Flux creep in the Tl-1223 high temperature superconductor

The relaxation of the irreversible magnetization has been measured by a Faraday magnetometer for the weakly anisotropic polycrystalline Tl0.5Pb0.5Sr2Ca2Cu3O9 (Tl-1223) and a highly anisotropic Tl2Ba2Ca2 Cu3O10 (Tl-2223) single crystal. From these data, the effective activation energy, Ueff(j)=Ul(jc/j)μ, is determined by the method of Maley. A comparison of the irreversibility data at 77 K for different high-temperature superconductors (HTSC) indicates that the irreversibility field is quickly decreasing with increasing distance between the superconducting CuO2-layers. In agreement with this tendency, Ueff is higher for the one-intermediatelayer compound Tl-1223 than for the highly anisotropic two-intermediate-layer Tl-2223 compound. The exponent μ≡d log(Ueff)/d log(j) saturates for Tl-1223 at a value of μ≈1.4 near jc and decreases with decreasing current down to 0.3. On the other hand μ increases for Tl-2223 with increasing current from 0.1 up to 2.5 and shows no tendency to saturate. A comparison with data for Y-123 and Bi-2212 taken from literature, indicates that the flux creep in Tl-1223 is 3-dimensional like in Y-123. Therefore it can be explained by the 3D collective creep model for not too small currents. For Tl-2223, 2-dimensional behaviour is observed.