Irreversibility fields of the high-Tc superconductors Hg-1212 and (Hg,Tl)-1212

Irreversibility fields, Bi are investigated for several polycrystalline samples of the high-Tc superconducting systems Hg-1212 and (Hg,Tl)-1212 in magnetic fields up to 14 T. In addition, the critical temperature Tc and the critical fields Bc1 and Bc2 are determined using the so-called differential paramagnetic effect for the determination of Bc1. AC as well as dc susceptibility measurements are performed with special attention to the temperature dependence of the irreversibility field, Bi(T), which above approximately 1 T coincide for both types of measurements and can be described by a model for diffusive flux motion. Below 1 T more than one loss mechanism seems to exist. The ac data in this region are in good agreement with the Bean model. The description of the irreversibility line in the whole temperature range, however, fits well to the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. In the case of Tl substitution Bi is enhanced by some degrees