Thesis (Ph. D.)--Florida State University, 2006. / Advisor: Peng Xiong, Florida State University, College of Art and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed June 9, 2006). Document formatted into pages; contains xiv, 78 pages. Includes bibliographical references.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references. Also available in electronic version.

Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: John Q. Xiao, Dept. of Physics and Astronomy. Includes bibliographical references.

In this thesis, the potential of all-spinel oxide tunnel junctions in the field of spintronics has been investigated. In particular, the suitability of the metallic and superconducting LiTi2O4 as non-magnetic electrode in an almost defect-free CoFe2O4-based spin-filter tunnel junction has been explored. For this purpose, high-quality spinel LiTi2O4 and CoFe2O4 thin films have been deposited by pulsed laser deposition on MgAl2O4 substrates. Both films were extensively characterised in terms of structural, surface, magnetic and transport properties. The LiTi2O4 showed metallic and superconducting properties, and the CoFe2O4 had insulating and ferromagnetic properties. A careful tuning of the different growth conditions of these oxides followed in order to grow CoFe2O4/LiTi2O4 bilayers in which LiTi2O4 maintains its metallic and superconducting properties and CoFe2O4 its insulating ferromagnetic characteristics. Transport measurements at low temperature have been carried out to explore details of the tunnelling in symmetric tunnel junctions of the form LiTi2O4/CoFe2O4/LiTi2O4. The measured current–voltage characteristics of these junctions revealed clear Josephson junction behaviour due to superconductivity of the LiTi2O4 electrodes. This conclusive evidence of the tunnel nature of these junctions proves that LiTi2O4 can be used as bottom electrode in all-spinel oxide tunnel junctions.

This dissertation reports on a detailed systematic study of the investigation into using Indium Oxide based materials in next generation spin-transport electronic applications. Initial studies focused on the optimisation of the electrical properties of Indium Oxide (In2O3) and Tin(Sn)-doped Indium Oxide (ITO) thin films grown using DC magnetron sputtering. The manipulation of various deposition parameters allowed the electrical properties to be tuned effectively. With the desire to create multi-functional spintronic devices, a dilute magnetic oxide system is developed where the In2O3 and ITO matrices are doped with low levels of transition metals, in particular, Co. Using a number of characterisation techniques, the origins of the magnetic response in these thin films is explored in great detail. In particular, powerful probes such as x-ray and optical magnetic circular dichroism are utilised. The major finding from these investigations is that the magnetism does not necessarily emanate from the Co dopants alone. In fact, Co dopants give a strictly paramagnetic response, suggesting that the magnetism observed may be a result of polarised electrons in localised donor states in the In2O3 and ITO hosts. Therefore, we believe that the origins of magnetism in these films is related to a hybridisation and charge transfer of electrons from a broad donor/defect-derived impurity band to a band of unoccupied 3d states at the Fermi level. The emergence of a very weak magnetic signal in pure ITO raises further questions as to the true origins of the ferromagnetic behaviour and supports a defect-related mechanism. To explore the suitability of ITO for a future in spintronics further, the performance of some metal ferromagnet/oxide multilayered structures was investigated. The investigations revealed a significant contribution to both the magnetic and magnetotransport properties from a superparamagnetic component giving some insight into the importance of the quality of interfaces between the metal ferromagnet/oxide layers and heterostructures. Using a three-dimensional focused-ion beam etching technique to fabricate submicronspin-valve devices with ITO spacer layers, current-perpendicular-to-plane magnetoresistance measurements were carried out to estimate the spin diffusion length of ITO at room temperature. In conjunction with a simplified Valet-Fert model, a spin asymmetry ratio for Co of 0.55 and spin diffusion length of 6±1 nm in semiconducting ITO at room temperature was estimated. These findings imply that spin information can be conserved and transported through In2O3 and ITO even up to and beyond room temperature.

The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1-xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ~2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ~30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga,Mn)As layer. By employing the k . p semiconductor theory approach (performed by our collaborators in Institute of Physics, ASCR, Prague), including strong spin-orbit coupling effects in the host semiconductor valence band, a change in sign of Kc at hole density of approximately 1.5x1020 cm-3 is observed. Below this density, the [110]/[1⁻10] magnetization directions are favoured, consistent with experimental data. A double-gated FET, with an ionic-gel top-gate coupled with a p-n junction back-gate based on the same material, was also employed in an attempt to achieve larger effects through gating. It reaffirms the results obtained and demonstrates enhanced gating effects on the magnetic properties of (Ga,Mn)As.

The area of spintronics has witnessed tremendous progress in the 21st century. During the 1980s and 1990s, the Giant and Tunnel Magnetoresistance effect dominated spintronics research with room-temperature ferromagnets(FM), metals, and insulators. Since then, the observation of several spin-based phenomena in heavy non-magnetic materials such as the Spin Hall Effects and the discovery of topological insulators have broadened the scope of spintronics research. Strong spin-orbit coupling (SOC) in topological insulators is expected to induce strong spin-orbit torques on proximal magnetic moments. This has motivated much recent interest in FM/TI systems with applications in spintronic memory and sensing. In this work, magnetron sputtered large-area Co/Bi2Se3 thin films were investigated with a custom-built magnetotransport setup. When current is passed predominantly through the Co layer we observe typical Co anisotropic magnetoresistance, but by promoting higher current density through the interface, we observe unidirectional magnetoresistance with a much larger change in resistance than Co alone. We also observe an unusual inverse current dependence. To test the contributions of each constituent material, Co/Ta and Cr/Bi2Se3 were prepared as controls, and similar observations were made in both materials, but not Co/Cr, suggesting that high SOC in the overlayer may be the driving force. While a proper understanding of our magnetoresistance data is not available at this point, the results broadly highlight the exciting prospects of observing novel phenomena in bilayer spintronic systems at room temperature.

No description available.

Every engineer that has worked on designing an integrated circuit has to leverage an under- standing of device physics. Understanding device physics is essential when optimizing a design for speed, power, etc. These characteristics affect the bottom line when considering an integrated circuit used in a particular application. In order for there to be an under- standing of device physics, there must be a device model that is developed for a device of interest. The development of a device model often involves utilizing fundamental physical equations in a manner that is solvable by either analytical or numerical means. This typically begins by simplifying fundamental physical equations, possibly spanning multiple domains, and considering the physical quantities of interest. In order to make simplifications, assumptions about the underlying physics must be made. It is the process of transitioning from known physics laws to simplified mathematical models that a device modeler spans. This thesis will cover the device modeling aspects of a new classification of computing devices, spintronics. It will begin by stating the physical assumptions necessary for the operation of spintronic devices. Then it will go the process of deriving the underlying physical equations and stating them in a tractable form with the appropriate boundary conditions. Then these equations will be manipulated and mapped into an equivalent circuit. The equivalent circuits will them be validated against analytical solutions provided from other works. It will then finish by providing example devices that can be simulated with the develop device models, and some optimization results are proposed based off a simplified circuit model.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22161号 / 工博第4665号 / 新制||工||1728(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 白石 誠司, 教授 藤田 静雄, 准教授 掛谷 一弘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM