Flux Pinning by Columnar Defects along a-axis in a-axis Oriented YBCO Thin Films

Columnar defects (CDs) along a-axis were installed into an in-plane aligned a-axis-oriented YBa₂Cu₃O_y film by using heavy-ion irradiation, in order to clarify the influence of CDs on flux pinning properties around B // ab. The critical current density J_c at B // c significantly decreases after the irradiation, since the CDs act as a channel for flux creep along their length. The magnetic field dependence of J_c at B // ab, on the other hand, is weakened by the introduction of CDs. This means that the CDs can trap flux lines at B // ab, whereas naturally growth defects working as effective ab-correlated PCs exist. The trapping angle of CDs along the a-axis is extremely narrow (∼5.8°) in comparison with CDs along the c-axis and is roughly coincides with other ab-correlated PCs. The small trapping angle of CDs along the a-axis is attributed to strong line tension energy of flux lines with elliptical core around B // ab.