Magnetic measurements on single crystals YBa₂Cu₃O_7−δ

We have measured ac susceptibility and magnetization for single crystals of YBa₂Cu₃O_x, which exhibit twinning of the a- and b-axes. The field-dependent Meissner effect is about 10% of the shielding effect. Critical-current and flux-pinning-force densities were calculated from the magnetization loops, which have been measured both parallel and perpendicular to the c-axis. We find an anisotropy in the critical current and the flux-pinning force. Field dependence of the pinning force, which is likely due to pinning at the twin boundaries, in fields close to B_c2 is compared with the flux pinning in NbN, where it is limited by the flux line shear mechanism.