Multiscale Multimodel Simulation of Micromagnetic Singularities
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Beschreibung
During the last decades, the research on fundamental magnetic structures, like domain
walls, spinwaves and vortices, resulted in a detailed understanding of the magnetization
dynamics in ferromagnetic materials, without which the development of modern
storage devices would not have been possible. On the pathway to this level of understanding
micromagnetic simulations played an important role due to their ability
to reproduce experimental results in great detail and, especially, to predict magnetic
patterns and their dynamic properties. An example of the predictive power is the
research eld of vortex dynamics. Another fundamental magnetic structure is the
Bloch point, which is particularly complex since the description of the processes and
energy terms responsible for its formation lie within the scope of the continuum theory
of micromagnetism, but the study of its detailed properties requires a dierent
framework. In terms of topology and concerning the exchange energy density, the
Bloch point displays a point singularity in the theory of micromagnetism.
Bloch points are not a marginal phenomenon; they play, e.g., an important role
as transient structures during the switching of vortex cores and reside inside of the
archetypal example of vortex domain walls in solid cylindrical nanowires. In the
1960s, E. Feldtkeller and W. Döring described and characterized Bloch points with
the then available methods, yet their dynamics eluded a detailed description, since on
one hand a large volume is necessary to stabilize a Bloch point structure and on the
other hand an atomistic description of its center is required. To solve this problem we
developed a multiscale multimodel simulation framework in the context of this thesis,
which is able to detect automatically Bloch points as well as other micromagnetically
critical structures. In that simulation kit we apply a classical Heisenberg model to
the critical regions, while using the framework of micromagnetism for the remaining
sample, which is discretized with nite elements. The program allows not only for a
static examination of Bloch points residing in a localized Heisenberg approximated
region, but also for dynamic simulations due to its ability to detect regions of interest
automatically as well as to track them with the multimodel region.
The simulations within this thesis focus on ferromagnetic cylindrical nanowires
with vortex domain walls. The simulations describe the depinning eld necessary
to trigger a propagation of the domain wall with the Bloch point in its center and
the impact of the relative orientation of the lattice to the Bloch point propagation
direction. In addition, we could identify dierent propagation patterns of the structure
consisting of domain wall and Bloch point. In addition to regimes with a continuous
domain wall movement, this thesis highlights and discusses several complex modes of
domain wall/Bloch point propagation. In particular, we nd a propagation regime
in which the Bloch point and domain wall propagate with constant velocity above
the minimum spin wave phase velocity. This velocity remains constant within a
broad interval of external eld strength. Using analytic calculations we could ascribe
this maximum velocity, which is a feature of potential interest from a technological
perspective, to an intrinsic property of the Bloch point.
Autor*in
Christian Andreas