Computersimulationen zur Dynamik und Thermodynamik von Domänenwänden in nanostrukturierten Ferromagneten
The observation of magnetic nanostructures is a highly topical field of research in recent years. Due to new developments regarding their controlled fabrication and characterization these structures play an important role for basic research as well as for applications in the area of information technology. The focus of prior theoretical observations was mainly on the description of domain structures and domain dynamics within such nanostructures. Effects of a finite temperature were here usually neglected. Within this dissertataion the influence of the temperature will be investigated for the case of domain walls. It will be shown, that a finite temperature leads to novel and interesting effects, like new domain wall shapes or the super-paramagnetic behaviour of a vortex core. It was not before recently that these kinds of theoretical investigations were made possible with the development of new numerical techniques. These are, e.g., the numerical solution of the Landau-Lifshitz-Gilbert equation with Langevin dynamics, the heat bath Monte Carlo simulation with quantified time step, the implementation of the fast Fourier transformation method to implement long-range dipolar interactions, the local mean field method, and Green's function methods. In this work, all the above methods will be used to describe the dynamics and thermodynamics of domain wall structures. For the case of domain walls dynamics it will be shown that all of the three equations for domain wall velocities which can be found in the literature (the equations after Walker, after Slonczewski and after Landau and Lifshitz) can be relevant in certain limits. Surprisingly, it is shown that for the case of vanishing damping a domain wall can still move, at the same time emitting spin waves. Considering thermodynamics it will be shown that the magnetization component inside a transverse domain wall disappears at a temperature below the Curie temperature T_C where the magnetization component inside the domains disappears. The same effect can be observed in other domain wall structures, like, e.g. a vortex structure. In this case the magnetization inside the vortex core disappears below T_C. Besides this thermodynamic effect a dynamical effect exists: the vortex core shows a super-paramagnetic behaviour similar to that found in nanoparticle. The investigation of FePt nanoparticles is also part of this dissertation. Within a collaboration with Seagate Research in Pittsburgh, PA, ab-initio calculation have been performed in order to derive and parameterize an effective spin model including an anisotropic long-range exchange interaction. The comparison of the simulation results for the thermally activated magnetization dynamics of FePt nanoparticles with the Neel-Brown model illustrates the limits of macro-spin models and underlines the importance of atomistic calculations.
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