In this thesis, we have analyzed one-dimensional fermion-spin interactions in the dilute limit. The two cases we analyze represent different paradigms. For the first part, we look at the existence of spins for all sites as an effective model to describe the rearrangement of core electrons within the dynamic Hubbard model. Within this model, the behavior of electrons and holes will be compared in the presence of fermion-spin coupling and on-site repulsion. It will be shown that in this framework, electrons and holes behave differently and even though electrons experience increased repulsion, holes show attraction for a range of on-site repulsions. The characteristics of the interaction show effective nearest-neighbor attraction though no such term exists within the model. By the analysis of dynamic properties, two regions of interaction are identified. The gradual change from weak to strong coupling of fermions is presented. The effect of introducing on-site repulsion for both ranges of coupling is presented for both the dynamic Hubbard model and electron-hole symmetric version. For the second case involving fermion-spin interaction, we look at the interaction of a fermion with spins existing only for a small portion of the lattice, representing a coupled magnetic layer that an itinerant fermion interacts with through Heisenberg-like spin flip interaction. The interaction represents a spin-flip interaction of a spin current and magnetic layer. This interaction has been extensively studied for its relevance to computer hard drives both experimentally and theoretically. Most theoretical descriptions utilize the semi-classical Landau-Lifshitz-Gilbert (LLG) formalism. However, with recent improvements in experimental methods with very small magnetic layers and very fast real time measurements, quantum effects become more pronounced. We present quantum mechanical results that show considerable modification to spin-flip interaction. We identify a set of conditions that exhibits the existence of an emerging bound state for the spin current both numerically and analytically. The bound state is a quantum mechanical state and cannot be achieved with a classical picture. We present results in a one-dimensional lattice for a spin-1/2 system, and generalize our arguments to higher dimension and spins with S > 1/2.

Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements. This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, &gt;0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism. / <p>QC 20121119</p>

Graduation date: 1992

Roller gear cam (RGC) mechanisms are widely used in modern industries. While operated at high speeds, the indexing precision of these devices will be influenced by their dynamic characteristics. In this study, the Bond Graph Method is employed to construct a mathematical model to simulate the dynamic responses of RGC mechanisms. Meanwhile, associated experiments will be made to verify the effectiveness of this model. The influences of torque compensation mechanisms and different cam motion curves affecting the dynamic response of a RGC mechanism are observed. And the results show that the torque compensation mechanism is effective to the dynamic performance, and high-speed stability of a RGC mechanism. Besides, the simulated results also indicate that the kinematic properties of cam motion curves have certain influences on the residual vibrations of the system.

HVDC is usually used to link two AC power systems with same or different system frequencies. Nevertheless, even the two AC power systems linked have the same rated system frequency, the actual frequencies of the two AC systems may be different from time to time due to different load conditions. As a result, asynchronous operation occurs to the HVDC system, which leads to a lot of harmonics to be induced. The frequencies of the main harmonic are within several to several tens Hz (i.e. sub-harmonics), which coincide with the turbine resonant frequency range. Therefore, it has the potential of producing the sub-synchronous resonance phenomena. Usually, the sub-synchronous resonance arising from the excitation by the sub-harmonics currents persists only a very short period of time, thereby the induced fatigue loss would not so serious. However, due to the cumulating characteristics, the fatigue loss may reach the dangerous degree if the shafts and blades are persistently subjected to such resonance excitations. According to such a situation, the fatigue life expenditure of the turbine shafts and blades are evaluated in the thesis. It is anticipated that the potential danger of the turbine-generators in conjunction with the operations of HVDC system can be found.

Roller Gear Cam mechanism¡]RGC¡^has been used widely in different automation mechanisms and all kinds of orientation mechanism. High speed and high accuracy of the RGC system is a tendency in high production automation. The interaction between the driving speed and torque of a high speed RGC system is investigated in this work. The effect of adding a torque compensation cam¡]TCC¡^on the improving of indexing precision of a RGC system is investigated in this thesis. The dynamic responses of a RGC system driven by a DC motor are conducted. Dynamic equations of the intermittent- motion of a RGC driven system are derived by using the Lagrange¡¦s equation with the assumption of dual-stiffness. Furthermore, the effect of adding a torque compensation mechanism¡]TCM¡^ such as torque compensation cam¡]TCC¡^ or idle wheel on the improving of indexing precision of a RGC system is investigated in this work. The sixth order Runge-Kutta iteration method is employed in the system¡¦s responses simulation. Variations of the driving torque, driving speed and residual vibration of a RGC system with different torque compensation devices are analyzed in this research. The simulated and measured results indicate that a RGC system attached with a TCC can improve its speed and torque fluctuation at the designed speed significantly. However, this compensation effect is quite sensitive to the driving speed. On the contrary, the compensation effect introduced by using an idle wheel is not so sensitive to the speed as the TCC does. The low cost and easy design are also the favorable factors for using an idle wheel to instead of an expensive TCC device.

