Dynamics of Magnetic Vortices in Soft Ferromagnetic Nanodisks driven by Spin-polarized Currents: Micromagnetic Numerical and Analytical Calculations

Abstract

This thesis deals with dynamics of magnetic vortices in soft ferromagnetic nanodisk driven by spin-polarized currents with micromagnetic simulations and analytical calculations. For the numerical calculations, the Landau-Lifshitz-Gilbert equation is solved by the Object Oriented Micromagnetic Framework (OOMMF) and the LLG Micromagnetics Simulator. For the analytical calculations, Thieles equation of motion in a single vortex model is used. We observed sizable eigenfrequency shifts in spin-polarized dc-current-driven vortex gyrotropic motions in a soft magnetic nanodot, and clarified the underlying physics through micromagnetic numerical calculations. It was found that the vortex eigenfrequency is changed to higher (lower) values with increasing Oersted field strength associated with the out-of-plane dc current for the vortex chirality parallel (antiparallel) to the rotation sense of the Oersted field circumferential in-plane orientation. We studied magnetic vortex oscillations associated with vortex gyrotropic motion driven by spin-polarized out-of-plane dc current by analytical and micromagnetic numerical calculations. Reliable controls of the tunable eigenfrequency and orbital amplitude of persistent vortex oscillations were demonstrated. This work provides an advanced step toward the practical application of vortex oscillations to persistent vortex oscillators in a wide frequency range of 10–2000 MHz and with high q factor. Finally, we reported on an observation of transitions of the fourfold degenerate state of a magnetic vortex in soft magnetic nanodots by micromagnetic numerical calculations. The quaternary vortex states in patterned magnetic dots were found to be controllable by changing the density of out-of-plane dc or pulse currents applied to the dots. Each vortex state can be switched to any of the other states by applying different sequence combinations of individual single-step pulse currents. Each step pulse has a characteristic threshold current density and direction. This work offers a promising way for manipulating both the polarization and chirality of magnetic vortices.