| This dissertation describes several projects related to micromagnetic simulations of magnetization dynamics. In the first part we will describe several improvements implemented in the micromagnetics code that has been developed by our research group. The most time-consuming part of a micromagnetic calculation is the computation of the magnetostatic field at each computational cell due to each of the other cells. Straightforward calculation requires a time of order N2, where N is the number of cells in the system. There are several methods to decrease the calculation time: Ewald summation, Fast Fourier transform and Fast Multipole method (FMM). The last has several advantages over the first two: the computation time is proportional to N and the method does not impose any additional requirements, such as a regular grid of cells, on the system. However, the adoption of the FMM by the micromagnetic community has been slowed because of some problems in its conventional implementation, such as complexity and difficulty in treatment of nearby cells and boundary conditions. This dissertation describes the solution to these problems.;An important characteristic of a recording process is the switching time, which shows how long it takes to write a bit of information. Switching via spin-wave instability is one of the fastest switching mechanisms, which is not fully understood. This part of the dissertation studies this type of switching applied to longitudinal switching, in which the magnetization of the bit lies in the plane of the film, and perpendicular switching, in which the magnetization is perpendicular to the plane of a thin magnetic film. A new visualization scheme, which quantifies the spin-wave instability for each possible wave vector, is introduced and used to study the magnetization dynamics during switching.;The third part of the dissertation deals with a very new and promising type of switching, which occurs when an electric current is passed between two ferromagnetic layers, separated by exchange-break layer. Unless the current is very large, the switching is thermal in nature and takes too long for a conventional micromagnetics simulation. The solution to this problem lies in statistical theory based on Fokker-Planck equation applied to this switching. The switching probabilities, dwell times, and switching currents are calculated and compared to experimental data and single-domain micromagnetic simulation. |
