Flux Pinning Properties and Irreversibility Field Curves of the High-concentration BaHfO₃-Doped SmBa₂Cu₃O_y Thin Films

Abstract

For the improvement and stable use of various superconducting applications using REBa₂Cu₃O_y (REBCO) coated conductors, the enhancement of the critical current density (J_c) in high magnetic fields is essential. It is known that BaMO₃ (BMO; M = Zr, Sn, Hf) nanorods can improve the in-field J_c of REBCO films. Furthermore, the density and formation can be controlled by the deposition conditions such as deposition temperature and BMO content. In this study, our aim was to improve the J_c of SmBa₂Cu₃O_y (SmBCO) films in high magnetic fields. We tried to introduce high-density BaHfO₃ (BHO) nanorods into SmBCO films by increasing the BHO content. We fabricated 2.3, 4.9 and 7.2 vol.% BHO-doped SmBCO films at the same deposition temperature, and compared the geometric formation of nanorods and flux pinning properties. From TEM observation, the densities of BHO nanorods increased with increases in BHO content, and the diameters of all nanorods were almost the same (i.e., approximately 12 nm). The highest density and matching field were 1711 /μm² and 3.54 T in a 7.2 vol.% sample. The flux pinning force of the sample with a higher matching field was effective in higher fields in the magnetic field dependence of J_c. This was in good agreement with theories and many reports. In irreversibility field lines, a unique behavior, which probably originated in the thick diameters, was observed around the low magnetic field.