TlBa2Ca2Cu3Oy superconducting films on MgO with different morphologies

Abstract

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 °C. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities Jc. Critical current density Jc determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of Jc is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 °C. In these samples, the substrate’s flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 °C and showing rough surface and Ca-segregates, Jc was lower. However, Jc was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended.