We present an investigation of two non-equilibrium systems: spatial many-species predator-prey games and systems of interacting magnetic skyrmions. We numerically study two predator-prey systems characterized by nested pattern formations. We first consider a six species game in which spiral patterns spontaneously form within coarsening domains. Through a systematic investigation of relevant correlation functions, the interface width, and other quantities, we show that the non-trivial in-domain dynamics affect the coarsening process and the interfacial properties. The exponents which govern domain growth, aging, and interface fluctuations differ from those expected from curvature driven coarsening. The response to perturbations of the reaction rates is also studied. Furthermore, we introduce a nine species model characterized by nested spiral pattern formations. Quantitative evidence of the existence of two length and time scales associated to the spiral levels is presented in the form of correlation lengths and a temporal Fourier analysis of the species densities. A generalized interaction scheme is proposed for dynamically generated hierarchies. Magnetic skyrmions are particle-like spin configurations found in certain chiral magnets. We study the effect of the Magnus force on the relaxation dynamics through Langevin molecular dynamics simulations. The Magnus force enhances the disorder of the system at high noise strengths while we observe a dynamic regime with slow decaying correlations at low noise strengths. The different regimes are characterized by changes in the aging exponent. In general, the Magnus force accelerates the approach to the steady state. In the presence of quenched disorder, we find that the relaxation dynamics are more robust in systems with a strong Magnus force. We also examine periodically driven skyrmion systems and show that a transition from reversible to irreversible flow exists in the presence of attractive defects. The Magnus force enhances the irreversible regime in this case. The work on predator-prey systems was supported by the U.S. National Science Foundation through Grant No. DMR-1606814 whereas the work on skyrmions was supported by the US Department of Energy, Office of Basic Energy Sciences (DOE-BES), under Grant No. DE-FG02-09ER46613. / Doctor of Philosophy / We present an investigation of two non-equilibrium systems: spatial many-species predator- prey games and systems of interacting magnetic skyrmions. We numerically study two predator-prey systems characterized by nested pattern formations. We first consider a six species game in which spiral patterns spontaneously form within coarsening domains. Through a systematic investigation of relevant correlation functions, the interface width, and other quantities, we show that the non-trivial in-domain dynamics affect the coarsening process and, to a greater extent, properties at the interface between competing groups of species. The exponents which govern domain growth, aging, and interface fluctuations are shown to differ from those expected in typical games of competition. We also study the change of the system due to a perturbation of the reaction rates, which could represent an abrupt change in the environment. Furthermore, we introduce a nine species model characterized by the emergence of nested spiral pattern formations. Quantitative evidence of the existence of two distinct spiral levels is presented. We also propose a generalized interaction scheme for dynamically generated spiral hierarchies. Magnetic skyrmions are particle-like spin configurations found in certain chiral magnets. We study the effect of the Magnus force on the dynamic properties of skyrmion systems through particle-based simulations. The Magnus force enhances the disorder of the system at high noise strengths while accelerating the formation of the triangular lattice at low noise strengths. We find that, in general, the Magnus force accelerates the approach to the steady state. In the presence of randomly placed attractive pinning sites, we find that a strong Magnus force can prevent caging effects and allow skyrmions to more easily move around pinning sites. We also examine periodically driven skyrmion systems and show that a transition from reversible to irreversible flow exists in the presence of attractive defects. The Magnus force is shown to enhance the irreversible regime in this case. The work on predator-prey systems was supported by the U.S. National Science Foundation through Grant No. DMR-1606814 whereas the work on skyrmions was supported by the US Department of Energy, Office of Basic Energy Sciences (DOE-BES), under Grant No. DE-FG02-09ER46613.

