Thesis (Ph. D.)--University of Iowa, 2008. / Thesis supervisor: Markus Wohlgenannt. Includes bibliographical references (leaves 82-88).

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Lee, Kok-Meng; Committee Member: Allen, Mark; Committee Member: Hesketh, Peter; Committee Member: May, Gary; Committee Member: Sitaraman, Suresh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

This dissertation describes an experimental study on the patterning of thin films and spin valve devices. Initially the change in the magnetisation reversal of ferromagnetic Ni80Fe15Mo5 thin films was investigated as the shape anisotropy was increased using optical lithography to pattern wire arrays. These structures show a progressive increase in coercivity and a transition between single and two-stage reversal with increasing milling depth. A similar patterning technique was applied to unpinned (Ni80Fe20/Cu/Ni80Fe20) pseudo spin valve (PSV) structures in order to enhance the coercivity of one of the ferromagnetic layers. The increased coercivity induced by micropatterning changed the natural similarity of the magnetic layers and the structure exhibited a small spin valve response. These initial measurements were carried out with separate milling and electrical characterisation steps. However, it was decided that it would be ideal to design a technique to do in-situ magnetoresistance measurements during milling. This meant that the samples could be milled and characterised in the same step, leading to a much cleaner and more efficient process. In-situ magnetoresistance measurements were carried out during micropatterning of PSV devices, and the measurements showed the evolution in the electrical response as wire structures were gradually milled through the thickness. Contrary to what was expected, the structures showed a maximum spin valve response when fully milled through. The effect of further increasing the shape anisotropy by reducing the wire width, and changing the material properties in the PSV structure has also been investigated. MR measurements were taken as the temperature was increased from 291K to 493K, and the results show that the patterned PSV structures have a better thermal stability than exchange biased spin valves with an IrMn pinning layer. The experiment was extended to the nanoscale, and the results show that a significant increase in MR is not observed despite the fact that the magnetic configuration tends more towards single domain. This is thought to be due to an increase in the initial resistance of the structures. A small increase in MR was observed as the wire width was decreased from 730 to 470 nm, although the spin valve response is heavily dependent on the gallium dosage density during patterning in the Focused Ion Beam (FIB). Micromagnetic simulations were carried out, which agree with the experimental results and showed the change in the magnetisation reversal from rotation to switching as the dimensions were reduced on the nanoscale.

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.

The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage.   The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase.   Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena.   The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions. / QC 20100819

No description available.

Oxides of manganese have received considerable attention lately, mainly because of the colossal magnetoresistance they exhibit. After a careful interpretation of the large body of available experimental results, the paramount importance of intrinsic inhomogeneities to the understanding of these materials, can no longer be ignored. A scenario, based on the competition between different ordered phases which are mixed by the intrinsic disorder, is proposed. Several quantities that follow from this scenario can be evaluated and are found to correspond to experiments. / Dissertation / PhD

Wong Chung Ki. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 93-94). / Text in English; abstracts in English and Chinese. / Wong Chung Ki. / Abstract --- p.i / Acknowledgement --- p.iii / Contents --- p.iv / List of Figures --- p.viii / List of Tables --- p.xii / Chapter Chapter 1. --- Introduction --- p.1 / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Double exchange model --- p.3 / Chapter 1.3 --- This project --- p.5 / Chapter Chapter 2. --- The generalized double exchange model --- p.6 / Chapter 2.1 --- Assumptions of the model --- p.6 / Chapter 2.2 --- Transfer process of the mobile electrons --- p.7 / Chapter 2.3 --- Double exchange model --- p.9 / Chapter 2.4 --- The generalized double exchange model --- p.10 / Chapter 2.5 --- Previous results on the model --- p.13 / Chapter 2.5.1 --- Solution for two localized spins --- p.13 / Chapter 2.5.2 --- Ground state configuration for one doping electron and the electron concentration at half fill --- p.13 / Chapter 2.5.3 --- The planar spiral state and the canted ferromagnetic state --- p.14 / Chapter Chapter 3. --- Ground state calculations --- p.16 / Chapter 3.1 --- Minimization of ground state energy --- p.17 / Chapter 3.2 --- Energy and spin configuration --- p.18 / Chapter 3.3 --- Occupation number of mobile electrons --- p.19 / Chapter 3.4 --- Rotated occupation number of mobile electrons --- p.19 / Chapter Chapter 4. --- One-dimensional ground state results --- p.21 / Chapter 4.1 --- Zero superexchange coupling --- p.21 / Chapter 4.1.1 --- Total energy of the system --- p.21 / Chapter 4.1.2 --- Spin configuration and nearest neighbor spin-spin correlation --- p.23 / Chapter 4.1.3 --- Occupation number --- p.29 / Chapter 4.2 --- Antiferromagnetic superexchange coupling --- p.34 / Chapter 4.2.1 --- Total energy of the system --- p.35 / Chapter 4.2.2 --- Spin configuration and nearest neighbor spin-spin correlation --- p.36 / Chapter 4.2.3 --- Occupation number --- p.39 / Chapter 4.3 --- Discussions on the one-dimensional results --- p.43 / Chapter Chapter 5. --- Two-dimensional ground state results --- p.46 / Chapter 5.1 --- Zero superexchange coupling --- p.46 / Chapter 5.1.1 --- Total energy of the system --- p.46 / Chapter 5.1.2 --- Localized spin configuration --- p.47 / Chapter 5.1.3 --- Occupation number --- p.51 / Chapter 5.2 --- Nonzero superexchange coupling --- p.54 / Chapter 5.2.1 --- Total energy of the system --- p.54 / Chapter 5.2.2 --- Spin configuration --- p.54 / Chapter 5.2.3 --- Occupation number --- p.57 / Chapter 5.3 --- Discussions --- p.59 / Chapter Chapter 6. --- Finite temperature calculations on the model --- p.62 / Chapter 6.1 --- Quantum Monte Carlo simulation --- p.62 / Chapter 6.2 --- Heat capacity and magnetic susceptibility --- p.63 / Chapter 6.3 --- Localized spin-spin correlation --- p.64 / Chapter Chapter 7. --- Results at finite temperature --- p.65 / Chapter 7.1 --- Two-dimensional results without superexchange interaction --- p.65 / Chapter 7.1.1 --- Fourier transform of the spin-spin correlation --- p.65 / Chapter 7.1.2 --- Occupation number --- p.68 / Chapter 7.2 --- Two-dimensional results with nonzero superexchange interaction --- p.73 / Chapter 7.2.1 --- Fourier transform of the spin-spin correlation --- p.73 / Chapter 7.2.2 --- Occupation number --- p.74 / Chapter 7.3 --- Three-dimensional results with anisotropic exchange interaction --- p.77 / Chapter 7.3.1 --- Fourier transform of the spin-spin correlation --- p.77 / Chapter 7.3.2 --- Occupation number --- p.79 / Chapter 7.3.3 --- Magnetization --- p.81 / Chapter 7.3.4 --- Heat capacity --- p.81 / Chapter 7.3.5 --- Magnetic susceptibility --- p.82 / Chapter 7.3.6 --- Discussions --- p.83 / Chapter Chapter 8. --- Conclusions --- p.85 / Chapter Appendix A. --- Details on Euler rotation of the localized spin --- p.87 / Chapter Appendix B. --- Details on the calculation of the occupation number --- p.89 / Chapter B.1 --- Occupation number --- p.89 / Chapter B.2 --- Rotated occupation number --- p.90 / Chapter Appendix C. --- Fourier transform of the spin-spin correlation at T≠0 --- p.92 / Bibliography