Browsing by Subject "Magnetostriction"
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Item Asymptotic models in magnetostriction with application to design of sensors.(2012-04) Krishnan, Shankar NarayanMagnetostrictive wires of diameter in the nanometer scale have been proposed for application as acoustic sensors [Downey et al., 2008], [Yang et al., 2006]. The sensing mechanism is expected to operate in the bending regime. In the first part of this work, we derive a variational theory for the bending of magnetostrictive nanowires starting from a full 3-dimensional continuum theory of magnetostriction. We recover a theory which looks like a typical Euler-Bernoulli bending model but includes an extra term contributed by the magnetic part of the energy. The solution of this variational theory for an important, newly developed magnetostricitve alloy called Galfenol ¡ cf. [Clark et al., 2000] ¢ is compared with the result of experiments on actual nanowires ¡ cf. [Downey, 2008] ¢ which shows agreement. In the next part of this thesis, Multilayered wires of diameter in the nanometer scale with periodic layering of non-magnetic copper and ferromagnetic galfenol segments are studied. The numerical computation of the physics of magnetization for such geometries is very costly computationally. We use the theory of periodic homogenization to understand the overall behavior of such structures. We first determine a “homogenized theory” after which this “homogenized model” is used to study the nucleation and stability of staturated states. Thus we get a broad generalization of what is known in the magnetic literature as the “fanning model” first introduced in [Jacobs and Bean, 1955] for a chain of spheres geometry. Some further numerical work on computing M vs H curves for such geometries is also presented.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 Development and Characterization of Magnetostrictive GaFe and Plasmonic Gold Thin Films(2015-04) Estrine, EliotAs device sizes continue to shrink into the nano-scale, material development becomes increasingly important. This presents new deposition and characterization challenges which must be overcome to produce the next generation of devices. Magnetostrictive GaFe (galfenol) is one such material in which development of deposition and characterization techniques is necessary to enable new MEMS devices. In addition, plasmonic gold Near Field Transducers (NFTs) used in Heat Assisted Magnetic Recording (HAMR) require new characterization options to understand device failure modes as well as new gold deposition processes to improve device reliability. While these applications are very different, the underlying material deposition and characterization challenges involving thin film crystallinity are very similar. Magnetostriction measurements of electrodeposited galfenol show that it is possible to achieve thin films of this material over a wide range of compositions using electrodeposition. In addition, grain refinement in gold was achieved through alloying which shows the potential to create more robust thin films while maintaining gold's desirable plasmonic properties. Finally, advanced characterization processes using Electron Back Scatter Diffraction (EBSD) were also developed to analyze thin film crystal structure and its role in NFT stability. These results will further progress in the fields of MEMS and HAMR as well as provide the basis for identifying and solving materials challenges in the future.Item Modelling of galfenol nanowires for sensor applications.(2010-05) Narayan, Krishnan ShankarGalfenol is a new magnetostrictive material with potential applications in sensors for acoustic waves. The purpose of the present investigation is to study the properties of galfenol nanowires of nanometer range diameter relevant to sensing of acoustic waves using the phenomenon of magnetostriction. In this endeavor we study first the basic energetics for this material. Then we investigate the macro-scale behaviour for galfenol using the theory of "Large body limits in Ferro-magnetism" of Desimone [DS93]. Subsequently we look at the existence and stability of single domain states in galfenol nanowires. The theoretical predictions are then verified by numerical methods.