In the scope of this thesis spin transfer torque (STT) driven switching and resonances in point contact experiments are investigated. In the first part, the focus is on STT driven switching events in magnetic devices with different tilt of the magnetization with respect to the thin film sample plane. Varying tilt is reached by different magnetic multilayers as Co/Ni and Co/Pt and the e efficiency of STT is compared by measuring the magneto resistance (MR) traces. As expected it was observed that tilting the magnetization of one layer with respect to the other, can improve STT efficiency. This was confirmed by micromagentic simulations using OOMMF. In the second part of this thesis, STT driven resonances in an exchange-biased spin valve (EBSV) were investigated by applying ac (microwave) and dc currents while sweeping the applied magnetic field. The resulting magnetodynamics were observed by measuring the rectified voltage which appears across the sample. To characterize the sample first the well known and understood ferromagnetic resonance (FMR) was excited. After that the power of the applied ac current was increased and a second resonance at a smaller magnetic field could be observed. This resonance structure was investigated and shown to be due to parametric resonance. This non-linear excitation appears in oscillator systems, if one or both parameter (damping, eigen frequency) oscillate in time. In the STT driven resonance experiments, the accurrent causes the damping to oscillate and therefore drives the system into parametric resonance. / text

This research focuses on device design and process integration of high density nonvolatile memory devices. Research was carried out to improve scaling of floating gate memories by increasing charge density as well as spin-based memories by reducing critical switching current. This work demonstrates fabrication of CMOS-compatible nonvolatile hybrid memory device using fullerene molecules as a floating gate. Molecules have dimensions of several Angstroms resulting in an electron density of ~10¹³ cm⁻² or higher. In hybrid MOSCAPs, fullerenes were encapsulated between inorganic oxides, i.e. SiO₂ as a tunnel oxide and HfO₂ as a control oxide. Introduction of a high-k material as a control oxide improves capacitive coupling between control gate and floating gate as well as the program/erase efficiency. The MOS capacitors demonstrate nonvolatile memory operation at room temperature. The device data infers that program/erase mechanism in fullerene devices is Fowler-Nordheim tunneling; however, retention is determined by trap-assisted tunneling. The next part of the work focused on spin-transfer-torque (STT) based magnetic memory. Spin-based memory has the unique potential to be the universal memory because of its high density, fast switching, and nonvolatility. This work presents STT switching of perpendicular magnetic anisotropy (PMA) spin-valves with tilted magnetization using point contact measurement. The PMA materials have high coercivity resulting in good retention and tilted magnetization induces precessional switching resulting in a lower switching current density. First, micromagnetic simulations were performed for spin-valves with tilted magnetization and precessional switching was observed to reduce the switching current. Then, spin-valve structures were fabricated by e-beam evaporation. The structure consisted of Co/Pt and Co/Ni layers, where the thickness of the layers was optimized to obtain different amount of tilt in magnetization. Point contact measurements of tilted spin-valves show STT switching, where the switching field of the free layer varies with the magnitude and sign of the applied current. The observed STT effect is stronger in a 45° tilted spin-valve compared to a 12° tilted device presumably due to the tilted spin polarization. However, tilting introduces nonuniform effective field and canting of the domains which affect the STT. / text

No description available.

Osseointegrated transfemoral implants have been introduced as an alternative to conventional, socket-based prostheses for above knee amputees. This method is showing great promise; however, the frequency of device failure is of concern. As a means to understand some of these failures, this study investigates the mechanical behaviour of three osseointegrated transfemoral implant systems; an in-house prototype design and two assemblies comprised of implant-grade parts provided by Integrum AB, Sweden. A series of mechanical tests were administered to evaluate the effects of tightening specifications and connection geometry on the material response, and mechanical behaviour of the assemblies. The tests included torque-elongation measurements, torque to failure tests, and strain analyses during loading. This research was carried out in an attempt to better understand the behaviour of components under loading, in order to optimize the assembly specifications and improve the useful life of the system. The results obtained confirm that the Integrum assemblies exhibit improved performance when tightened to a level beyond the current specified tightening torque of 12 Nm. Increased tightening torque provides an improved connection between components, leading to increased torque retention, decreased peak tensile strain values and a more gradual, primarily compressive distribution of strains throughout the assembly. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2009-09-29 12:14:18.438