This thesis explores the growth, the nano-fabrication and the study of the magneto-
transport properties of Superconductor/Ferromagnet/ Superconductor (SFS) struc-
tures from complex oxides such as the high Tc superconducting cuprate YBa2Cu3O7
(YBCO) and the ferromagnetic manganites La2=3Ca1=3MnO3 and La2=3Sr1=3MnO3
(LCMO and LSMO), deposited with the pulsed laser deposition (PLD) technique.
The present work has been possible thanks to the collaboration between the \Mag-
netism and Superconductivity" Group at the University of Fribourg, in Switzerland,
and the \Experimentell Kondenserade Materiens Fysik" Group at Stockholm Uni-
versity, in Sweden.
Earlier, the two research groups in Fribourg and Stockholm had studied SFS
structures from YBCO/LaMnO3/YBCO multilayers with 20 nm thick ferromagnetic
and insulating LaMnO3 barriers, and obtained signs of an unconventional spin-triplet
current across these structures. This nding motivated the present thesis work with
a focus on two main aspects.
Firstly, to explore other candidate materials suitable as barriers and optimise
their growth conditions as to maintain a large ferromagnetic moment and thus a
high spin polarisation of the charge carriers. Secondly, to study what happens when
the thickness of the ferromagnetic and insulating LaMnO3 barrier is reduced well
below 20 nm to enable larger supercurrents.
It has been shown for a series of YBCO/LCMO multilayers that the ferromag-
netic moment of LCMO depends critically on the PLD growth conditions as well
as on the thickness and even structural details of the YBCO layer on which they
are grown. Furthermore, a protocol has been established to grow heterostructures
with strongly ferromagnetic manganite layers embedded in thick YBCO layers by
optimising the PLD growth conditions and by substituting the bottom YBCO layer
with a Co and Ca substituted version of YBCO that has a tetragonal structure
(tYBCO) instead of the orthorhombic one of plain YBCO.
Devices suitable for perpendicular magneto-transport measurements have been
nano-fabricated from YBCO / manganite / YBCO multilayers with 10 nm thick
LCMO and LSMO layers as the F barriers. While no clear indications of a spin-
triplet component of the superconducting order parameter have been obtained yet,
a negative and hysteretic magneto-resistance has been observed that is indicative
of a strong ferromagnetic order in the thin manganite barrier. The latter suggests a
potential memory functionality of such structures that could be exploited in future
spintronic memory devices.
Moreover, devices have been fabricated on SFS structures with a reduced thick-
ness of the LaMnO3 barrier of 10 nm and 5 nm. These samples were grown prior
to the beginning of this PhD work using non optimised growth conditions, and it
was found that the ferromagnetic properties of these LaMnO3 barriers are strongly
deteriorated. It remains to be seen whether the ferromagnetic order of such thin
LaMnO3 layers can be also recovered by using the optimised growth conditions as
for LSMO and LCMO.
|speaker||Roberto De Andres Prada
directeur : Prof. Christian Bernhard, DepPH, Université de Fribourg
co-directeur : Prof. Vladimir Krasnov, Université de Stockholm
|Contact||Département de Physique
Roberto De Andres Prada
026 300 90 61 ou 74