Growth and Characterization of Ferromagnetic Nanostructures for Device Applications
Nanotechnology offers tremendous potential for future technology. A better understanding of ferromagnetism, half-metallicity and magnetostriction on the smallest scales are expected to improve technological performance. Processing and characterization of nanostructured materials, therefore, are central in modern solid state physics. One of the techniques to grow nanostructuresis electrospinning which offers simplicity, low cost and exibility. This dissertation focuses on functionally different colossal magnetoresistance strontium and calcium doped lanthanum manganites and magnetostrictive iron-gallium nanowires. We have tailored the conventional electrospinning method to grow high quality manganites and iron-gallium nanowires below 100 nm in diameter for the first time. We have shown that these ferromagnetic nanostructures can be grown parallel to one another for understanding the basic physics as well as for fabricating advanced device structures. Single nanowire devices were fabricated by electron-beam lithography. Details of the growth, morphology, structure and magnetic characterization, device fabrication and performance of the devices will be discussed in this thesis. Manganite nanowires have been grown by anodized alumina oxide template and by the pulsed laser deposition methods by other groups, nanowires grown from these techniques are difficult. to use for device fabrication. Devices based on electrospun manganite and iron-gallium nanowires have shown improved electrical and magneto-transport properties. Half-metallic lanthanum-strontium manganite nanowires with 80 - 300 nm in diameters display enhanced magnetoresistance behavior at room temperature and the large magnetoresistance is exhibited at low magnetic field which will play an important role for creation of novel-next generation devices. Although inter-metallic, bulk iron-gallium is one of the highly attractive magnetostrictive materials, at nano level, it is very rarely studied due to difficulty of growing high-quality materials. Alumina templates or by melt-spinning are only methods applied so far to grow iron-gallium wires. Melt-spinning yields microscale ribbons and the growth using templates results in nanowires with varying stoichiometry over the entire length of the nanowire. This work has produced high quality, stoichiometric, parallel bundles of iron-gallium nanowires with diameters varying from 80 - 500 nm. These iron gallium nanowires exhibit enhanced magnetic properties. Devices with bundles of iron-gallium nanowires are explored for various applications such as submarine sonar, as highly sensitive magnetic sensors and nanoscale valves.
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