Superconducting Properties of Magnetron Sputtered Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O Thin Films
Thin films of high temperature superconducting Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O have been made using multiple source magnetron sputtering. We will discuss preparation and annealing and how they affect the superconducting properties. The Bi-based films form in at least two compounds, a lower Tc phase with a 2212 composition and Tc0 ≈ 80 K and a higher Tc phase with a nominal composition of 2223 and a Tc ≈ 105 K. The higher Tc phase is very difficult to form so that a complete superconducting path exists, requiring long annealing times at temperatures somewhat below the formation temperature. Although superconducting properties are less sensitive to stoichiometry than YBa2Cu3O7-x, annealing temperatures must be held in a narrow range around 865°C to get the best results. Films prefer growing on (100) MgO in a highly textured polycrystalline form, with the c-axis perpendicular to the plane of the film. The Tl-based films form at least three compounds (2212, 1223, 2223) with Tc’s ranging from ≈ 100 K to 114 K and grow best on (100) and polycrystalline Y-ZrO2 substrates. Annealing conditions are particularily difficult because of the volatility of thallium and require samples to be annealed in sealed capsules with a controlled atmosphere of thallium and oxygen. Upper critical field measurements show a large anisotropy (≈15 for Bi-Sr-Ca-Cu-O and ≥70 for T1-Ba-Ca-Cu-O) in the parallel to perpendicular field slope ratio as might be expected from a highly textured anisotropic structure.
KeywordsCritical Field Superconducting Property YBCO Film Resistive Transition Thin Film Geometry
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