Large fishtail effect in DyBa₂Cu₃O_7-δ single crystals containing columnar defects

Using capacitance torque magnetometry, hysteresis loops were measured on a Pb-ion irradiated DyBa₂Cu₃O_7-δ single crystal as a function of magnetic field, temperature and angle Θ between the field direction and the c axis of the sample. The superconducting current densities, j_c (T, B, Θ), determined using the extended Bean model, show a clear fishtail effect at temperatures above 30 K despite the large enhancement of j_c due to the columnar tracks. The enhancement of j_c due to irradiation was magneto-optically determined to j_c(φt)/j_c(0) = 10 at T = 30 K. The pinning energy U₀ is, however, only slightly increased by the irradiation. Our measurements indicate clearly that the fishtail effect is a general feature of the high-T_c superconductors and not related to a special arrangement of pinning centres.