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Title: Magnetic structures and proximity effects in rare-earth/transition metal ferromagnetic and superconductor systems
Author: Higgs, Thomas David Charles
ISNI:       0000 0004 7426 5458
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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The antiferromagnetic coupling between a rare-earth (RE) and a tran- sition metal (TM) ferromagnet can be exploited to engineer normal state and superconducting functional devices. RE/TM ferromagnetic multi- layers were previously used as spin-mixers to generate spin-triplet su- percurrents. This was possible due to magnetic inhomogeneity present in the devices, however the precise nature of the inhomogeneity was not understood. Here we present a comprehensive study of the Ni/Gd/Ni system using a powerful element-specific measurement technique: x-ray magnetic circular dichroism. In order to analyse the experimental results we present a novel model based on the Stoner-Wohlfarth model, which shows that significant inhomogeneity exists at the Ni/Gd interfaces due to the competition between the exchange energies within the system and the Zeeman energy of the applied magnetic field. The experiment and model together provide a complete overview of the Ni/Gd/Ni system due to the breadth of temperatures and thicknesses studied. The knowledge gained from this work is then applied to designing and test- ing new spin valves based on the intrinsic inhomogeneity at the RE/TM interface, and both Ni/Gd- and Gd/Ho-based devices show reversible magnetic switching behaviour which alters the superconducting critical temperature.
Supervisor: Robinson, Jason Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: magnetism ; superconductivity ; thin-film ; xmcd ; proximity effect ; ferromagnet ; antiferromagnet ; python ; rare-earth ; transition metal ; holium ; gadolinium ; nickel ; niobium ; exchange bias