The temperature and field dependence of magnetization and conductivity of amorphous Ge doped with Gd $\left(a\text{−}{\mathrm{Gd}}_x{\mathrm{Ge}}_{1-x}\right)$ has been measured for a wide range of $x$ $(0.08<x<0.25)$ near the metal-insulator transition. Magnetization and magnetic susceptibility measurements show strong magnetic interactions and a low temperature spin-glass freezing. High field magnetization and susceptibility per Gd atom in the paramagnetic state are significantly suppressed below that of noninteracting Gd, as observed previously for $a\text{−}\mathrm{Gd}\text{−}\mathrm{Si}$ alloys. However, unlike $a\text{−}\mathrm{Gd}\text{−}\mathrm{Si}$, the low field susceptibility does not fit a Curie-Weiss law and shows no significant dependence on composition. Conductivity measurements show that Gd causes localization of charge carriers below a characteristic temperature $T^{*}$, which also marks the onset of significant negative magnetoresistance. Both $T^{*}$ and the magnitude of the MR are significantly lower in $a\text{−}\mathrm{Gd}\text{−}\mathrm{Ge}$ than in comparable $a\text{−}\mathrm{Gd}\text{−}\mathrm{Si}$ alloys. It is proposed that the large effects of the host matrix (Ge vs Si) are due to differences in both the band gap and dielectric constant, which cause changes in screening, thereby altering the effect of Gd magnetic moments on the localization of carriers and on the indirect mediated Gd-Gd exchange interactions.

• Received 2 July 2007

DOI:https://doi.org/10.1103/PhysRevB.76.184440