UC San Diego

Nanoscale magnetic materials and devices are at the heart of memory and recording technologies ranging from magnetic hard drives to spintronic devices, such as magnetic random access memory (MRAM) and spin transfer torque oscillators. Advanced development of these technologies requires comprehensive computational tools. This dissertation presents a theoretical and micromagnetic study of challenges faced when considering interactions between applied fields and spin-polarized currents with nanomagnetic materials. The study is about solving the generally non-linear Landau-Lifshitz-Gilbert (LLG) equation using its linearized version. The approaches include using a linearized eigenvalue framework, solving a source-excited linearized LLG equation, and using a harmonic balance approach for the study of the higher-harmonic generation in weakly-nonlinear magnetization dynamics problems. The dissertation starts with an introduction to micromagnetics and modeling of spin-torque-driven devices. The following chapters present the eigenvalue based micromagnetic framework for spin-torque-driven devices. It presents an analysis related to the MRAM switching properties, including the critical current, switching time, and magnetization time evolution. It also introduces an optimization approach based on the eigenvalue analysis to reduce the critical current in MRAM. It then extends the eigenvalue analysis to the Fokker-Planck equation framework to study of non-switching probability, namely write error rate, under finite temperature. Next, the dissertation presents a solver for the linearized LLG equation with under time-harmonic applied fields, describing its formulation, numerical implementation, results, and analysis. The linearized LLG equation solver is finally extended to create a harmonic balance solver, which represents the solution as a set of multiple frequency components with an iterative process, which allows computing the excitation coefficients of these components. All the codes are developed in the finite element method framework, which is flexible in handling complex materials and devices, and it is integrated with the high-performance micromagnetic FastMag framework.