Comparison of flux pinning in single crystals and melt-textured YBa₂Cu₃O_7-x

Flux pinning properties of single crystals and melt-textured samples of YBa₂Cu₃O_7-x were studied for external magnetic field oriented parallel and perpendicular to the CuO₂ planes (ab-planes). For H ∥ ab vortex behaviour is mainly determined by a very effective intrinsic pinning mechanism. The irreversibility line is located in the region of higher fields and temperatures in comparison with the irreversibility line for H ⊥ ab. In a single crystal for H ⊥ ab, we observed an increase in the critical current density j_c with magnetic field - the so-called "fishtail effect" - in a very broad temperature range. For H ∥ ab, this effect is observed only at temperatures close to T_c, when the intrinsic pinning is much weaker.