Bondarenko A. Nonlinear dynamics of strongly-bound magnetic vortex pairs

This work is concerned with investigation of nonlinear phenomena in vertically stacked pairs of magnetic vortices. Exact form of response to circularly-polarized electromagnetic field was obtained for arbitrary strength of the excitation field. It is shown that this nonlinear frequency response exhibits a foldover and an isolated resonance which explains sudden appearance of a response on a frequency of the mode which is not excited because of the stack symmetry. Intrinsic bistability of the structure was discovered and researched as a candidate for being a non-volatile memory register. The rates of thermal switching were investigated to find the optimum operating DC bias conditions. Parameters of the short pulses for the switching between states were considered, over 90 % switching probabilities were achieved on real samples with pulses only few nanoseconds long. Extreme sensitivity of the system to presence of defects in magnetic films was demonstrated. Model was built to describe the changes in dynamical characteristics of the vortex pair while pined on a defect. The defects are considered to plausibly come from either the class of film parameter inhomogenеities or small geometry changes. The possibility to use the model for characterization of defects with indestructive, simple measurements in real samples was shown on experiment.