|Title: ||Control of Superconductivity in Cuprate/Manganite Heterostructures|
|Authors: ||Pang, Brian SiewHan|
|Issue Date: ||Jun-2004|
|Abstract: ||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.|
|Appears in Collections:||Theses - Materials Science and Metallurgy|
Theses - Device Materials Group
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