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ISBN 978-3-8439-0395-0

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978-3-8439-0395-0, Reihe Physik

Andreas Vogel
Magnetization dynamics in coupled ferromagnetic micro- and nanostructures

156 Seiten, Dissertation Universität Hamburg (2012), Softcover, A5

Zusammenfassung / Abstract

The magnetization dynamics on the nanosecond and subnanosecond time scale in coupled ferromagnetic micro- and nanostructures are investigated in frequency space and in real space. Broadband ferromagnetic-resonance (FMR) measurements using a vector-network analyzer with a frequency range from 10 MHz to 26.5 GHz and magnetic transmission x-ray microscopy with a spatial resolution of typically 25 nm and a temporal resolution below 100 ps are applied. A focus is on the understanding of the stray-field mediated coupling between neighboring structures and its influence on the magnetization dynamics. The gyroscopic motion of vortices in pairs, chains, and arrays of coupled disk-shaped and square-shaped structures is investigated. FMR transmission spectra of arrays reveal a dependence of the vortex-resonance frequency on the inverse sixth power of the normalized center-to-center distance between the elements and on the size of the array. Using x-ray microscopy, the trajectories of coupled vortex gyrations in pairs are directly observed in real space. The interacting vortices are shown to behave like coupled harmonic oscillators and distinct normal modes for an in-phase and antiphase motion of the vortex oscillators are identified. The transfer of an excitation signal along a chain of up to five vortices is demonstrated and a chain of three vortices is switched back and forth between a transmitting and a locking state. Besides elements in the vortex state, pairs of single-domain rectangles are investigated where the dipolar coupling is likewise shown to have an influence on the FMR mode. The cone angle of the magnetization precession is determined and a measurement setup is established which allows to simultaneously perform FMR and transport measurements. In the last part, the suitability of a concept to create well-defined confining potentials for domain walls between opposing magnetic domains in nanowires via the local modification of magnetic properties without geometric constrictions is verified. Implantation of chromium ions is used to reduce the saturation magnetization. Field-driven pinning and depinning at the so-called magnetic soft spots is observed using x-ray microscopy and the confining potential is shown to be tunable via the chromium ion fluence applied to induce the soft spots. The shape of the potential is characterized via micromagnetic simulations and electrical measurements. Reliable depinning via single current pulses is demonstrated.