Established Universal Properties of Linear Magnetoresistance in

1. Hybrid Ferromagnet/Semiconductor Nanodots and NanowiresDon Heiman, Northeastern University, DMR 0907007 This program focuses on advancing novel nanodot and nanowire composites of semiconductors and ferromagnets, which could be crucial for the development of future devices for information processing and storage. Established Universal Properties of Linear Magnetoresistance in Strongly Disordered Semiconductors The magnitude of the linear magnetoresistance (LMR) in MnAs/GaAs composites is numerically equal to the carrier mobility over a wide range and is independent of carrier density. This behavior is complementary to the Hall effect that is independent of the mobility and dependent on the carrier density. The LMR appears to be insensitive to the details of the disorder and points to a universal explanation of classical LMR that can be applied to other material systems. H.G. Johnson, S.P. Bennett, R. Barua, L.H Lewis, and D. Heiman, Phys. Rev. B 82, 085202 (2010) Method for Determining Magnetic Nanoparticle Size Distributions from Thermomagnetic Measurements Thermomagnetic measurements are used to obtain the size distribution and anisotropy of magnetic MnAsnanoparticles. An analytical transformation method is described which utilizes temperature-dependent zero-field cooling (ZFC) magnetization data to provide a quantitative measurement of the average diameter and relative abundance. This analytical technique holds promise for rapid assessment of the size distribution of nanoparticles. R.S. DiPietro, H.G. Johnson, S.P. Bennett, T. Nummy, L.H Lewis, and D. Heiman, Appl. Phys. Lett. 96, 222506 (2010) The magnetoresistance, percentage change of resistance with magnetic field, versus applied magnetic field in a composite film of MnAsnanoparticles in a GaAs matrix. The MR increases to 900% at low temperatures. The MR is found to be proportional to the carrier mobility times the magnetic field, so that universally, MR (T) = μ(T) H. Distribution in particle size of MnAsnanoparticles in a composite film of GaAs. The data points represent particle size obtained from the zero-field cooling magnetization. f (V) ~ d/dT (T * MZFC) The histogram was obtained from SEM images, and the solid curve is a log-normal fit to the data.

2. Hybrid Ferromagnet/Semiconductor Nanodots and NanowiresDon Heiman, Northeastern University, DMR 0907007 Much of the research shown on the previous slide was conducted by undergraduate students. The first authors of the publications, Hannah Johnson and Robert DiPietro, were undergraduate physics majors working on the Co-op program at Northeastern. The students spent 6 months working full-time in my lab. In total, four undergraduate students contributed to the research and are co-authors, Hannah Johnson, Robert DiPietro, Steve Bennett, and Tom Nummy. I have since added another undergraduate student, Tom Cardinal, to the project. Several of these students have been funded through REU supplements. Steve Bennett has moved up to a masters program and continues to work on projects in collaboration with Prof. Latika Menon. Robert DiPietro has graduated, was hired at Lincoln Labs, and expects to pursue a PhD at MIT. The research experience that these undergraduate students have accomplished has proven to be vital to advancing their careers in research.