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Title: Domain walls in spin-valve nanotracks : characterisation and applications
Author: Sampaio, Joao Miguel Ramos Melo
ISNI:       0000 0004 2710 2146
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Magnetic systems based on the manipulation of domain walls (DWs) in nanometre-scaled tracks have been shown to store data at high density, perform complex logic operations, and even mechanically manipulate magnetic beads. The magnetic nano-track has also been an indispensable model system to study fundamental magnetic and magneto-electronic phenomena, such as field induced DW propagation, spin-transfer torque, and other micromagnetic properties. Its value to fundamental research and the breath of potentially useful applications have made this class of systems the focus of wide research in the area of nanomagnetism and spintronics. This thesis focuses on DW manipulation and DW-based devices in spin-valve nanotracks. The spin-valve is a metallic multi-layered spintronic structure, wherein the electrical resistance varies greatly with the magnetisation of its layers. In comparison to monolayer tracks, the spin-valve track enables more sensitive and versatile measurements, as well as demonstrating electronic output of DW-based devices, an achievement of crucial interest to technological applications. However, these multi-layered tracks introduce new, potentially disruptive magnetic interactions, as well as fabrication challenges. In this thesis, the DW propagation in spin-valve nanotracks of different compositions was studied, and a system with DW propagation properties comparable to the state-of-the-art in monolayer tracks was demonstrated, down to an unprecedented lateral size of 33nm. Several DW logic devices of variable complexity were demonstrated and studied, namely a turn-counting DW spiral, a DW gate, multiple DW logic NOT gates, and a DW-DW interactor. It was found that, where the comparison was possible, the overall magnetic behaviour of these devices was analogous to that of monolayer structures, and the device performance, as defined by the range of field wherein they function desirably, was found to be comparable, albeit inferior, to that of their monolayer counterparts. The interaction between DWs in adjacent tracks was studied and new phenomena were observed and characterised, such as DW depinning induced by a static or travelling adjacent DW. The contribution of different physical mechanisms to electrical current induced depinning were quantified, and it was found that the Oersted field, typically negligible in monolayer tracks, was responsible for large variations in depinning field in SV tracks, and that the strength of spin-transfer effect was similar in magnitude to that reported in monolayer tracks. Finally, current induced ferromagnetic resonance was measured, and the domain uniform resonant mode was observed, in very good agreement to Kittel theory and simulations.
Supervisor: Cowburn, Russell ; Branford, Will Sponsor: EU Marie Curie project SpinSwitch
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral