Realisation of De Gennes’ Absolute Superconducting Switch and Magnetic Exchange Field Effects at Metallic and Superconductor Interfaces

Abstract

This thesis presents a systematic experimental study on the magnetic exchange field (MEF) effects at interfaces between a chalcogenide ferromagnetic insulator (EuS) and both superconducting and metallic thin-films exhibiting spin-orbit coupling. Firstly, we demonstrate an absolute superconducting switch, confirming a prediction by Pierre-Gilles de Gennes in 1966. This is achieved using a thin-film multilayer of EuS/Au/Nb/EuS, where Au is a metal with strong spin-orbit coupling, and Nb is an s-wave superconductor. The original de Gennes’ superconducting switch, consisting of a ferromagnetic insulator/superconductor/ferromagnetic insulator (FI/S/FI) trilayer, is characterised by the complete suppression of superconductivity at all temperatures when the magnetisations of the two FI layers are parallel-aligned (P), due to the addition of MEFs at both FI/S interfaces. Conversely, for antiparallel-aligned (AP) magnetisations, the superconducting state is recovered as the MEFs cancel each other out. While superconducting switches have been previously demonstrated, this thesis marks the first experimental realisation of an absolute superconducting switch that achieves complete switching between superconducting ‘on’ and resistive ‘off’ states. A theoretical analysis using the Green’s function method has been conducted to investigate the MEFs at the EuS/Nb and EuS/Au interfaces. Our findings confirm that the superconducting switch performance is enhanced by the Au spacer, attributed to a larger MEF at the EuS/Au interface than at the EuS/Nb interface. This enhancement by a large MEF at EuS/Au interfaces is further confirmed in the context of a metallic spin switch, as detailed subsequently. Moreover, we report re-entrant superconductivity in EuS/Nb/EuS superconducting switches with an out-of-plane magnetic field. We also investigate vortex dynamics in EuS/Nb wires using superconducting-quantum-interference-device-on-tip (SOT) microscopy. An asymmetry in vortex flux flow is demonstrated and manipulated by fringing fields generated by EuS, which causes a non-reciprocity of the critical supercurrent density, i.e., we observe a superconducting diode-like effect. Next, we demonstrate the MEF effects at interfaces between EuS and metallic Au thin-films by investigating the longitudinal and Hall magnetoresistance (MR) in EuS/Au/EuS switches of varying Au thicknesses. We observe that the MEF suppresses the localisation effect in thin Au spacers (dAu < 6 nm), resulting in the P-state resistance being larger than that the AP-state resistance (i.e., RP > RAP), as predicted by the Maekawa-Fukuyama theory. For thick Au spacers (dAu > 6 nm), we observe a giant-magnetoresistance-effect-like phenomenon (RAP > RP), attributed to spin-dependent electron scattering at the EuS/Au interface. Furthermore, we identify a significant anomalous-Hall-like spin Hall magnetoresistance (SMR) in patterned EuS/Au/EuS structures, verifying the presence of a large MEF at EuS/Au interfaces. We demonstrate for the first time that a trilayer (FI/N/FI) configuration enhances SMR in comparison to a bilayer (N/FI) structure, confirming the original SMR theory predicted by Chen et al. Additionally, this thesis reports the MR behaviour of EuS/Nb/EuS superconducting switches in a resistive state at temperatures nearly ten times above the superconducting transition temperature, linking the MR to superconducting fluctuations within nearly nanometre-thick Nb thin-films. Throughout this thesis, we use consistent values to evaluate the MEFs at the EuS/Au and EuS/Nb interfaces, with κEuS/Au = 1.5 meV·nm and κEuS/Nb = 1.2 meV·nm at 2 K, respectively.