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Title: Understanding and controlling dynamically-induced stochastic domain wall behaviours in magnetic nanowires
Author: Omari, Khalid
ISNI:       0000 0004 5992 0223
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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Stochastic pinning of domain walls (DWs) in Permalloy (Py) nanowires is a significant challenge that needs to be overcome to produce reliable magnetic nanowire-based devices for memory/logic applications. In this study, stochastic interactions of vortex DWs (VDWs) with intrinsic defects and artificial notch-shaped defects have been studied with the aim of understanding their complexity and proposing methods to control them. VDW pinning/depinning behaviour in double and single notches showed complex behaviour in real measurements when compared with quasi-static simulations. Systematic study was performed on VDW interactions with double and single notches with a range of depths and nanowires’ thicknesses. Results showed that multi-mode stochastic pinning/depinning behaviour occurred for the vast majority of systems. However, for large single notches in thick nanowires (t=40nm), single-mode depinning-field-distributions were observed. It was shown that convergence to single depinning-mode was due to the nature of Walkerbreakdown transformations that preserve the vortex shape of VDWs, and to the specifics of DWs interaction with notch and edge roughness. The dependence of stochastic pinning/depinning behaviours on DW injection processes was also studied. It was found that DWs injected into nanowires using pulses through orthogonal current-line nucleation exhibited more deterministic pinning/depinning behaviour than those injected from nucleation pads. This may be attributed to the process by which DWs depin from the nucleation pad junction. Additionally, it is shown that pinning/depinning stochasticity can be mitigated by passing VDWs through sharp bends that act as chirality rectifiers. For certain nanowire/notch geometries, this reduces the number of accessible pinned DW configurations to a single configuration; thus, producing a single-mode depinningfield- distribution. Finally, micromagnetic simulations are used to demonstrate feasibility of DW logic architecture where bits are encoded using VDW chirality. Designs are proposed for NAND, AND, NOR, OR and FAN-OUT gates. All gates work by manipulating the interaction of VDWs with Y-shaped junctions and/or notches.
Supervisor: Hayward, Thomas ; Allwood, Dan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available