Flux pinning centres correlated along the c-axis in PLD-YBCO films

Abstract

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ψ curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 µm−1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a–b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50–60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 µm−2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a–b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.