Abstract
The mixed-state magnetization M(H||c,T) of a Bi-2212 single crystal has been investigated with high resolution using a SQUID magnetometer. In the high-temperature region (50 K < T < Tc = 80.2 K), we found that the slope ∂M/∂H|T vs. H shows a positive step at Htrans(T) ≈ H0×(1 − T/Tc)n with H0 = 2340 Oe and n = 1.28. This observation is compatible with a first-order phase transition with a distribution of internal fields, and is attributed to the melting of the 3D vortex lattice. The estimated entropy jump is 1 kB/vortex/layer CuO. However, when T is lower than 50 K, we observe radical changes in M(H); the 3D melting line divides into a decoupling line at a temperature-independent field and the onset of the irreversibility.