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978-3-8439-2328-6, Reihe Physik
Stochastic and coherent dynamics of individual magnetic domains and domain walls
186 Seiten, Dissertation Universität Hamburg (2015), Softcover, A5
Recent concepts for prospective memory and logic devices based on magnetic domains and associated domain walls in micro- and nanostructures have strongly motivated the investigation of magnetic relaxation dynamics on microscopic length and time scales. In this thesis, relaxation processes with nontrivial stochastics are studied experimentally for ferromagnetic micro- and nanostructures in a temperature range from liquid-helium to room temperature.
The damping in an individual magnetic domain, as the collective relaxation of a coherently excited canonical ensemble of magnetic moments, is investigated by means of broadband ferromagnetic resonance spectroscopy. From the resonant response of the magnetic system to external stimuli, the temperature dependence of the saturation magnetization is assessed and reveals at low temperatures deviations from the conventional theoretical description, the Bloch T3/2 law. Different relaxation channels are identified, particularly intrinsic and extrinsic damping processes that are quantitatively evaluated as a function of temperature. In principle, an increase of extrinsic damping with a simultaneous decrease of intrinsic damping with decreasing temperatures can be understood in terms of a reshaped energy landscape of the magnetic system, where at low temperatures surface and interface defects are more prominent.
The stochastic depinning of individual domain walls from a structural defect in a ferromagnetic nanowire, as the relaxation of a quasiparticle from a metastable state, is studied by means of ballistic Hall micromagnetometry. The stochastics are discussed in terms of the Néel-Brown model and complementary Markov models, which represent either elementary trivial depinning or elementary nontrivial depinning schemes, respectively. In the experiments and simulations, intermediate pinned states of successive depinning schemes are identified that precede the depinning event and characterize individual depinning paths. Moreover, first experimental approaches to further investigate domain-wall depinning perturbed or assisted by external harmonic excitation have been successfully implemented.