Spin-glass freezing and RKKY interactions near the metal-insulator transition in amorphous Gd-Si alloys

Magnetization measurements of giant negative magnetoresistive amorphous Gdx Si1-x (0.04 < x < 0.19) show strong mixed ferromagnetic and antiferromagnet ic interactions and spin-glass freezing. The interactions appear to be an R KKY-like indirect exchange mediated by the conduction electrons, but are st rikingly independent of their localization at the metal-insulator (MI) tran sition, presumably because the localization length exceeds the inter-Gd dis tance. The susceptibility per Gd atom in the paramagnetic state shows a non trivial dependence on composition, with a maximum at the MI transition.