IRREVERSIBILITY LINES OF BI-2212 SINGLE-CRYSTALS IRRADIATED BY 6-GEV-PB IONS

6 GeV-Pb ion irradiation effects on the irreversibility line (IL) of Bi-2212 single crystals have been studied as a function of the induced columnar defect density. The crystals were irradiated parallel to the c-axis and the magnetic study of the IL was carried out with the field applied along this same direction. In a broad range of intermediate temperatures, we observe that the IL position is mainly determined by the value of the track density. The irreversibility fields in this domain correspond to a vortex density close to the track density, and the temperature dependence is weak. At lower temperatures, the ILs tend to join the position prior to irradiation and, at higher temperatures, a net change in the B(irr)(T) dependence is observed. This crossover at high temperatures could be interpreted as the result of a 3D-2D transition in the vortex lattice.