Spin-triplet proximity effects in magnetically controlled heterostructures with s-wave superconductors

Abstract

Historically superconductivity and ferromagnetism were largely seen as antagonistic order parameters due to the rapid depairing of conventional spin-singlet Cooper pairs in the presence of a magnetic exchange field. Over the last decade-and-half, experimental evidence has emerged for the generation of spin-triplet Cooper pairs, which have parallel aligned spins, and are therefore compatible with ferromagnetism. This thesis explores the generation of triplet Cooper pairs in various superconductor/ferromagnet (S/F) heterostructures by systematically engineering the magnetic texture at S/F interfaces. These investigations include measurements of the superconductor critical temperature, TC, and point-contact Andreev reflection (PCAR) spectroscopy. The first two chapters of this thesis cover the basic physics underlying triplet pair generation and experimental methods. The following four chapters cover experimental results. Chapter 3 investigates S/F/F superconducting spin valves (SSVs), in which the thin film superconductor is proximity-coupled to two ferromagnets. Evidence of triplet generation is provided through TC measurements as a function of the magnetisation angle between the two ferromagnets. A maximum suppression in TC was obtained when the two ferromagnets were orthogonal in Nb/Py/LCMO SSVs with the transition metal Ni80Fe20 (Py) and the highly spin-polarised manganite La2/3Ca1/3MnO3 (LCMO). To optimise the performance of such SSVs further, experiments in Chapter 4 sought to increase the coercivity mismatch between the two F layers within a superconducting spin valve. This is achieved by using a synthetic antiferromagnet heterostructure, comprising two transition metal ferromagnets separated by an ultrathin Ru layer. The second results part of this thesis builds a framework for investigating intrinsic triplet generation in metallic and oxide superconductors. In Chapter 5 PCAR was performed on Ho/Nb bilayers to explore the role of magnetic phase on triplet generation. Finally in Chapter 6 the development of all-oxide SSVs is initiated. In these structures, triplets are generated at the S/F interface and their propagation is determined by the mutual orientation of the ferromagnets.