Micromagnetic modeling and demonstration of wide bandwidth and ultralow power skyrmion-based spintronic devices and circuits

The scaling of microelectronics faces two major issues: memory bottleneck and power consumption. These issues prompted the proposal of in-memory computation that encodes information in spins. Using spins allows for nonvolatile logic and data manipulation functionalities. Skyrmions, which are nanoscale, topologically protected, chiral and surface spin textures, could be used for these functions. Skyrmions can be driven by charge or spin currents and have stability against stray magnetic fields and thermal noise. Using magnetic films that stabilize skyrmions might reduce the power consumption with respect to conventional microelectronics by several orders of magnitude. Despite the progress in experimental demonstrations and theoretical studies on skyrmion initialization, detection, and manipulation, using skyrmions for their promising digital logic applications has remained elusive. A set of new device designs based on skyrmion logic processing, and the investigation of their device physics is necessary. In this thesis, we designed a comprehensive skyrmion logic gate system, which includes a skyrmion clock generator and the essential connecting blocks including the duplicator, junction, and deflector. Then, all the logic gates AND, OR, inverter, NAND, NOR, XOR, and XNOR gates have been designed and their functionalities have been verified using computational micromagnetics. Each block has been investigated thoroughly in terms of their energy consumptions, Joule heating, temporal delay and transient response, bandwidth, cascadability, stability against thermal noise and external magnetic fields as well as functional sensitivity with respect to the geometric (sidewall roughness, notch size/feature, channel width), material (Gilbert damping, non-adiabaticity of spin transfer torques) and temperature variations. Skyrmions need to be generated and propagated using charge currents for integrated ultra-wideband spintronics. We introduce a device design for initialization and generation of periodic skyrmions from 114 MHz to 21 GHz using spin-polarized direct current. We demonstrate in micromagnetic simulations that skyrmion generation frequencies can be controlled reversibly over more than seven octaves of frequencies by changing DC current density. Thus, a non-volatile current-driven digital clock source has been established. The inverter can be driven with spin polarized current pulses, operate with wide bandwidth, low energy consumption (~1350 kBT/bit at room temperature), small footprint (~300 nm), no or very limited need for external magnetic fields, cascadability and room temperature thermal stability owing to substrate's thermal conduction. Using magnetic insulators to eliminate Joule heating hints that the power consumption could be even further reduced by 3-4 orders of magnitude. These analyses suggest that the inverter block could be cascaded and operated as part of digital spintronic circuits without loading or thermal drift effects. We presented a bit-slice circuit for a digital skyrmion full adder. This circuit could in principle be scaled to larger numbers of bits. As the skyrmion logic circuit designs might be integrated to electronic design automation workflows, a new protocol for the emulation of skyrmion signals with a tiled geometry is devised. The study presents a new comprehensive emulation and simulation software, featuring different elements of micromagnetic modeling, a Python package for automated simulation script development, and a library of block sets. A user-friendly web-based version of the emulator has been developed. The findings presented in this study suggest that skyrmionics has reached a milestone enabling a new area of ultralow power and ultra-wideband digital skyrmionics for in-memory computation.