REVERSIBLE MAGNETIZATION AS A FUNCTION OF THE OXYGEN CONCENTRATION IN BI-2212 SUPERCONDUCTING CERAMICS

We investigate the reversible magnetization in the mixed state of a batch of Bi2.12Sr1.90Cu1.96O8+delta(Bi-2212) ceramics with different oxygen concentrations delta and superconducting transition temperatures T-c between 50 and 92 K. Additional data for a single crystal (T-c = 85 K) demonstrate the validity of the effective-mass model with essentially infinite anisotropy. The magnetization curves M(T) for different fields cross in a single point {M* (delta); T* (delta)} for all T-c's greater than or equal to 65 K. The saturation magnetization M*(delta) is found to be proportional to T*(delta). This agrees with existing theories if the superconducting volume fraction, presumably 100%, does not vary with delta The systematic suppression of the Meissner effect that is observed with decreasing T-c is therefore attributed to enhanced pinning. For T-c = 50 K, there is no single crossing point. This is attributed to a crossover to 3D in the strongly overdoped state. The penetration depth lambda(T) is evaluated from lim(T-->0) partial derivative M/partial derivative ln H. The systematic dependence of lambda(0) versus delta indicates that the carrier density increases with oxygen content, as expected.