Semester

Summer

Date of Graduation

2012

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

David Lederman.

Abstract

The central theme of this work is the engineering of devices and materials that exhibit spin dependent phenomena. In particular, the spin orientation of charge carriers can play a central role in transport, especially in magnetic or other spin correlated media. Propagation of charge carriers with net spin results in a transfer of angular momentum that can excite static and dynamical states in active device elements. To utilize such phenomena in practical devices, new mew means of device characterization and optimization must be developed. To that end, we have performed experiments which elucidate some of the mechanisms underlying spin dependent transport phenomena.;We report the observation of hysteretic synchronization of point contact spin torque nano-oscillators (STNOs) by a microwave magnetic field. The hysteresis was asymmetric with respect to the frequency detuning of the driving signal, and appeared in the region of a strong dependence of the oscillation frequency on the bias current. Theoretical analysis showed that hysteretic synchronization occurred when the width of the synchronization range, enhanced by the oscillator's nonlinearity, became comparable to the dissipation rate, while the observed asymmetry was a consequence of the nonlinear dependence of frequency on the bias current.;Another emergent phenomenon was a series of fractional synchronization regimes in a STNO driven by a microwave field. These regimes are characterized by rational relations between the driving frequency and the frequency of the oscillation. Analysis based on the phase model of auto-oscillator indicates that fractional synchronization becomes possible when the driving signal breaks the symmetry of the oscillation, while the synchronization ranges are determined by the geometry of the oscillation orbit. Measurements of fractional synchronization were utilized to obtain information about the oscillation characteristics in nanoscale systems not accessible to direct imaging techniques.;Oxidation in magnetic nanosystems can result in changes of the magnetic ordering of active layers in devices, resulting in degraded device performance. We demonstrate that magnetic multilayer nanopillars can be efficiently protected from oxidation by coating with silicon. Both the protected and the oxidized nanopillars exhibited an increase of reversal current at cryogenic temperatures. However the magnetic excitation onset current increased only in the oxidized samples. We show that oxidized nanopillars exhibit anomalous switching statistics at low temperature, providing a simple test for the quality of magnetic nanodevices.;We studied exchange bias in magnetic multilayers incorporating antiferromagnet CoO doped with up to 35 atomic percent of Pt. The exchange bias increased with doping in epitaxial films, but did not significantly change in polycrystalline films at the lowest measured temperature of 5 K, and decreased at higher temperatures. We explain our results by the increased granularity of the doped antiferromagnetic films, resulting in simultaneous enhancement of the uncompensated spin density and reduction of the magnetic stability of antiferromagnetic grains.;Finally, we demonstrate the growth of Bi2Se3, a material known as a topological insulator (TI). The structural and electronic properties of Bi2Se3 films grown on Al2O 3 (110) by molecular beam epitaxy were investigated. The epitaxial films grew in the Frank-van der Merwe mode and were c-axis oriented. They exhibited the highest crystallinity, the lowest carrier concentration, and optimal stoichiometry at a substrate temperature of 200 .C determined by the balance between surface kinetics and desorption of selenium. The crystallinity of the films improved with increasing selenium/bismuth flux ratio.

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