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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    3D Magnetic Imaging using SQUIDs and Spin-valve Sensors
    (2016) Jeffers, Alex; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa2Cu3O7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from this inverse technique that demonstrate the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin-valve sensors I typically applied currents of 1 mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1 mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.
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    Design and Testing of a Galfenol Tactile Sensor Array
    (2006-12-20) Hale, Kathleen; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The smart material Galfenol, Fe(100-x)Ga(x), where 15<x<28, offers a unique combination of mechanical and magnetostrictive properties that are expected to lead to its use in new sensor and actuator concepts. This thesis seeks to determine if Galfenol can be used to develop a 2-dimensional array of force sensors as part of a 3-D magnetic circuit that, if properly scaled, could mimic the tactile force sensing capabilities needed for use in robotic grippers, prosthetic devices, and robotic surgery. This concept takes advantage of the fact that Galfenol is not brittle and its permeability has high sensitivity to mechanical loads. The hypothesis is that applying stress or force to one or more of the Galfenol rods will produce changes in Galfenol's permeability which will produce changes in the flux density distribution in the magnetic circuit that can be used to determine information about both the load's magnitude and location. The studies performed demonstrated that the decrease in permeability of a loaded rod results in complex changes in magnetic flux. Results from this thesis include recommendations for modifications to better match the rod flux density to the applied load levels and prevent rod top separation.
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    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.