Thesis (Ph. D., Electrical Engineering)--University of Idaho, May 2007. / Major professor: Gregory W. Donohue. Includes bibliographical references (leaves 145-160). Also available online (PDF file) by subscription or by purchasing the individual file.

Thesis (M.S.)--Florida State University, 2005. / Advisor: Dr. Kartik Gopalan, Florida State University, College of Arts and Sciences, Dept. of Computer Science. Title and description from dissertation home page (viewed June 8, 2005). Document formatted into pages; contains ix, 41pages. Includes bibliographical references.

Although more sophisticated designs of associative memories are not yet economically practical, with the dynamic advances in integrated circuitry currently taking place, the day appears not long off for an economical sophisticated associative memory to become a reality. This describes a general outline of a sophisticated DLM, but it also describes the actual logic involved in a building a working model. The design process involves formulating a set of commands sufficient to perform the desired algorithms, developing the logic necessary to implement these commands, and finally constructing a working model to test the logic.

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / Includes bibliographical references. / M.S.

Bibliography: p. 76-77. / Title page reads: Final report ESL-FR-361. / NASA Research Grant NSG-496. M.I.T. Project DSR 76152.

Thesis (M.S.)--Boise State University, 2009. / Includes abstract. Includes bibliographical references (leaves 124-131).

Thesis (M.S.)--Boise State University, 2009. / Title from t.p. of PDF file (viewed June 1, 2010). Includes abstract. Includes bibliographical references (leaves 124-131).

Research in future generation computing is focused on reducing energy dissipation while maintaining the switching speed in a binary operation to continue the current trend of increasing transistor-density according to Moore’s law. Unlike charge-based CMOS technology, spin-based nanomagnetic technology, based on switching bistable magnetization of single domain shape-anisotropic nanomagnets, has the potential to achieve ultralow energy dissipation due to the fact that no charge motion is directly involved in switching. However, switching of magnetization has not been any less dissipative than switching transistors because most magnet switching schemes involve generating a current to produce a magnetic field, or spin transfer torque or domain wall motion to switch magnetization. Current-induced switching invariably dissipates an exorbitant amount of energy in the switching circuit that nullifies any energy advantage that a magnet may have over a transistor. Magnetoelastic switching (switching the magnetization of a magnetostrictive magnet with voltage generated stress) is an unusual switching paradigm where the dissipation turns out to be merely few hundred kT per switching event – several orders of magnitude less than that encountered in current-based switching. A fundamental obstacle, though, is to deterministically switch the magnetization of a nanomagnet between two stable states that are mutually anti-parallel with stress alone. In this work, I have investigated ways to mitigate this problem. One popular approach to flip the magnetizations of a nanomagnet is to pass a spin polarized current through it that transfers spin angular moment from the current to the electrons in the magnet, thereby switching their spins and ultimately the magnet’s magnetization. This approach – known as spin transfer torque (STT) – is very dissipative because of the enormous current densities needed to switch magnets, We, therefore, devised a mixed mode technique to switch magnetization with a combination of STT and stress to gain both energy efficiency from stress and deterministic 180o switching from STT. This approach reduces the total energy dissipation by roughly one order of magnitude. We then extended this idea to find a way to deterministically flip magnetization with stress alone. Sequentially applying stresses along two skewed axes, a complete 180o switching can be achieved. These results have been verified with stochastic Landau-Lifshitz-Gilbert simulation in the presence of thermal noise. The 180o switching makes it possible to develop a genre of magneto-elastic memory where bits are written entirely with voltage generated stress with no current flow. They are extremely energy-efficient. In addition to memory devices, a universal NAND logic device has been proposed which satisfies all the essential characteristics of a Boolean logic gate. It is non-volatile unlike transistor based logic gates in the sense that that gate can process binary inputs and store the output (result) in the magnetization states of magnets, thereby doubling as both logic and memory. Such dual role elements can spawn non-traditional non-von-Neumann architectures without the processor and memory partition that reduces energy efficiency and introduces additional errors. A bit comparator is also designed, which happens to be all straintronic, yet reconfigurable. Moreover, a straintronic spin neuron is designed for neural computing architecture that dissipates orders of magnitude less energy than its CMOS based counterparts. Finally, an experiment has been performed to demonstrate a complete 180o switching of magnetization in a shape anisotropic magnetostrictive Co nanomagnet using voltage generated stress. The device is synthesized with nano-fabrication techniques namely electron beam lithography, electron beam evaporation, and lift off. The experimental results vindicate our proposal of applying sequential stress along two skewed axes to reverse magnetization with stress and therefore, provide a firm footing to magneto-elastic memory technology.

