Enhancement of the fishtail effect in the DyBa₂Cu₃O_7-δ single crystal at magnetic field tilted from the c-axis

We studied angular and temperature dependences of the irreversible magnetisation on a twin-free DyBa₂Cu₃O_7-δ single crystal both before and after irradiation with Pb-ions. The irradiation produced columnar tracks along the c-direction which increased significantly the induced current density j_s. The fishtail effect observed before irradiation was shifted to higher temperatures but survived. At high fields (above 2.4 T) the magnetic hysteresis loops became flat, their height being nearly field independent. In the high-field range a distinct difference in the angular dependences of the unirradiated and irradiated sample was observed whereas at low fields the angular behaviour was qualitatively the same for both samples. This fact points to the qualitative difference in the flux pinning processes governing the high-field and central peak regions. The differences imply that the central peak might be mainly due to the in-plane components of the internal field in contrast to the high-field range where the c-axis component of the internal field predominates.