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Thomas Jourdan

Multiscale approach for magnetism. Application to structural heterogeneities and magnetic singularities

Published on 29 October 2008


Thesis presented October 29, 2008

Abstract:
This thesis concerns the development of numerical methods to determine magnetic equilibrium configurations, and their use for systems with structural heterogeneities and magnetic singularities.
We first describe an algorithm based on a fast multipole method which permits to compute efficiently the dipolar field created by atomic spins in the framework of the classical Heisenberg model.
Using this model, we study the interaction of magnetic domain walls with structural defects in thin layers of FePt. We consider the case of antiphase boundaries and microtwins. We analyse the values of the depinning fields and compare them with experimental data.
Then we describe a multiscale method that we have developed. This method allows us to simulate a system with the Heisenberg model and with the micromagnetic model in a consistent formalism.
The last part of this thesis is devoted to the study of large systems with steep variations of magnetisation: vortex in a magnetic element, configurations with a Bloch point in a cube, magnetic bubble in a thin layer of FePd. In the latter case, results are compared with recent observations by Lorentz microscopy.

Keywords:
Heisenberg model, micromagnetics, multiscale method, fast multipole method, structural defect, domain wall, vertical Bloch line, Bloch point, magnetic singularity

On-line thesis.