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DTSTART;VALUE=DATE:20191111T171500
DTEND;VALUE=DATE:20191111T171500
UID:7005@agenda.unifr.ch
DESCRIPTION:This thesis explores the growth, the nano-fabrication and the study of the magneto-\ntransport properties of Superconductor/Ferromagnet/ Superconductor (SFS) struc-\ntures from complex oxides such as the high Tc superconducting cuprate YBa2Cu3O7\n(YBCO) and the ferromagnetic manganites La2=3Ca1=3MnO3 and La2=3Sr1=3MnO3\n(LCMO and LSMO), deposited with the pulsed laser deposition (PLD) technique.\nThe present work has been possible thanks to the collaboration between the \Mag-\nnetism and Superconductivity" Group at the University of Fribourg, in Switzerland,\nand the \Experimentell Kondenserade Materiens Fysik" Group at Stockholm Uni-\nversity, in Sweden.\nEarlier, the two research groups in Fribourg and Stockholm had studied SFS\nstructures from YBCO/LaMnO3/YBCO multilayers with 20 nm thick ferromagnetic\nand insulating LaMnO3 barriers, and obtained signs of an unconventional spin-triplet\ncurrent across these structures. This nding motivated the present thesis work with\na focus on two main aspects.\nFirstly, to explore other candidate materials suitable as barriers and optimise\ntheir growth conditions as to maintain a large ferromagnetic moment and thus a\nhigh spin polarisation of the charge carriers. Secondly, to study what happens when\nthe thickness of the ferromagnetic and insulating LaMnO3 barrier is reduced well\nbelow 20 nm to enable larger supercurrents.\nIt has been shown for a series of YBCO/LCMO multilayers that the ferromag-\nnetic moment of LCMO depends critically on the PLD growth conditions as well\nas on the thickness and even structural details of the YBCO layer on which they\nare grown. Furthermore, a protocol has been established to grow heterostructures\nwith strongly ferromagnetic manganite layers embedded in thick YBCO layers by\noptimising the PLD growth conditions and by substituting the bottom YBCO layer\nwith a Co and Ca substituted version of YBCO that has a tetragonal structure\n(tYBCO) instead of the orthorhombic one of plain YBCO.\nDevices suitable for perpendicular magneto-transport measurements have been\nnano-fabricated from YBCO / manganite / YBCO multilayers with 10 nm thick\nLCMO and LSMO layers as the F barriers. While no clear indications of a spin-\ntriplet component of the superconducting order parameter have been obtained yet,\na negative and hysteretic magneto-resistance has been observed that is indicative\nof a strong ferromagnetic order in the thin manganite barrier. The latter suggests a\npotential memory functionality of such structures that could be exploited in future\nspintronic memory devices.\nMoreover, devices have been fabricated on SFS structures with a reduced thick-\nness of the LaMnO3 barrier of 10 nm and 5 nm. These samples were grown prior\nto the beginning of this PhD work using non optimised growth conditions, and it\nwas found that the ferromagnetic properties of these LaMnO3 barriers are strongly\ndeteriorated. It remains to be seen whether the ferromagnetic order of such thin\nLaMnO3 layers can be also recovered by using the optimised growth conditions as\nfor LSMO and LCMO.
SUMMARY:Pulsed laser deposition and nanofabrication of mesoscopic devices based on cuprates and manganites
CATEGORIES:Soutenance de mémoire/thèse
LOCATION:PER 08\, auditoire 0.51\, Chemin du Musée 3\, 1700 Fribourg
URL;VALUE=URI:https://agenda.unifr.ch/e/fr/7005
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