Control of Superconductivity in Cuprate/Manganite Heterostructures
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  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.