Superconducting Proximity Effects and Josephson Coupling in CoFeB Structures

Abstract

This thesis presents experimental results that assess the suitability of a ferromagnetic cobalt-iron-boron alloy (CoFeB) for further research and application in ‘superconducting spintronics’ - the field that aims to utilise the distinct physics arising at interfaces between superconducting and magnetic thin films for the development of functional memory and logic devices. Following a review of the current field and an initial materials characterisation, CoFeB is assessed alongside the superconductor niobium (Nb) in two archetypal superconducting spintronic devices. The first of these is the superconducting spin-valve, in which a modification of the superconducting critical temperature of up to 23 mK is observed concurrent with infinite magnetoresistance. This is the first demonstration of complete switching between the resistive and superconducting states of Nb depending on relative ferromagnet magnetisation directions in a Nb-3d ferromagnet system. The potential for further optimisation of these devices is demonstrated by study of devices grown at elevated temperatures. Device optimisation via thermal treatment is also investigated in Nb/CoFeB/Nb ferromagnetic Josephson junctions - the second device type studied in this work. The key parameters of this particular materials system are characterised for the first time, and an increase in the Josephson critical current following thermal annealing up to a maximum of ≈ 670% is demonstrated, which is attributed to an increase in the electron mean free path. The results show CoFeB to be a promising material candidate for the development of more complex superconducting spintronic devices that may have tunable properties via annealing.