The interaction between superconductivity and non-collinear ferromagnetism has been investigated in hybrid structures. The nature of the interaction between the two can convert spin-singlet Cooper pairs from a conventional superconductor into spin-triplet pairs within the ferromagnet. These can survive over longer distances in the ferromagnetic material, which is a clear signature of a novel electronic state. This thesis primarily describes efforts to realise and study these fascinating long-range spin-triplet correlations in superconductor-ferromagnet hybrid structures composed of conventional superconducting Niobium (Nb) and ferromagnetic Permalloy (Py). A disk geometry has been used to promote a magnetisation vortex in the ferromagnet, whose naturally non-collinear magnetisation makes it ideal for spin-triplet pair generation. Firstly, two distinct superconductor-ferromagnet hybrid arrays have been fabricated in which the subsystems are either electronically or magnetically coupled, and the superconducting vortex behaviour investigated by scanning Hall probe microscopy. Distinct vortex nucleation patterns occur in different hybrid arrays and unconventional vortex behaviour near the Py disks has been observed, manifested as an apparent intervortex attraction. These interesting findings motivated the fabrication of planar superconductor-ferromagnet Josephson junctions with a disk-like geometry, which exhibited anomalous electrical magnetotransport phenomena, indicating the possible role of spin-triplet supercurrents. Secondly, a commercial second generation high-temperature superconducting (2G-HTS) tape has been investigated by scanning Hall probe microscopy in order to obtain a deeper understanding of the vortex pinning energy landscape, which is critical to underpin future developments. The magnetic flux front propagation in the tape has been mapped, showing very distinct behaviours at different temperatures. Our results have been compared to a classical critical state model, and find that this only describes the observed vortex behaviour well at temperatures closer to Tc. In addition, fishtail-like magnetisation peaks have been observed that have been linked to a ‘vortex matching’ effect for pinning sites near the edge of the tape.
|Date of Award||4 Sep 2019|
|Supervisor||Simon Bending (Supervisor)|
- Scanning Hall probe microscopy
- Hybrid structures
- flux lines