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
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.
University of Minnesota Ph.D. dissertation. January 2012. Major: Physics. Advisor: Paul A. Crowell. 1 computer file (PDF); ix, 113 pages, appendices A-C.
Magnetic vortex dynamics: non-linear dynamics, pinning mechanisms, and dimensionality crossover..
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