Large negative magnetoresistance and anomalous magnetic properties are found in amorphous Si doped with magnetic rare earth ions near the metal-insulator transition. The resistivity below 50 K rises orders of magnitude above that of comparable composition nonmagnetic alloys and is strongly reduced by a magnetic field. Magnetization measurements show noninteracting moments at high temperature which develop antiferromagnetic interactions below 50 K. We suggest that these results are due to formation below 50 K of a dense concentration of magnetic polarons which localize conduction electrons. [S0031-9007(96)01722-X]

We explore the influence of local magnetic moments on transport and magnetic properties in a new class of diluted magnetic semiconductors, amorphous RExSi1−x (RE=Gd, or Tb, 0.1<x<0.15). As a comparison, nonmagnetic a-YxSi1−x was also studied. The Gd, Tb, and Y ions have similar size and electronic structure, except for the 4f electrons in Gd and Tb. For 50 K<T<300 K, the conductivity σ rises linearly with increasing T with nearly identical slopes for these three alloys. Below 50 K, they have dramatically different conductivities. a-YxSi1−x shows the usual metal-to-insulator transition observed in doped semiconductors as the Y content is lowered. For a-GdxSi1−x and a-TbxSi1−x, however, σ rapidly decreases such that compositions that are metallic in a-YxSi1−x are insulating for a-GdxSi1−x and a-TbxSi1−x. Applying a magnetic field H has the usual relatively small effects on σ of a-YxSi1−x but produces a large increase in conductivity for the a-GdxSi1−x alloys. For example, in a-Gd0.12Si0.88, σ increases by 60 times at T=2.4 K and H=6 T. This giant negative magnetoresistance drops as T rises, disappearing above 50 K. Magnetization M as a function of H shows no hysteresis. M(H/T) for different fields and temperatures even for a single sample do not fit a Brillouin function, showing no paramagnetism or superparamagnetism. The magnetic susceptibility is much smaller than expected for isolated moments, indicating antiferromagnetic interactions between local moments. However, no Curie, Neel, or spin-glass freezing temperatures are observed. The magnetization and transport data are presumably the result of an interaction between nearly localized conduction electrons and the local moments of the Gd(J=7/2) and Tb (J=6) ions, i.e., a magnetic polaron forms that increases the conduction electron effective mass, causing it to localize. The amorphous nature of these alloys makes conduction significantly different than in the usual crystalline diluted magnetic semiconductors.