition, presumably because the localization length exceeds the inter-Gd distance. The susceptibility per Gd atom in the paramagnetic state shows a nontrivial dependence on composition, with a maximum at the MI transition.

• Received 1 July 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.5411