UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Characterization of Bending Magnetostriction in Iron-Gallium Alloys for Nanowire Sensor Applications(2008-11-21) Downey, Patrick; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research explores the possibility of using electrochemically deposited nanowires of magnetostrictive iron-gallium (Galfenol) to mimic the sensing capabilities of biological cilia. Sensor design calls for incorporating Galfenol nanowires cantilevered from a membrane and attached to a conventional magnetic field sensor. As the wires deflect in response to acoustic, airflow, or tactile excitation, the resultant bending stresses induce changes in magnetization that due to the scale of the nanowires offer the potential for excellent spatial resolution and frequency bandwidth. In order to determine the suitability for using Galfenol nanowires in this role, the first task was experimentally characterizing magnetostrictive transduction in bending beam structures, as this means of operation has been unattainable in previous materials research due to low tensile strengths in conventional alloys such as Terfenol-D. Results show that there is an appreciable sensing response from cantilevered Galfenol beams and that this phenomenon can be accurately modeled with an energy based formulation. For progressing experiments to the nanowire scale, a nanomanipulation instrument was designed and constructed that interfaces within a scanning electron microscope and allows for real time characterization of individual wires with diameters near 100 nm. The results of mechanical tensile testing and dynamic resonance identification reveal that the Galfenol nanowires behave similarly to the bulk material with the exception of a large increase in ultimate tensile strength. The magnetic domain structure of the nanowires was theoretically predicted and verified with magnetic force microscopy. An experimental methodology was developed to observe the coupling between bending stress and magnetization that is critical for accurate sensing, and the key results indicate that specific structural modifications need to be made to reduce the anisotropy in the nanowires in order to improve the transduction capabilities. A solution to this problem is presented and final experiments are performed.Item CRYOGENIC ATOMIC FORCE MICROSCOPE FOR CHARACTERIZATION OF NANOSTRUCTURES(2005-07-28) Li, Changyi; Yang, Chia-Hung; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design and applications of a cryogenic atomic force microscope (AFM) for characterization of nanostructures. The cryogenic AFM with a conductive tip can measure DC current through nanostructures. We use quartz tuning fork (QTF) as the force sensor. Unique coarse z motor design provides reliable autoapporach in the Z direction. AFM imaging with 10nm horizontal and ~2 angstrom vertical resolution has been achieved. We have used this AFM in the current-voltage characterization of diodes, and, with a modified sensing mechanism, electrical force microscopy (EFM) and magnetic force microscopy (MFM) have been demonstrated.Item Spin injection and detection in copper spin valve structures(2005-01-25) Garzon, Samir Y; Webb, Richard A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We report measurements of spin injection and detection in a mesoscopic copper wire from which the electron spin relaxation time and the spin current polarization in copper can be found. Spin injection is realized by applying a voltage to drive a current from a ferromagnet into the normal metal, while spin detection is done using transport measurements. Precession of the spin of the injected electrons due to an external magnetic field is also studied. The existence of a previously unobserved spin signal which vanishes at low temperatures but increases nonlinearly above 100K is reported and a possible explanation for its origin, based on interfacial spin-flip scattering, is suggested. Multiple cross checks to test the possibility of artifacts as an origin of this signal are discussed. An alternative spin detection method using magnetic force microscopy (MFM) is also studied. This method measures the magnetic field produced by the injected spins directly, so the spin coherence length and the spin current polarization can be extracted directly without the need of a particular transport model, avoiding issues like contact resistance and interface scattering. The MFM method can also be useful for measuring the spin polarization of currents in semiconductors and semiconductor heterostructures, which is important for the development of spintronics.