Reversible magnetization and irreversibility line of tri-layer superconductor Ba2Ca2Cu3O6(O,F)2 with Tc ∼ 108 K

We report magnetization measurements of grain-aligned Ba2Ca2Cu3O6(O,F)(2) with T-c similar or equal to 108 K. The interlayer distance of the material is the shortest among known tri-layer superconductors. Unexpectedly, the magnetization data show that the coupling strength between CuO2 layers is rather weak. A direct reflection of the weak coupling is highly suppressed irreversibility line, i.e. a broad reversible region in H-T plane. The decoupling field obtained from the irreversibility line is less than 0.1 T, which is comparable with that of quasi two-dimensional superconductor Bi2Sr2CaCu2O8+delta. Comparison of data with the Hao-Clem model gives characteristic parameters [xi(ab)(0) and lambda(ab)(0)] and the critical fields [Hc(0) and H-c2(c)(0)]. A large value of penetration depth, lambda(ab)(0) = 240 nm reflects a small carrier concentration in CuO2 planes, and explains the reason of the weak interlayer coupling. (C) 2004 Elsevier Ltd. All rights reserved.