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 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.