Control of Superconductivity in Cuprate/Manganite Hetrostructures
Pang, Brian SiewHan
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Research has shown that the spin alignment in an adjacent ferromagnet is capable of suppressingsuperconductivity. In this project, devices incorporating cuprate/manganite heterostuctreswere successfully fabricated to study the eﬀects of spin transport on the high temperature superconductor,YBCO. Deposition of such oxide ferromagnet/superconductor(F/S) multilayersusing the ‘eclipse’ pulsed laser deposition(PLD) technique was also examined. Reproducible multilayerswith ultrathin repeats were deposited, which exhibited superconducting and magneticproperties to minimum thicknesses of 3nm for both YBCO and LSMO.Using spin injection, via a ferromagnet, to create a spin imbalance in the superconductor, asuppression 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 nonequilibriumeﬀects, as it proved diﬃcult to eliminate the eﬀects of localized heating, currentsummation and magnetic ﬁeld. Interfacial studies of the device junction provided evidence ofan alternative currnent path at the interface.The control of superconductivity was also examined using F/S proximity eﬀects, which improvesthe understanding of how magnetic and superconducting materials coexist. We observedthat oxide F/S samples deposited by high O2 sputtering  and ‘eclipse’ PLD were similar, andthat 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. TheCurie temperatures of the multilayers were suppressed with increasing manganite thickness becauseof structural eﬀects, and also with increasing thickness of the YBCO layer which reducedthe 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 isequivalent to a superconductor sandwiched within a spin valve where both parallel and antiparallelconﬁgurations 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 superconductivitywas suppressed when the ferromagnets were in the parallel conﬁguration .From the onset of superconductivity, when the normal metallic behaviour of YBCO switchesto superconductivity, a magnetoresistance(MR) peak was observed when the F layers were antiparallel.The MR eﬀect increased with decreasing bias current and temperature, characteristicof a pseudo-spin valve. The result is suggestive of spin transport across the YBCO spacer layer.
Materials Science and Metallurgy, University of Cambridge 2004
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