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Dipl.-Phys. Robert Wieser :
Dissertation angenommen durch: Universität Duisburg-Essen, Campus Duisburg, Fachbereich Physik, 2006-04-28
BetreuerIn: Priv. Doz. Dr. Ulrich Nowak , Universität Duisburg-Essen, Campus Duisburg, Fachbereich Physik, Theoretische Physik
GutachterIn: Priv. Doz. Dr. Ulrich Nowak , Universität Duisburg-Essen, Campus Duisburg, Fachbereich Physik, Theoretische Physik GutachterIn: Prof. Dr. rer. nat. Peter Entel , Universität Duisburg-Essen, Campus Duisburg, Fachbereich Physik, Theoretische Physik
Schlüsselwörter in Englisch: domain walls, Green's function,
local mean field, Landau-Lifshitz-Gilbert equation, micromagnetism,
computer simulation, nanoparticle
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Abstrakt in Englisch
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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