Magnetic Moments in Amorphous Semiconductors

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TECHNICAL EXPLANATION This project will investigate the effect of adding magnetic moments to semiconductors on the electrical and magnetic properties of the resultant materials. The focus will be on amorphous semiconductors where a wide variety of dopants can be readily introduced without damaging the structure. Amorphous Si, Ge, and various forms of C doped with magnetic ions such as rare earth elements, Fe and Mn will be prepared and their electronic, magnetic, thermodynamic, and structural properties measured. The influence of the phonon stiffness and the semiconductor band gap on these properties will be determined. In amorphous C, the bonding can and will be varied from graphitic to diamond-like, allowing the band gap to be tuned from near zero to large. The interaction between the added local moments and the spontaneous moments formed at the Metal-Insulator transition as electrons localize will be studied. The research will provide a phenomenological underpinning for a microscopic model for the three dimensional Metal-Insulator transition, and an understanding of the effect on both transport and magnetic properties of introducing local magnetic moments into a semiconductor. The project will provide research training and education for two graduate students and an estimated 5-6 undergraduates, including exposure to industry and to the national laboratories through the PI's collaborations with the magnetic recording industry and with the National High Magnetic Field Lab and Lawrence Berkeley Lab. NON-TECHNICAL EXPLANATIONThe deliberate addition of magnetism or "magnetic moments" to semiconductors produces effects that are extremely large. In particular the change in the electrical resistance when such a material is placed in a magnetic field (magnetoresistance) can be enormous. This project will investigate the effect of adding magnetic atoms to amorphous (non-crystalline) semiconductors Si, Ge, and C, deliberately creating and studying new materials not previously made. Films of amorphous Si, Ge, and C doped with magnetic ions such as rare earth elements, Fe, and Mn will be prepared. Their electronic, magnetic, thermodynamic, and structural properties will be measured over a wide range of temperature, magnetic field, and doping levels. The research will attempt to determine the principles governing the large effect of magnetic moments in semiconductors. An increased understanding of these effects may point the way to making the very large magnetoresistance of use to a developing technology that will use the magnetic property of the electron, its "spin" in addition to its charge, i.e. spin electronics. In addition this research aims to increase our understanding of why magnetic moments play such a crucial role in materials of current interest, such as the high temperature superconductors. The project will provide research training and education for graduate and undergraduate students, and provide them with an exposure to industry and to the national labs through collaborations. Thus, the students will be well prepared for future careers in academia, or industrial or government laboratories.