Flux pinning effects of twin boundaries studied with unidirectionally twinned YBCO films

We have investigated the flux pinning effects of the twin boundary (TB) of YBCO, using TB-aligned YBCO films on YAlO3 and NdGaO3 (0 0 1) substrates. We observed a large anisotropy in critical current densities J(c) and normal-state resistivities, when four-probe transport measurements were performed with currents in different directions. To our surprise, along TBs J(c) was lower and the normal-state resistivity was higher. We observed microcracks perpendicular to the TBs, and found that the observed large anisotropy in transport properties is due to microcracks. It is strongly suggested that the previously reported anisotropic transport properties in unidirectionally twinned YBCO films [Phys. Rev. Lett. 77 (1996) 3913] are caused by microcracks and/or related planar defects, not by TBs. We have observed homogeneous Jc in YBCO films without apparent microcracks whose TBs are aligned but not perfectly. (C) 2002 Elsevier Science B.V. All rights reserved.