Control of Superconductivity in Cuprate/Manganite Heterostructures

Change log
Pang, Brian SiewHan 

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 currnent 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 [1] 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 [2]. 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.

Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge