Field-angular dependence study of the critical current density in (RE)Ba2Cu3O7 films with nanoprecipitates larger than normal-core diameter of a quantized flux line

Highlights: • J_c(H, θ) was studied in (RE)BCO films containing relatively large nanoprecipitates. • Films containing a high density of nanoprecipitates showed flat J_c(θ) near H // c. • Films containing less density of nanoprecipitates showed broad J_c(θ) peak at H // c. • Different behaviors of J_c(θ) are qualitatively explained by simple core-pinning models. • The origin of the butterfly-shaped J_c(θ) curves at high magnetic fields is explained. -- Abstract: The magnetic-field angle dependence of the critical current density J_c(H, θ) reflects the background flux-pinning mechanisms in high-J_c (RE)Ba₂Cu₃O₇ (RE: rare-earth elements) thin films, where θ is the angle between the applied magnetic field H and the c-axis of (RE)BCO. It is because the pinning of flux lines by appropriate crystalline defects determines J_c. Yamasaki and coworkers found that nanoprecipitates larger than normal-core diameter of a quantized flux line caused broad J_c(θ) peaks centered on the c-axis in moderate magnetic fields of μ₀H = 0.5–2 T. These broad peaks have been reasonably explained by a simple theoretical model that considers the linear summation of core pinning interactions and the angular dependence of the coherence length [H. Yamasaki, K. Ohki, H. Yamada, Y. Nakagawa, Y. Mawatari, Supercond. Sci. Technol. 21 (2008) 125011]. On the other hand, several research groups have recently observed almost flat J_c(θ) curves at H // c in (RE)BCO thin films containing a high density of nanoprecipitates whose sizes are 1–4 times larger than the normal-core diameter. Some of these films showed constant J_c(θ) values at H // c that are as high as those at H // ab. The origin of the different behaviors of the J_c(θ) curves is qualitatively explained by simple core-pinning models, which have been modified by introducing the concept of flux bending that has been reported in recent theoretical works on the strong flux pinning of nanoparticles.