Browsing by Subject "Nanowire"
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Item Constricted current perpendicular to plane (CPP) magnetic sensor via electroplating.(2011-01) Huang, XiaoboElectrochemically deposited magnetic nanowires have gained increasing attention since current perpendicular to the plane giant magnetoresistance (CPP-GMR) was observed in multilayered nanowires. Magnetic nanowires have potential for fundamental studies, including measuring spin diffusion lengths and understanding the mechanisms of the electron spin transfer. They also have great potential technological applications as CPP-GMR sensors, magnetic random access memory (MRAM), and next generation magnetic recording heads. Small diameter nanowires are desired in order to have large current density per device and a high areal density for device arrays, for example, 2 Tb/in2 media. In this research, E-beam lithography, nano-imprinting, and self-assembled nanoporous alumina templates (AAO) were studied to achieve as small diameter nanopores as possible. AAO templates with 10 nm diameter were fabricated using both Al foils and Al thin films. Very small diameter (10 nm) CPP-GMR Co/Cu nanowires were fabricated into AAO templates using electrochemical deposition. The magnetic transport properties of these multilayered and trilayered Co/Cu nanowires were investigated. It was found that nanowire anisotropies parallel and perpendicular to the nanowires were dependent on the thicknesses of Co and Cu layers. GMR of 19% was achieved with 10 nm diameter nanowires at room temperature. The magnetic free layers were as thin as 4.5 nm with GMR of 18%. Spin transfer torque switching current densities were measured to be 106 - 108A/cm2. The measurement of spin transfer torque was conducted numerous times with high repeatability in the critical switching currents from parallel to antiparallel alignment (JP-AP) and slight variations in back (JAP-P). Small resistance area products (RA) of 0.003 ohmµm2 were achieved with trilayers that had 40ohm total resistance. All of results in this study show that nanowires with 10 nm diameters have potential application as next generation CCP-GMR sensors and spin transfer torque MRAM.Item Controlled electrochemical synthesis of giant magnetostrictive iron-gallium alloy thin films and nanowires.(2012-04) Reddy, Kotha Sai MadhukarMagnetostrictive Galfenol (Fe1-xGax, x = 10% - 40%) alloys have generated tremendous interest in recent times because of their potential as functional materials in various micro- and nano-electromechanical systems (MEMS/NEMS)-based transducers and sensors. Among the giant magnetostrictive alloys, Terfenol-D (Tb1-xDyxFe2) has the largest magnetostriction, but its brittle nature limits its applications. In contrast, the next best magnetostrictive alloy, Galfenol, is highly malleable and ductile while having the tensile strength of Iron. Electrochemistry is an economical route to fabricate 'very thick' films (upto several microns) or high-aspect ratio structures like nanowire arrays. However, the highly electropositive nature of gallium makes it very difficult to electrodeposit from aqueous solutions, similar in behavior to other non-ideal elements like molybdenum, phosphorus, tungsten etc. As a result, Fe1-xGax alloy plating has been severely plagued by non-repeatability in compositions from growth to growth, lack of uniformity in filling of pores when growing nanowires in nanoporous templates, undesired secondary hydrogen evolution reactions etc. In this study, a thorough understanding of the complex interplay between various deposition parameters (pH, overpotential, concentration, hydrodynamic conditions) was achieved, leading to an understanding of the deposition mechanism itself, thus allowing excellent control and ability to tune the alloy compositions. Arrays of nanowires were fabricated with alternating segments of the magnetostrictive alloy Fe1-xGax and Cu in nanoporous anodic aluminum oxide (AAO) templates. A novel rotating disk electrode-template (designed in-house) was used to optimize the nanowire length distributions by controlling the various aspects of electrodeposition like nucleation, kinetics and mass-transfer. Extensive structural characterization was done by X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), and magnetic characterization by vibrating sample magnetometry (VSM). Furthermore, of excellent promise in semiconductor spintronics, the feasibility of fabricating epitaxially nucleated Fe1-xGax thin films on GaAs having the desired (001) texture was demonstrated. Structural characterization using microdiffraction, high resolution ω - 2θ and rocking curve analysis revealed that the films grown on GaAs(001) are highly textured with <001> orientation along the substrate normal, and the texture improved further upon annealing at 300 °C for 2 hours in N2 environment. This was in contrast to films grown on polycrystalline brass substrates which exhibited undesired <011> texture out-of-plane. Rocking curve analysis on Fe1-xGax/GaAs structures further confirmed that the <001> texture in the Fe1-xGax thin film was indeed due to epitaxial nucleation and growth. A non-linear current-voltage plot was obtained for the Fe1-xGax/GaAs Schottky contacts, characteristic of tunneling injection, and showed improved behavior with annealing.Item Modeling of continuum transport and meso-scale kinetics during solution crystal growth(2014-05) Wang, WeiSolution crystal growth is widely applied in many industries and fundamental research, and it is employed to crystallize materials ranging from inorganic molecules, small organic molecules, to large organic molecules. However, despite the broad application, fundamental factors regarding this crystal growth process are not well understood. In this thesis, numerical models are developed to study the influences of macro-scale mass transfer limitations and meso-scale growth kinetics on solution crystal growth. A parallel, finite element model is implemented to compute three-dimensional fluid flow and mass transfer during crystal growth and is especially applied to the growth systems in Atomic Force Microscopy fluid cells. This work assesses the parametric sensitivity of growth conditions to factors such as the strength of flow, the frequency of scanning motion, the size of the crystal, and the kinetics of the growing surface. Accounting for such effects will be very important to understand solution crystal growth and to interpret AFM measurements of growth dynamics. Additionally, a simplified two-dimensional numerical model focused on the region near the growing crystal surface and the AFM cantilever was developed based on the calculated results of the three-dimensional model. With this two-dimensional model, we provide basic understanding of the fluid flow and mass transfer where the AFM measurements were made, and simplified the revision of AFM measurements interpretation.A fundamental theoretical model based on the phase-field approach is developed to simulate nano-scale island growth and spiral step growth on crystal surfaces in a supersaturated liquid and is validated by comparison to zinc oxide nanowires synthesis experiments. Results obtained by this work help to explain how experimental factors affect the crystal growth and crystal microstructures and the correlation between island growth and spiral growth mechanisms.