Comparison of the critical current anisotropy in YBaCuO and BiSrCaCuO films

Laser deposition using an excimer laser was used to prepare thin single-crystal YBa₂Cu₃O_7-x and Bi₂ Sr₂CaCu₂O_x (BSCCO) films on <100> SrTiO₃ and LaAlO₃ substrates. The CuO planes of the films are always parallel, and the c-axes are perpendicular to the film plane. The critical current density j _c was measured in the whole superconducting temperature range in magnetic fields up to 7 T as a function of the angle θ between the field direction and the c-axis. At 4.2 K, j_c(B⊥c) is almost field-independent (up to 7 T), and, at 7 T, it is about one order of magnitude larger than j_c(B||c) for both types of superconductors. This anisotropy increases markedly with increasing temperature, also at lower fields, for the BSCCO films. It decreases for the YBaCuO films, as long as j_c(B) is not limited by the irreversibility field, leading to a crossover point in j_c(B) at 77 K, where below 2 T, j _c(B||c) is even larger than j _c(B⊥c). The extremely high pinning force for the B⊥c configuration is explained by an intrinsic pinning due to a periodic variation of the order parameter perpendicular to the CuO layers. At higher temperatures, planar defects which lie parallel to the c-axis like twin boundaries or stacking faults become effective for YBaCuO and lead to a reduced difference of j _c(B||c) and j_c( B⊥c). The absence of these planar defects might be a reason for the very low j_c(B||c ) values in BSCCO films at higher temperatures, although the critical current density at 4.2 K and zero field (1×10⁷ A/cm²) can be very high