YBa ₂ Cu ₃ O _7−x films with Ba ₂ Y(Nb,Ta)O ₆ nanoinclusions for high-field applications

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.