Flux pinning properties of c-axis correlated pinning centres in PLD-YBCO films

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle psi-dependence of the critical current density J(C) and the E-J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The psi-dcpendence of J(C) showed large peaks when magnetic field B was applied parallel to the c-axis (B parallel to c), and we observed two types of J(C)-peak: that is, a broad peak for the films deposited at small D (50-60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E-J-characteristics followed the power law, E proportional to J(n), and the psi-dependence of the n-value also showed broad peaks around B parallel to c for the films deposited at D = 50-60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around B parallel to c may be attributed to a high density of elongated precipitates, and the harrow-angle pinning effect around B parallel to c may be attributed to dense planar defects parallel to the c-axis.