The vortex decoupling transition in Bi2212 crystals with columnar defects

Two Bi2Sr2CaCu2O8+δ single crystals were characterized, before and after irradiation with 200 MeV Ag ions under fluence of 5 × 1010 ions/cm2. Columnar defects were produced along the c-axis direction in one crystal, and in a direction tilted by an angle of 30° with respect to the c-axis for the other crystal. The 3D-2D vortex decoupling lines were significantly shifted to higher temperatures, after irradiation of the crystals, for magnetic fields smaller than the matching value BΦ ≈ 1 T. For B > 3 T the decoupling lines for the irradiated crystals tend smoothly to the same behavior observed in the original crystals, up to the maximum probed field of 5 T. Interestingly the vortex pinning by columnar defects was found to be almost independent of the applied magnetic field direction, as revealed by the width of magnetization loops.