Flux pinning in *YBCO superconductor.

Abstract

The critical current density, <italic>J_c</italic>, is an engineering parameter of paramount importance for superconductors. Enhancement of <italic> J_c</italic> in magnetic field <italic>B</italic>, 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 YBa₂Cu₃O_7-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 <italic> in vacuo</italic>. XRD and HRTEM indicated that the nanostructures were Y₂O₃. <italic>J_c</italic> 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, <italic>et al.</italic> 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 <italic>T_c</italic> (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 <italic> T_c</italic> (at 75 K). These results were the first transport measurements demonstrating flux pinning in a ferromagnet-superconductor multilayer.