Magnetic vortex dynamics: non-linear dynamics, pinning mechanisms, and dimensionality crossover.

Abstract

The dynamics of a magnetic vortex, which is the simplest realization of a domain structure, are influenced profoundly by non-linear effects at both large and small amplitudes. For example, a strongly driven magnetic vortex is unstable with respect to internal deformation, leading to reversal of its core magnetization. At small amplitudes, a second class of non-linear phenomena are associated with pinning of the vortex core. The pinning of magnetic vortices is closely related to the pinning of domain walls in ferromagnetic films. For both cases, however, the absence of an appropriate characterization tool has limited the ability to correlate the physical and magnetic microstructures of ferromagnetic films with specific pinning mechanisms. Given this range of phenomena, there is also an acute need for a global picture of vortex dynamics over a wide range of excitation amplitudes and frequencies. In this dissertation, I show a global phase diagram of vortex dynamics in permalloy (Ni80Fe20) disks by probing the response spectrum over four orders of magnitude in excitation power. A clear boundary separates pinned and unpinned dynamics in a phase space of amplitude and frequency. I also discuss a highly quantitative analysis of the pinning potential for defects, and how it can be used to trace the dynamics of a single vortex from deep in the pinning regime to the onset of core reversal. Regarding the pinning mechanism, I show that the pinning of a magnetic vortex is strongly correlated with surface roughness, and I make a quantitative comparison of the pinning energy and spatial range in films of various thickness. The results demonstrate that thickness fluctuations on the lateral length scale of the vortex core diameter, i.e., an effective roughness at a specific length scale, provide the dominant pinning mechanism. I argue that this mechanism will be important in virtually any soft ferromagnetic film. Finally, I show the dynamics of a magnetic vortex cross over from two-dimensional (2D) to three-dimensional (3D) with increasing disk thickness. A 2D mode of the vortex dynamics is the lowest frequency excitation below the crossover region, above which a 3D mode becomes the lowest frequency excitation.