Artificial pinning centres in YBa2Cu3O7-δ thin films by Gd2Ba4CuWOy nanophase inclusions

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, Jc), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). Jc of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased Jc. The relative enhancement in Jc increases with further increase in temperature. Jc of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of δTc-type.