Micromagnetic modelling of imperfect crystals
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Date
29/11/2018Author
Ó Conbhuí, Pádraig
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Abstract
In paleomagnetism, practical measurements are rarely made using perfect, isolated,
single-phase, ferromagnetic crystals. Experimental observations are typically
made using magnetic materials formed by a variety of natural processes.
In this thesis, we will look at bridging the gap between current numerical modelling
capability and experimental observations.
First, we work towards micromagnetic modelling of multi-phase magnetic materials,
including magnetostriction, embedded in a rocky matrix, along with
crystal defects. We present a derivation of the Boundary Element Method formulation
used by the micromagnetics package, MERRILL, and provide an extension
of this from single-phase materials to multi-phase. After discussing issues
with previous approaches to modelling magnetostriction, we derive and
present a more robust and flexible approach. This model of magnetostriction is
suitable for non-uniformmagnetizations, for multi-phase materials, and for arbitrary
boundary conditions, and can be incorporated into MERRILL.We then
outline a method for extending our model to materials embedded in an infinite
elastic matrix of arbitrary elasticity. Finally, we present a method for modelling
the magnetic response of a material due to crystal defects, along with
a concrete example of a magneto-dislocation coupling energy at a magnetite-ilmenite
boundary where stress due to lattice misfit is eased by regular edge
dislocations.
Second, we work towards being able to verify micromagnetic models against
nano-scale experimental data. To do this, we present two techniques for simulating
electron holograms from micromagnetic modelling results, a technique
capable of imaging magnetic structures at the nano-scale. We also present
example electron holograms of commonly occurring magnetic structures in
nano-scale rock and mineral magnetism, and highlight some distinguishing
features, which may be useful for interpreting experimental electron holography
data.