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Title: Domain wall dynamics and resonant modes of magnetic nanostructures
Author: Albert, Maximilian
ISNI:       0000 0004 6422 2278
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2016
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In this work we present finite element-based simulations of magnetic nanostructures using the micromagnetic software packages Nmag and Finmag developed at the University of Southampton. As part of this work the package Finmag has been extended with the implementation of an eigenvalue-based method to compute resonant modes in magnetic nanosystems. The details of this implementation are discussed, including certain complications encountered in the context of a finite element discretisation scheme. The implementation is verified using results from an independently published study on eigenmodes of an elliptical nanodisc. We present two studies of domain walls in magnetic nanowires. The first one investigates field-driven domain wall motion in nanowires with edge roughness. A new roughness model is introduced which allows the systematic study of how edge roughness features influence the domain wall motion compared to the case of a smooth nanowire. While the large-scale behaviour, such as the asymptotic domain wall velocity, is largely unaffected by the roughness, it introduces marked local alterations to the domain wall trajectories and can lead to dynamic pinning, both below and above the Walker breakdown. It is shown that the effective pinning strength of the roughness features is strongest when their size is comparable to the size of the domain wall. The second domain wall study investigates different types of resonant modes (translational, breathing and twisting modes) of transverse domain walls pinned at notches in a magnetic nanowire. The different sensititivies of each mode type on the nanowire and notch geometry are investigated in detail. It is found that the translational and twisting mode respond relatively strongly to changes in the notch geometry, while the breathing mode is fairly robust to changes in the notches’ size, making it a promising candidate for applications. We finally present a study of resonant modes in an elliptical magnetic nanodisc representing the free layer of a spin-torque nano-oscillator. We demonstrate that the resonant frequencies and spatial mode profiles are altered in the presence of a magnetic nanoparticle. The dependence of the frequency shifts on the nanoparticle position and material parameters is studied systematically. It is shown that these frequency shifts exceed achievable linewidths in state-of-the-art spin-torque oscillators and that they can be maintained over large external field ranges (owing to to the fact that they are a direct response to the stray field of the nanoparticle and do not rely on changes to the magnetic ground state of the disc). This opens up promising applications for novel nano-sensing devices using frequency-based detection schemes.
Supervisor: Fangohr, Hans Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available