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

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)Ba 2 Cu 3 O 7 ( 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 μ 0 H  = 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.