Magnetic properties of the Bi1.95Sr2.05-xLaxCuOy (Bi-2201) superconducting phase

We have investigated the reversible mixed-state magnetization M of three lanthanum substituted Bi1.95Sr2.05-xLaxCuOy?,, (Bi-2201) ceramic samples having different critical temperatures T-c ranging from 20.0 to 35.5 K. As for the Bi2Sr2CaCu2O8+delta (Bi-2212) phase, we found that anisotropy of Bi-2201 is large. A manifestation of this anisotropy is the field independent magnetization M* observed at a temperature T*. In the framework of the London model, and including thermal fluctuations of vortices, we found for the temperature dependence of the penetration depth lambda(ab)(T) = lambda(ab)(0)[1-(T/T-c0)(n)](-1/2), with n approximate to 1.7 and lambda(ab)(T = 0) approximate to 4000 Angstrom. The estimated upper critical fields mu(0),H-c2,H-c,,, are of the order of 10 T. We observe a peculiar negative slope partial derivative M/partial derivative T at low temperature and sufficiently high external magnetic field. This feature seems to he a characteristic of the Bi-2201 phase. However, we do not know whether it is associated with the superconducting mixed-state. A small amount of magnetic impurities could also be responsible for this behavior. Finally, the behavior of the reversible magnetization of the Bi-2201 samples investigated, which are situated at the optimal and in the overdoped region, did not indicate any unusual temperature dependence for the upper critical field H-c2,H-c. (C) 1997 Elsevier Science B.V.