Le sujet de cette thèse concerne la dynamique auto-entretenue excitée par transfert de spin de vortex couplés, dans des structures de type nano-piliers vannes de spin (Py/Cu/Py). Un premier objectif a été de comprendre les processus de transport polarisé en spin et de transfert de spin associés à des configurations d’aimantation fortement non-homogènes. Cette étude a permis d‘identifier et ainsi de précisément contrôler les configurations magnétiques à base de vortex, et en particulier d’observer l’influence du transfert de spin sur les mécanismes de renversement du cœur de vortex. En combinant des calculs analytiques et des simulations micro-magnétiques, nous avons également pu déterminer les conditions sur les paramètres relatifs des deux vortex (chiralités et polarités) pour obtenir des oscillations gyrotropiques couplées auto-entretenues de deux vortex dans un pilier unique. Un cas très intéressant est prévu pour les piliers de plus grands diamètres (typiquement supérieurs à 200nm) pour lesquels le courant critique est réduit potentiellement à zéro. Les résultats expérimentaux confirment les prédictions sur l’existence d’une dynamique couplée de vortex, avec des largeurs de raies atteignant 200kHz, un record à champ nul (soit un facteur de qualité Q ≈ 5000, un ordre de grandeur plus grand que pour les auto-oscillations de vortex unique) et diminuant même jusqu’à 50kHz sous champ extérieur. Un second objectif de ce travail a été l’étude de la synchronisation de deux auto-oscillateurs à transfert de spin à base de vortex. Nous avons démontré que le verrouillage des phases par couplage dipolaire de deux oscillateurs identiques peut être théoriquement obtenu indépendamment des paramètres des deux vortex. Toutefois un couplage trois fois plus important est prévu dans le cas de vortex de polarités opposées. Du point de vue expérimental, des premiers résultats ont permis de démontrer une faculté de synchronisation de deux oscillateurs présentant un écart en fréquence atteignant jusqu'à 10% de leurs fréquences d'auto-oscillation. Ce travail de thèse, qui s’inscrit dans l’effort de recherche mené pour améliorer les performances rf des nano-oscillateurs à transfert de spin, a permis d’illustrer que l’excitation de modes d’aimantations couplées est une voie à poursuivre dans le but d’aboutir à des largeurs de raies de plus en plus faibles. / My PhD work is dedicated to the spin transfer induced self-sustained dynamics of two coupled vortices, in nano-pillars spin-valves structures (Py/Cu/Py). A first objective was to understand the spin-polarized transport processes as well as spin transfer mechanisms associated to highly non-homogeneous magnetic configurations. This study allows me to identify and then precisely tune the vortex based magnetic configurations, and notably to observe the influence of spin transfer on reversal mechanisms of the vortex core. Combining analytical calculations and micro-magnetic simulations, we determine the conditions on relative parameters for the two vortices (chiralities and polarities) necessary to obtain self-sustained gyrotropic oscillations of the coupled vortices in a single pillar. A very interesting case is predicted for the pillars with larger diameters (typically over 200nm) for which the critical current is reduced to zero. The experimental results confirm the predictions that a coupled dynamics exists with linewidths as narrow as 200kHz, that is a record at zero field (corresponding to a quality factor Q ≈ 5000, an order of magnitude over the self-sustained oscillations of a single vortex), and even down to 50kHz under external field.A second objective was to investigate the synchronization of two vortex based spin transfer oscillators. We demonstrate theoretically that the phase locking through dipolar coupling of two identical oscillators can be achieved for any parameters of the two vortex. However, the coupling is three times stronger when vortices have opposite core polarities. From an experimental point of view, the synchronization capability for two oscillators having a frequency mismatch reaching up to 10 % of the auto-oscillation frequency has been demonstrated. This work, being part of the research effort made to improve the rf properties of spin transfer nano-oscillators emphasizes how the excitation of coupled magnetizations modes is important to reach lower and lower linewidths.

L'étude du déplacement des parois de domaines magnétiques au moyen d'un courant électrique, par couple de transfert de spin, a généré beaucoup d'intérêt ces dernières années, notamment depuis que de nouveaux dispositifs de mémoires magnétiques utilisant cet effet ont été proposés. Récemment, un nouveau mécanisme capable de propager les parois sous courant avec une grande efficacité a été mis en évidence dans les matériaux tri-couches à anisotropie perpendiculaire et à fort couplage spin-orbite. La compréhension de ce mécanisme, appelé couple de spin-orbite, reste néanmoins loin d'être acquise, tout comme son effet sur la propagation des parois de domaines.L'objectif de ce travail de thèse était d'étudier l'influence de ce couple de spin-orbite sur la dynamique des parois. Pour cela, j'ai étudié expérimentalement le déplacement de paroi sous l'action d'un courant et d'un champ magnétique dans une tri-couche de Pt/Co/AlOx en présence d'un champ magnétique planaire, utilisé pour modifier la structure interne de la paroi et ainsi moduler l'action du couple de spin-orbite sur la dynamique de celle-ci. Ce travail a permis de mettre en évidence l'existence d'un effet asymétrique dans la dynamique de la paroi pour ce type de système.Pour expliquer ce résultat, nous avons proposé une nouvelle structure de paroi dans les matériaux ultra-minces à anisotropie perpendiculaire, résultant de l'interaction Dzyaloshinskii-Moriya. En combinant des calculs analytiques et des simulations micro-magnétiques, la dynamique d'une telle paroi a été étudiée et comparée aux résultats expérimentaux. Enfin, toujours dans le but d'expliquer l'effet asymétrique observé expérimentalement, une seconde interprétation basée sur la présence d'un mécanisme d'amortissement anisotrope a également été proposée. / The study of current-induced magnetic domain wall motion through spin transfer torque has attracted a lot of attention in recent years, especially since new magnetic memories devices based on this effect have been proposed. Recently, a new mechanism allowing for highly efficient current-induced domain wall motion has been discovered in ultrathin asymmetric materials with perpendicular magnetic anisotropy and high spin-orbit coupling. However this mechanism, named spin-orbit torque, and its effect on domain wall motion are not yet well understood.The objective of this work was to study the influence of this spin-orbit torque on domain wall motion. For that, I have studied field- and current-induced domain wall motion in Pt/Co/AlOx trilayer, in the presence of an in-plane magnetic field. This work allowed highlighting the existence of an asymmetric effect in the domain-wall dynamics of this system.In order to explain this result, we have proposed a new kind of domain wall structure, resulting from Dzyaloshinskii-Moriya interaction in materials with perpendicular magnetic anisotropy and high spin-orbit coupling. Using analytic calculations and micro-magnetic simulations, this domain wall dynamics has been studied and compared to the experimental results. Finally, a second approach based on the presence of an anisotropic damping mechanism has also been proposed to explain the asymmetric effect observed experimentally.