Vortex pinning properties at dc and microwave frequencies of YBa ₂ Cu ₃ O _7-x films with nanorods and nanoparticles

Abstract

Abstract YBa 2 Cu 3 O 7−x (YBCO) nanocomposites for wire applications need to operate in a broad range of frequencies, ranging from dc in magnets to GHz in cavities and screenings of future particle accelerators. We have investigated the in-field and angular vortex pinning performance in dc and at 50 GHz of two types of nanocomposites, pulsed laser deposition (PLD) YBCO with mixed Ba 2 YNbO 6 + Ba 2 YTaO 6 (BYNTO) nanorods and chemical solution deposited (CSD) YBCO with BaHfO 3 (BHO) nanoparticles (NPs), and the pristine counterpart films, grown on top of single-crystalline substrates. Transport measurements performed up to 9 T between 5 and 77 K show that CSD nanocomposites exhibit a smooth field decay and increased single-to-collective crossover field H * compared to pristine samples, associated to the enhanced isotropic pinning contribution induced by the NPs, while PLD films exhibit unchanged H * and superior critical current densities up to higher irreversibility fields, associated to the anisotropic contribution introduced by the rods. Microwave in-field measurements of the pinning constant k p revealed CSD NCs exhibit a qualitatively similar, but smoother k p ( H ) than pristine samples, whereas for PLD samples, a growing k p ( H ) dependence is observed as a result of the increased relevance of the stiffness of the fluxons pinned by nanorods.