ISBN 978-3-8439-2615-7

978-3-8439-2615-7, Reihe Physik

Christian F. Adolff
Self-organized state formation in magnonic vortex crystals

116 Seiten, Dissertation Universität Hamburg (2015), Softcover, A5

Zusammenfassung / Abstract

Complexity created by periodic arrangement of well-understood building blocks plays an important role in biochemistry, photonics, engineering and nanoelectrics. The periodic arrangement of atoms or molecules as basis determines the physical properties of crystals. With the flexibility of nanometer precise electron-beam lithography here magnetic interactions are engineered yielding two-dimensional magnonic crystals that benefit from the magnetic vortex core as crystal basis. Using scanning transmission X-ray microscopy at the MAXYMUS beamline at BESSY II in Berlin, Germany the magnonic crystal dynamics are imaged with time resolution in the sub-nanosecond regime and simultaneous spatial resolution on the nanometer scale. Self-organized vortex core state formation by adiabatic reduction of a high frequency magnetic field excitation is observed. The emerging polarization states are shown to depend on the frequency of excitation and the strength of the dipolar interaction between the elements. In spite of the complexity of the investigated system, global order caused by local interactions creates polarization states with a high degree of symmetry. An analytical dipole model and numerically solved coupled equations of motion are adopted to analytically describe the experimental results. The emerging states can be predicted by a fundamental stability criterion based on the excitability of eigenmodes in the crystal. Further experiments with ferromagnetic absorption spectroscopy are carried out that give insight into the characteristic frequencies of the vortex dynamics that are crucially influenced by the self-organized state formation. This is emphasized with experiments on benzene-like magnetic vortex molecules whose motions show strong similarities to the vibrational modes of the actual benzene molecule (C6H6). The symmetry of both systems determines the motions of the oscillators, i.e., the carbon atoms or the magnetic vortices. This allows to simplify the derivation of the fundamentally different dispersion relations depending on the previously tuned polarization state. The experiments confirm the calculations and prove that the magnetic vortex molecule features a reprogrammable band structure or dispersion relation.

Consequently, this work allows further research studies to tailor the characteristic properties of various magnetic vortex arrangements by tuning the polarization state.