Magnetic moments and interactions near the metal-insulator transition in amorphous magnetic semiconductors

We report magnetization, magnetic susceptibility, and specific-heat measurements of amorphous Gd_xY_ySi_1-x-y ternary alloy thin films near the metal-insulator (MI) transition as a function of temperature and applied magnetic field. Samples of the same magnetic moment concentration x but varying conduction-electron concentration x+y were measured to test the effect of the MI transition on the magnetic properties. The effective moment in the paramagnetic state (per Gd atom) shows a strong dependence on composition, with a peak at the MI transition, independent of the Gd/Y ratio. Addition of Y weakens the Gd-Gd magnetic interactions, consistent with an indirect RKKY-like exchange interaction, despite the localized or nearly localized nature of the conduction electrons. Specific-heat measurements show further evidence of weakened RKKY-like interactions in the y≠0 sample via a spin-glass peak that is shifted to lower temperature, narrowed, and more field dependent.