Local and Non-Local Magnetic Moments in Amorphous Silicon Near the Metal-Insulator Transition

Much of the success in understanding the properties of electrons in solid materials has been based on taking advantage of the symmetry and periodicity of a crystalline structure. However, it is well known that amorphous materials with non-periodic structures can also be metals, insulators, and semiconductors. The disorder of the amorphous structure causes some moderately well understood localization phenomena. Adding magnetic ions to an amorphous system results in dramatic and not yet understood effects, due to an interaction between the conduction electrons and the local magnetic moments. These local magnetic moments cause a shift in the metal-insulator transition that can be reversed by application of a magnetic field. The result is that very large negative magnetoresistance (many orders of magnitude) occurs in amorphous Si doped with magnetic rare earth ions (a-RE_xSi_1-x) for x<0.15. In zero applied magnetic field, these alloys are strongly insulating. Applying a magnetic field aligns the RE moments and delocalizes the electrons. We use magnetotransport, magnetic susceptibility, tunneling spectroscopy, IR absorption spectroscopy, and heat capacity measurements to understand the effect of magnetic moments on electrical transport in semiconductors and the effect of the semiconducting matrix on the magnetic moments.