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Note: thesis: Thesis (Ph. D.)--Brown University, 2018
Genre (aat): theses
Abstract: Magnetic tunnel junctions (MTJs) based on textured MgO insulating barrier are a class of quantum mechanical devices, increasingly used in technology industries as magnetic memory devices and magnetic sensors. However, improving MTJs' performance remains as a challenging task, particularly, the magnetic sensitivity has remained relatively steady for a while. New approaches are needed to make MTJs reach their potentials. One of the objectives of this work is to increase the field sensing capability of MTJs by reducing magnetic anisotropy of the constituent materials, and by implementing novel magnetic amplying components based on magnetic flux concentration. The second focus of this work is on the electrical junctions based on Mott insulators (MIs), another class of quantum materials. Due to the strong electron correlation effect in MIs, their heterojunctions have been proposed as ultrafast electrical rectifiers (e.g., diodes), with performance exceeding conventional semiconductor diodes. We have made a major attempt to fabricate high quality epitaxial MI heterojunctions, that have demonstrated the potential for future applications in electronics and in solar rectification devices. We have systematically studied many MgO-based MTJ multilayer structures. Each thin-film layer's contribution to the MTJ performance has been isolated and improved. MTJs with large tunneling magnetoresitance ratios (TMR) have been successfully fabricated using a magnetron sputtering system through the optimization of layer structures. The geometry of the MTJ sensor and magnetic annealing process have also been optimized for performance. We have introduced magnetic flux concentrator into our MTJs to significantly increase the magnetic sensing capability. The flux concentrator is a "soft" magnetic material with certain geometry working directly on the free layer of MTJ sensor as a magnetic field amplifier. Thin films of a special "soft" magnetic material (Co88Nb8Zr4) has been successfully fabricated utilizing our high vacuum magnetron sputtering technique, and characterized using magnetometry measurement. The "soft" magnetic concentrators have been added to the MTJ using lithography techinique. Furthermore, we have included additonal magnetic concentrators outside of the MTJ devices. With these approaches, we have increased the magnetic sensing capability of the MTJs significantly. Based on the same magnetron sputtering technique, we have successfully developed high-quality heterojunctions of epitaxial La1.9Sr0.1CuO4 (LSCO) thin films on a 0.5wt% Nb doped SrTiO3 (NSTO) single-crystal substrate. The heterojunction shows good rectification and expected temperature dependency. We have investigated the capacitance of the junctions over a broad frequency range and bias voltage range. The results were explained by considering the LSCO/NSTO junctions as a Schottky barrier with deep-level impurities. This doped Mott insulator system is quite different from traditional p-n heterojunctions and exhibited interesting interfacial properties. Our work has built a foundation for the further development of MI-based junctions as viable high frequency diodes and solar rectification energy conversion devices.
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Identifier: DOI: 10.26300/5hwy-2t84
Access Condition: rights statement (href="http://rightsstatements.org/vocab/InC/1.0/"): In Copyright
Access Condition: restriction on access: Collection is open for research.
Type of Resource (primo): dissertations