Magnetic-field angle dependent critical current densities and flux pinning in commercial YBCO tapes at liquid nitrogen temperatures

The magnetic-field angle dependence of critical current densities Jc(H,θ) in commercial YBCO tapes grown by MOCVD and MOD was examined at liquid nitrogen temperatures. We first measured Jc(H,θ) in MOCVD-YBCO tapes at 70 and 77.3 K in fields up to 2 T using both transport and inductive (the third harmonic voltage) methods and compared the results. It was observed that, in low magnetic fields, the transport measurements gave higher Jc than the inductive ones; however, in high fields they agreed well, which is well explained by the effects of weak links due to low-angle grain boundaries. We then investigated Jc(H,θ) in MOCVD- and MOD-YBCO tapes at 77.3 K in fields up to 2 T. All the tapes exhibited peaks at H//ab in the shape of Mount Fuji, which shows that small random pinning plays a major role. However, an anisotropic scaling analysis showed that the flux pinning mechanisms in those tapes were different, resulting in distinctive angular behaviors of Jc(H,θ). It is suggested that the difference in the flux pinning mechanisms of the two types of tapes came from the different sizes of point defects originating from the growing processes.