Successful PhD defense of Björn Niezielski

Congratulations to Björn Niedzielski!

November 28, 2023

Title: “Vortex states in superconducting/ferromagnetic hybrid systems

Superconductivity and ferromagnetism are macroscopic ordering phenomena that exclude each other under most conditions. On the other hand, the antagonistic behavior of the two phenomena can give rise to new and unexplored effects in artificially coupled superconductor/ferromagnet (SC/FM) hybrid systems.

This theoretical study focuses on the dynamics of superconducting vortices in coupled SC/FM bilayer systems of mesoscopic size. The behavior of such vortex states was investigated by solving the time dependent Ginzburg-Landau equations of superconductivity together with the Landau-Lifschitz-Gilbert equation of micromagnetism. Specifically we addressed four problems.

In the first research project we investigated the nucleation of superconducting vortex-antivortex pairs in electromagnetically coupled FM/SC bilayers. Furthermore, we studied the effect of external magnetic fields and geometric confinement on the resulting vortex systems. Thereby we found that the vorticity of the hybrid system can effectively be controlled with a small external magnetic field. In the second research project it was investigated how interfacial Rashba spin-orbit-coupling affects the vortex dynamics in a proximity-coupled SC/multiferroic bilayer. Here the magnetoelectric coupling between a superconducting thin film and a multiferroic oxide was used to generate supercurrents of different distribution and magnitude. Specifically it was shown how changes in the spin-system of the multiferroic can either stop or enhance the vortex motion in the SC layer. The third research problem considered how superconducting vortices react to a temperature gradient. We found that a local heating of the material is able to attract and confine single vortices. Furthermore, it was found that the origin of this attraction lies within a nonzero divergence of the supercurrent density driving the vortex like a conventional transport current. The last research project investigated how superconducting vortices can be utilized to build a magnonic crystal. Thereby it was found that the periodic stray field of a superconducting vortex lattice is able to modify the spectrum of magnonic excitations in a nearby magnetic wave guide. The results were confirmed to be in qualitative agreement with recent experiments on magnon-fluxon interaction. Furthermore, we investigated how the geometry of the wave guide and imperfections in the vortex lattice affect magnon propagation in the wave guide.

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