YBa <sub>2</sub> Cu <sub>3</sub> O <sub>7−x</sub> films with Ba <sub>2</sub> Y(Nb,Ta)O <sub>6</sub> nanoinclusions for high-field applications
G. Celentano, F. Rizzo, A. Augieri, A. Mancini, Valentina Pinto, A. Rufoloni, A. Vannozzi, Judith L. MacManus‐Driscoll, John Feighan, A. Kuršumović, Alexander Meledin, Joachim Mayer, Gustaaf Van Tendeloo
Abstract
Abstract The structural and transport properties of YBa 2 Cu 3 O 7− x films grown by pulsed laser deposition with mixed 2.5 mol% Ba 2 YTaO 6 (BYTO) and 2.5 mol% Ba 2 YNbO 6 (BYNO) double-perovskite secondary phases are investigated in an extended film growth rate, R = 0.02–1.8 nm s −1 . The effect of R on the film microstructure analyzed by TEM techniques shows an evolution from sparse and straight to denser, thinner and splayed continuous columns, with mixed BYNO + BYTO (BYNTO) composition, as R increases from 0.02 nm s −1 to 1.2 nm s −1 . This microstructure results in very efficient flux pinning at 77 K, leading to a remarkable improvement in the critical current density (J c ) behaviour, with the maximum pinning force density F p (Max) = 13.5 GN m −3 and the irreversibility field in excess of 11 T. In this range, the magnetic field values at which the F p is maximized varies from 1 T to 5 T, being related to the BYNTO columnar density. The film deposited when R = 0.3 nm s −1 exhibits the best performances over the whole temperature and magnetic field ranges, achieving F p (Max) = 900 GN m −3 at 10 K and 12 T. At higher rates, R > 1.2 nm s −1 , BYNTO columns show a meandering nature and are prone to form short nanorods. In addition, in the YBCO film matrix a more disordered structure with a high density of short stacking faults is observed. From the analysis of the F p ( H , T ) curves it emerges that in films deposited at the high R limit, the vortex pinning is no longer dominated by BYNTO columnar defects, but by a new mechanism showing the typical temperature scaling law. Even though this microstructure produces a limited improvement at 77 K, it exhibits a strong J c improvement at lower temperature with F p = 700 GN m −3 at 10 K, 12 T and 900 GN m −3 at 4.2 K, 18 T.