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Electrical transport in hybrid spintronic structures

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Title: Electrical transport in hybrid spintronic structures
Authors: Magnus, Fridrik
Item Type: Thesis or dissertation
Abstract: Injection and detection of spin polarised current in a metal/semiconductor device and the measurement of the degree of injected spin polarisation are two key issues in the development of hybrid spintronics. This thesis touches on both themes as it details the development of planar Andreev spectroscopy as a tool to measure injected spin and the electrical characterisation of MgO tunnel barriers for efficient spin injection and detection. Point contact Andreev reflection spectroscopy has been widely used tomeasure transport spin polarisation in magnetic materials. Planar Andreev structures have an advantage over point contacts as they offer greater control over interface quality and the possibility of spatially resolved information about the spin polarisation using nanojunction arrays. We find that planar junctions compare favourably to point contacts in that they can yield low interface barriers and minimal nonthermal smearing. We show that a low interface barrier is critical for accurate detection of spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatch contributes to the barrier. Furthermore, the fabrication method strongly affects all parameter values. For Pb/InAs planar junctions we demonstrate that the most feasible way to obtain interfaces suitable for spin detection is an “etch-back” processing strategy. The processing routes are shown to be scalable to nanoarray fabrication to allow measurement of spin accumulation. We also examine the electrical properties of ultrathin MgO barriers grown on InAs epilayers and the dependence of barrier characteristics on InAs surface pretreatment and growth conditions. Chemical pretreatment improves the yield of tunnel junctions and changes the roughness of the interface between the oxide and the semiconductor. Electrical characterisation confirms that tunnel barriers with appropriate values of interface resistance for efficient spin injection/detection have been achieved. Using the Rowell criteria and various tunnelling models we show that single step tunnelling occurs above 150 K and a thermal smearing model suggests that tunnelling is the dominant transport process down to 10 K.
Issue Date: Jul-2008
Date Awarded: Jan-2009
URI: http://hdl.handle.net/10044/1/4411
DOI: https://doi.org/10.25560/4411
Supervisor: Cohen, Lesley
Author: Magnus, Fridrik
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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