Recent theoretical studies have suggested that the observed suppression of superconductivity in superconductor/ferromagnet (S/F) heterostructures could be modulated by controlling the ferromagnetic exchange interaction in the superconductor. The exchange interaction in the superconductor is the sum of the exchange interaction from the ferromagnetic regions, which has a phase and magnitude that depends upon the direction that the ferromagnet is magnetised and the distance. As the exchange interaction has a phase it is possible that the contribution from two regions will cancel out. The exchange interaction, which can be viewed as an imbalance in the spin populations, suppresses superconductivity so any reduction in the exchange interaction will increase the superconducting transition temperature (Tc) of the heterostructure. Thus by changing the magnetisation of the ferromagnetic regions it is possible to control the exchange interaction in the superconductor and so the superconducting properties of the heterostructure. I have measured the superconducting properties of niobium/cobalt bilayers as a function of the applied magnetic field. I have observed that one component of the superconducting properties is controlled by the net magnetisation rather than the magnitude of the applied field. I have been able to show that this component of the observed change in the superconducting properties is not simply due to stray magnetic field but that the superconductivity is being actively controlled by means of the exchange interaction. This is the first experimental evidence that the superconducting properties of a S/F heterostructure can be controlled in this way, which opens up the possibilities for the construction of future devices.

Research has shown that the spin alignment in an adjacent ferromagnet is capable of suppressing superconductivity. In this project, devices incorporating cuprate/manganite heterostuctres were successfully fabricated to study the effects of spin transport on the high temperature superconductor, YBCO. Deposition of such oxide ferromagnet/superconductor (F/S) multilayers using the 'eclipse' pulsed laser deposition(PLD) technique was also examined. Reproducible multilayers with ultrathin repeats were deposited, which exhibited superconducting and magnetic properties to minimum thicknesses of 3nm for both YBCO and LSMO. Using spin injection, via a ferromagnet, to create a spin imbalance in the superconductor, a suppression of superconducting critical current was observed with increasing injection current. However, the exact cause of this suppression could not be solely attributed to spin-induced nonequilibrium effects, as it proved difficult to eliminate the effects of localized heating, current summation and magnetic field. Interfacial studies of the device junction provided evidence of an alternative current path at the interface. The control of superconductivity was also examined using F/S proximity effects, which improves the understanding of how magnetic and superconducting materials coexist. We observed that oxide F/S samples deposited by high O2 sputtering and 'eclipse' PLD were similar, and that Tc was clearly more suppressed in F/S compared to N(normal metal)/S systems. However, the magnetic moment and exchange coupling, two magnetic properties of significance in ferromagnets, did not, individually, have a major influence on the increased Tc suppression. The Curie temperatures of the multilayers were suppressed with increasing manganite thickness because of structural effects, and also with increasing thickness of the YBCO layer which reduced the coupling between manganite layers. To study the use of the spin-valve effect as a means to control high temperature superconductors, we fabricated an LSMO/YBCO/LC(0.3)MO pseudo spin-valve structure, which is equivalent to a superconductor sandwiched within a spin valve where both parallel and antiparallel configurations of the F layers can be achieved within a single magnetic field sweep. Previous research involving a metallic F/S/F/AF structure, showed that the superconductivity was suppressed when the ferromagnets were in the parallel configuration. From the onset of superconductivity, when the normal metallic behaviour of YBCO switches to superconductivity, a magnetoresistance (MR) peak was observed when the F layers were antiparallel. The MR effect increased with decreasing bias current and temperature, characteristic of a pseudo-spin valve. The result is suggestive of spin transport across the YBCO spacer layer.

Unconventional superconductivity (or superconductivity with a nontrivial Cooper pairing) is believed to exist in many heavy-fermion materials as well as in high temperature superconductors, and is a subject of great theoretical and experimental interest. The remarkable progress achieved in this field has not been reflected in published monographs and textbooks, and there is a gap between current research and the standard education of solid state physicists in the theory of superconductivity. This book is intended to meet this information need and includes the authors' original results.