ISBN 9783843927444

978-3-8439-2744-4, Reihe Physik

Max Hänze
Collective oscillations of magnetic vortices

142 Seiten, Dissertation Universität Hamburg (2016), Softcover, A5

Zusammenfassung / Abstract

Tailored ferromagnetic structures on micro- to nanometer length scales are promising candidates for future storage and logic devices based on spin-wave excitations and spin currents. The advances of such spintronic devices reduce the power consumption and overcome the operation speed limit of electric charge based devices. In this thesis the interaction of highly symmetric spin textures in the form of magnetic vortices are investigated. Their magnetization dynamics are studied at their genuine time, spatial, and spectral scales using the combination of two complementary measurement techniques, i.e., scanning transmission X-ray microscopy and ferromagnetic resonance spectroscopy.

The first part of this thesis focuses on the interaction of lateral arrangements of vortex structures. It is shown that in analogy to coupled classical harmonic oscillators, the motions of coupled magnetic vortices can be described by a system of eigenmodes. These eigenmodes depend on the polarizations of the vortices. Based on this dependence, a method is introduced that allows for the reliable manipulation of the polarizations with writing times of about 100\,ns. For increased sizes of the coupled arrangements a dispersion relation is determined. In such magnonic crystals the dispersion can be tuned at will in dependence on the polarizations. The investigated vortex crystals allow for the transmission of wave packets with group velocities of about 100 m/s.

In the second part the coupling of stacked magnetic vortices is investigated. Here, the interaction of the vortex cores yields an additional anharmonic contribution to the collective dynamics. Based on these findings consecutive experiments reveal the collective modes of three-dimensional vortex crystals. Finally, higher modes of excitation are observed in stacked vortices for frequencies up to 12 GHz. These excitations correspond to propagating spin waves. They can be understood by a non-reciprocal dispersion and allow for the manipulation of the relative polarizations on the sub-nanosecond time scale. The access to the third dimension in stacked vortices overcomes the limitations concerning the storage density in potential memory devices.