METAL-INSULATOR-TRANSITION AND GIANT NEGATIVE MAGNETORESISTANCE IN AMORPHOUS MAGNETIC RARE-EARTH SILICON ALLOYS

Large negative magnetoresistance and anomalous magnetic properties are found in amorphous Si doped with magnetic rare earth ions near the me tal-insulator transition. The resistivity below 50 K rises orders of m agnitude above that of comparable composition nonmagnetic alloys and i s strongly reduced by a magnetic field. Magnetization measurements sho w noninteracting moments at high temperature which develop antiferroma gnetic 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.