Flux pinning in *YBCO superconductor.

The critical current density, Jc, is an engineering parameter of paramount importance for superconductors. Enhancement of Jc in magnetic field B, allows the superconductor to generate larger magnetic fields and to carry more electrical current. This enhancement can be engineered by flux pinning, which is the main subject of the present work. The primary superconductor used in the studies was YBa 2Cu3O7-delta (YBCO). After providing an overview of superconductivity, flux pinning, and our experimental techniques, three experimental approaches designed to study enhancements in flux pinning via materials engineering are presented. Flux pinning in multilayer films of YBCO and Y2O3 nanostructures was explored. The nanostructured layer neither helped nor hindered the superconducting properties of the YBCO, for the conditions that were tested. The Y2O3 nanostructures were formed by a novel method, which was the pulsed laser ablation of a stoichiometric YBCO target in vacuo. XRD and HRTEM indicated that the nanostructures were Y 2O3. Jc measurements indicated compatibility between 6 nm or less Y2O3 nanostructured film with YBCO. Next, improvement in flux pinning by in a ferromagnet-superconductor multilayer was investigated using two ferromagnetic materials systems. The ferromagnetic materials studied included TbFe and CoPt, which when grown under certain conditions, possess uniaxial perpendicular magnetic anisotropy (UPMA). It was predicted by Bulaevskii, et al. that a ferromagnet with UPMA when applied on a superconductor, under certain conditions, would provide strong flux pinning. The amorphous TbFe films, which were grown by PLD and co-sputtering, were heavily oxidized, as confirmed by RBS. Oxidation was primarily attributed to oxygen incorporation during film growth. The TbFe showed weak UPMA at greater thickness (200 nm) and no UPMA at 100 nm, indicating a limiting oxide thickness. Flux pinning by 200 nm TbFe on YBCO showed weak enhancement. Sputtered CoPt multilayers showed significant UPMA, as confirmed by MFM and magnetic hysteresis. Strong flux pinning near Tc (86 K) was demonstrated by field-dependent transport measurements in a CoPt/YBCO bilayer, while no change in flux pinning was shown further away from Tc (at 75 K). These results were the first transport measurements demonstrating flux pinning in a ferromagnet-superconductor multilayer.PhDApplied SciencesElectromagneticsMaterials sciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123201/2/3068891.pd