REVERSIBLE MAGNETIZATION BELOW T(C) IN HIGH-QUALITY SUPERCONDUCTING CERAMICS

We have investigated the reversible magnetization below T(c) in high-q uality YBa2Cu3O7-delta (Y-123), YBa2Cu4O8 (Y-124), Y2Ba4Cu7O15+delta ( Y-247), and Bi2Sr2CaCu2O8+delta (Bi-2212), Tl2Ba2Cu1O6+delta (Tl-2201) and Tl2Ba2Ca1Cu2O8+delta (Tl-2212) ceramics. Except for the stoichiom etric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the nor mal-state susceptibility becomes temperature independent. Using the si mple London model, we are able to fit the reversible magnetization M(T , H) outside the region near T(c) with good accuracy for the nearly th ree-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into a ccount the fluctuations of vortices. An important result is that Y-123 exhibits a critical field clearly higher than those of the BiSrCaCuO or TlBaCaCuO phases. We obtain for the Y-123 phase a slope mu0 partial derivative H(c2,c)/partial derivative T\Tc almost-equal-to 4.3 T/K an d an extrapolated mu0H(c2,c)(0) almost-equal-to 280 T.