Comparison of the critical current anisotropy in YBaCuO and BiSrCaCuO films

Abstract

Laser deposition using an excimer laser was used to prepare thin single-crystal YBa/sub 2/Cu/sub 3/O/sub 7-x/ and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ (BSCCO) films on <100> SrTiO/sub 3/ and LaAlO/sub 3/ 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/sub c/ was measured in the whole superconducting temperature range in magnetic fields up to 7 T as a function of the angle theta between the field direction and the c-axis. At 4.2 K, j/sub c/(B perpendicular to c) is almost field-independent (up to 7 T), and, at 7 T, it is about one order of magnitude larger than j/sub 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/sub c/(B) is not limited by the irreversibility field, leading to a crossover point in j/sub c/(B) at 77 K, where below 2 T, j/sub c/(B//c) is even larger than j/sub c/(B perpendicular to c). The extremely high pinning force for the B perpendicular to 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/sub c/(B//c) and j/sub c/(B perpendicular to c). The absence of these planar defects might be a reason for the very low j/sub c/(B//c) values in BSCCO films at higher temperatures, although the critical current density at 4.2 K and zero field (1*10/sup 7/ A/cm/sup 2/) can be very high.