Buffer layers for Tl-2212 thin films on MgO and sapphire substrates

High quality Tl-2212 films can be readily grown on LaAlO₃ substrates, but the design of complex microwave devices in these films is made more difficult by the twinning of the substrate. CeO₂ buffer layers on sapphire have been previously demonstrated to offer excellent substrates for Tl-2212 films. We have been exploring alternative architectures for buffer layers on MgO and sapphire. Epitaxial Tl-2212 superconducting thin films have been successfully grown on Gd₂O₃-buffered MgO substrates by sputter deposition of a Tl-free precursor followed by an ex-situ thalliation anneal. The Tl-2212 films were aligned with the c-axis normal to the film surface, and also had excellent in-plane orientation with Tl-2212 [100] aligned with the Gd₂O₃ and MgO [110]. The critical temperatures of the films ranged from 100 - 103 K and transport and inductive critical current densities of 7.5 × 10⁵ A/cm² at 77 K have been achieved. We have also fabricated epitaxial GdAlO₃ buffer layers on sapphire substrates which support epitaxial growth of Tl-2212 but with relatively poor microstructure. High quality films have been prepared on the new (La,Sr)(Al,Ta)O₃ (LSAT) substrates.