Irreversibility line and second peak in the magnetization curves of Pb₂Sr₂Y_1-xCa_xCu₃O ₈ single crystals

The irreversibility field H_irr and the anomalous second peak in the magnetization curves have been studied from the magnetization measurements (H ∥ c-axis) for Pb₂Sr₂Y_1-xCa_xCu₃O ₈ single crystals with 0 ≦ x ≦ 0.5 and with the anisotropic parameter γ = (m*_c/m*_ab)^1/2 of 24 ≧ γ ≧ 5. The temperature dependence of H_irr is discussed in terms of flux creep and 2D or 3D vortex-lattice melting. The anomalous second peak is observed for all x values. The peak shape is similar to that of YBa₂Cu₃O_7-δ. The peak field H_p is dependent on temperature and proportional to (1 - T/T_c)^3/2 near T_c. Both H_irr and H_p at each reduced temperature are proportional to γ^-2. The origin of the second peak is discussed in terms of dimensional crossover of the vortices from 3D to 2D and collective pinning.