Reversible magnetization below <i>Tc</i> in high-quality superconducting ceramics

We have investigated the reversible magnetization below Tc in high-quality YBa2Cu3O7−δ (Y-123), YBa2Cu4O8 (Y-124), Y2Ba4Cu7O15+δ (Y-247), and Bi2Sr2CaCu2O8+δ (Bi-2212), Tl2Ba2Cu1O6+δ (Tl-2201) and Tl2Ba2 Ca1Cu2O8+δ (Tl-2212) ceramics. Except for the stoichiometric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the normal-state susceptibility becomes temperature independent. Using the simple London model, we are able to fit the reversible magnetization M(T, H) outside the region near Tc with good accuracy for the nearly three-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into account 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 μ0∂Hc2,c/∂T|Tc≈−4.3 T/K and an extrapolated μ0Hc2,c(0)≈280 T