Spin-mixing and transport through superconducting interlayers in ferromagnetic Josephson junctions

The creation of spin-polarization in the superconducting state raises possibilities for low-energy logic and memory elements. Supercurrents of spin-aligned triplet Cooper pairs (or ’triplet pairs’) can be generated at magnetically inhomogeneous superconductor/ferromagnet interfaces, and have a longer decay envelope in a ferromagnet than singlet Cooper pairs (or ’singlet pairs’). In this thesis I investigate ferromagnetic nanopillar Josephson devices with different types of ferromagnetic barriers,incorporating spin-mixer interfaces and Rashba spin-orbit coupling. The research focuses on equilibrium spin-transport through intermediary superconducting regions for which experimental and theoretical studies are limited. The behaviour of singlet and triplet pair components in Nb/Cr/Fe/Nb/Fe/Cr/Nb Josephson devices demonstrates that spin-aligned triplet pairs decay more rapidly than singlet pairs in the normal state of the central Nb layer. In the superconducting state of the central Nb layer, the results suggest that at least one of the singlet or spin-aligned triplet components must be non-zero throughout the barrier region in order to carry a Josephson supercurrent. Recent theoretical work predicts that spin-aligned triplets can also arise in the presence of both a ferromagnetic exchange field and a Rashba spin-orbit field. The thesis investigates this hypothesis using Nb/Pt/Co/Pt/Nb Josephson devices, but triplet supercurrents due to the presence of a spin-orbit field from Pt are undetected at zero voltage (equilibrium). These results indicate that out-of-equilibrium transport might be required to generate triplet supercurrents due to spin-orbit fields and/or a different device geometry (e.g. a planar geometry with current components parallel to the superconductor/ferromagnet interface). A recent ferromagnetic resonance experiment showed an enhancement of dynamic spin transport from a ferromagnet through an intermediary superconducting layer when a material with high spin-orbit coupling was present at a distance of around a coherence length from the ferromagnet. This was attributed to triplet pairing opening a triplet channel in the superconductor. Analogous equilibrium Josephson junction experiments presented here did not give rise to a spin-aligned triplet, indicating that the origin of enhanced spin-transport is dependent on non-equilibrium effects.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Devine-Stoneman, James

Advisor dc:contributor.advisor

• Robinson, Jason

• superconductivity
• Josephson junctions
• spintronics

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/aaced914-34eb-414d-a030-15e1d8e9c70a/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.74470

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/327019