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Title: Magnetization dependent effects in superconducting nanowires influenced by the ferromagnetic effect
Author: Wren, Thomas
ISNI:       0000 0004 8499 2990
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2016
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This thesis presents the experimental study of superconducting nanowires under the influence of ferromagnetic nano-structures. Placing superconducting and ferromagnetic materials in contact with one another causes their long range orders to compete. This manifests as the leakage of superconducting properties into the ferromagnet and the suppression of superconductivity in the superconductor near the interface, known as the proximity and inverse proximity eects, respectively. The experiments presented in this thesis aim to show that the inverse proximity eect is sensitive to the magnetization of the ferromagnet, specically that the suppression is weaker if the ferromagnet has an inhomogeneous magnetization. To do this, the magnetic vortex state in sub-micron nickel disks and L-shape domain wall traps were used as the inhomogeneous magnetizations. The magnetization in the nickel disks and L-shape domain wall traps were investigated using magnetic force microscopy (MFM), in situ MFM, magnetotransport, and modelling. Aluminium nanowires were deposited over the ferromagnets and low temperature transport measurements of the hybrid structures were performed. It is found that the superconductivity in the nanowire above the disks is suppressed, creating an SNS junction. The critical current is shown to be sensitive to the magnetic history of the disks. The critical current of the entire nanowire is found to be dependent on the properties of the hybrid junction. This long-range influence has a thermal origin due to Joule heating in the hybrid junction as demonstrated by use of heat sink structures and Andreev loop interferometers. Replacing the disk with Lshape domain wall traps shows that the suppression of superconductivity is weakest when a domain wall is placed beneath the nanowire. Lastly, comparison to theory indicates the junction length is proportional to temperature. The results presented demonstrate previously unknown complexity in the behaviour of so-called proximity junctions and a step toward magnetically controlled superconducting circuitry.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available