A new tetragonal Heusler compound Mn1.4PtSn is synthesized. Crystal growth techniques that require growth directly from melt, such as Bridgman method, always result in microtwinned crystals. To get microtwin free crystals, another technique, flux method is employed, where growth can be done far below the melting point and martensitic transition temperature. The flux method results in successful large microtwin free crystals of Mn1.4PtSn. The single-crystal diffraction is done on a small piece of single crystals of Mn1.4PtSn. From structural analysis, it is found out that the crystal structure of Mn1.4PtSn is the first tetragonal superstructure in the family of Heusler compounds. The superstructure reflections are clearly observed in the powder X-ray diffraction patterns. Direction-dependent magnetic properties are measured. The compound is found to undergo two magnetic transitions. First, at 392 K, which corresponds to Curie temperature and second, at 170 K, which corresponds to the spin-reorientation transition temperature. The saturation magnetic moment at 2 K is very large of 4.7 µB/f.u. The refinement of powder neutron diffraction reveals that in the temperature range of 170 to 392 K, the magnetic structure is collinear ferromagnet whereas below 170 K, it is a non-coplanar spin structure. The magnetic moment, obtained from refinement, is close to the saturation moment obtained from magnetization. The electric transport properties are studied along the different crystallographic directions of the compound. The longitudinal resistivity measurement indicates that the compound is metallic and reveals the magnetic transitions at the same temperature as seen in the magnetization. An overall negative magnetoresistance of 3 percent is found. The Hall resistivity measurements reveal the presence of a large topological Hall resistivity (THE) of 0.9 µΩ cm and -0.1 µΩ cm for the magnetic field applied along [100] and [001], respectively. Two types of contributions in the THE for the field along [100] are seen. One that follows the quadratic form of longitudinal resistivity and second, that is independent of longitudinal resistivity. Anomalous Hall conductivity is found to be 250 and 165 Ω-1cm-1 for the field along [100] and [001], respectively. This value is close to the value obtained from theoretical calculations. The topological Hall conductivity is found to be approximately the same as its anomalous analog. A new series of polycrystalline samples with iridium substitution at the place of platinum in Mn1.4PtSn are prepared. The structural characterization show the crystal structure of these compounds is the same as Mn1.4PtSn, therefore, they also possess the tetragonal superstructure form. Magnetic properties, along with powder neutron diffraction data, reveal that the magnetic structure changes from out-of-plane ferromagnet to in-plane ferrimagnet with Ir-substitution. All the compounds are found to have metallic character. A large anomalous Hall conductivity of 405 Ω-1cm-1 is found for compound Mn1.4Pt0.7Ir0.3Sn. Three new series of compounds are prepared as an attempt to fill the vacancies present in the crystal structure of Mn1.4PtSn with transition-metal elements cobalt, nickel, and copper. The tetragonal superstructure survives up to 0.2 cobalt addition, 0.4 nickel addition and 0.6 copper addition. Further addition of elements leads to transformation to the inverse cubic Heusler structure. The magnetic properties show that the compounds with tetragonal structure have spin-reorientation transition, which is absent in the compounds with cubic structure. A new compound Mn1.7Pt0.8In is discovered. The single crystals are prepared by flux-method. Upon structural analysis from single-crystal refinement, it is found that the crystal structure is 3 × 3 × 3 superstructure of a Heusler structure and is so far the largest discovered in the Heusler family of compounds. Two magnetic transitions are revealed in the magnetization measurements. First, at 330 K, which corresponds to Curie temperature and second, at 220 K, which corresponds to spin-reorientation transition. The magnetic moment is 0.4 µB/Mn at 2 K and 0.07 µB/Mn at 300 K. Such a low moment might be due to possible compensated ferrimagnetic structure. Therefore, the compound is a potential candidate for spintronics devices.

<div> <div> <div> <p>Process variations and increasing leakage current are major challenges toward memory realization in deeply-scaled CMOS devices. Spintronic devices recently emerged as one of the leading candidates for future information storage due to its potential for non-volatility, high speed, low power and good endurance. In this thesis, we start with the basic concepts and applications of three spintronic devices, namely spin or- bit torque (SOT) based spin-valves, SOT-based magnetic tunnel junctions and the magnetic skyrmion (MS) for both logic and machine learning hardware. </p> <p>We propose a new Spin-Orbit Torque based Domino-style Spin Logic (SOT-DSL) that operates in a sequence of Preset and Evaluation modes of operations. During the preset mode, the output magnet is clocked to its hard-axis using spin Hall effect. In the evaluation mode, the clocked output magnet is switched by a spin current from the preceding stage. The nano-magnets in SOT-DSL are always driven by orthogonal spins rather than collinear spins, which in turn eliminates the incubation delay and allows fast magnetization switching. Based on our simulation results, SOT-DSL shows up to 50% improvement in energy consumption compared to All-Spin Logic. Moreover, SOT-DSL relaxes the requirement for buffer insertion between long spin channels, and significantly lowers the design complexity. This dissertation also covers two applications using MS as information carriers. MS has been shown to possess several advantages in terms of unprecedented stability, ultra-low depinning current density, and compact size. </p><p><br></p><p>We propose a multi-bit MS cell with appropriate peripheral circuits. A systematic device-circuit-architecture co-design is performed to evaluate the feasibility of using MS-based memory as last-level caches for general purpose processors. To further establish the viability of skyrmions for other applications, a deep spiking neural network (SNN) architecture where computation units are realized by MS-based devices is also proposed. We develop device architectures and models suitable for neurons and synapses, provide device-to-system level analysis for the design of an All-Spin Spiking Neural Network based on skyrmionic devices, and demonstrate its efficiency over a corresponding CMOS implementation.</p> <div> <div> <div> <p><br></p><p>Apart from the aforementioned applications such as memory storage elements or logic operation, this research also focuses on the implementation of spin-based device to solve combinatorial optimization problems. Finding an efficient computing method to solve these problems has been researched extensively. The computational cost for such optimization problems exponentially increases with the number of variables using traditional von-Neumann architecture. Ising model, on the other hand, has been proposed as a more suitable computation paradigm for its simple architecture and inherent ability to efficiently solve combinatorial optimization problems. In this work, SHE-MTJs are used as a stochastic switching bit to solve these problems based on the Ising model. We also design an unique approach to map bi-prime factorization problem to our proposed device-circuit configuration. By solving coupled Landau- Lifshitz-Gilbert equations, we demonstrate that our coupling network can factorize up to 16-bit binary numbers. </p> </div> </div> </div> </div> </div> </div>