Improvement in J_c performance below liquid nitrogen temperature for SmBa₂Cu₃O_y superconducting films with BaHfO₃ nano-rods controlled by low-temperature growth

For use in high-magnetic-field coil-based applications, the critical current density (J_c) of REBa₂Cu₃O_y (REBCO, where RE = rare earth) coated conductors must be isotropically improved, with respect to the direction of the magnetic field; these improvements must be realized at the operating conditions of these applications. In this study, improvement of the J_c for various applied directions of magnetic field was achieved by controlling the morphology of the BaHfO₃ (BHO) nano-rods in a SmBCO film. We fabricated the 3.0 vol. % BHO-doped SmBCO film at a low growth temperature of 720 °C, by using a seed layer technique (T_s = 720 °C film). The low-temperature growth resulted in a morphological change in the BHO nano-rods. In fact, a high number density of (3.1 ± 0.1) × 10³ μm⁻² of small (diameter: 4 ± 1 nm), discontinuous nano-rods that grew in various directions, was obtained. In J_c measurements, the J_c of the T_s = 720 °C film in all directions of the applied magnetic field was higher than that of the non-doped SmBCO film. The J_c^min (6.4 MA/cm²) of the former was more than 6 times higher than that (1.0 MA/cm²) of the latter at 40 K, under 3 T. The aforementioned results indicated that the discontinuous BHO nano-rods, which occurred with a high number density, exerted a 3D-like flux pinning at the measurement conditions considered. Moreover, at 4.2 K and under 17 T, a flux pinning force density of 1.6 TN/m³ was realized; this value was comparable to the highest value recorded, to date