Properties, magnetic susceptibility, critical currents and irreversibility fields of (Tl0.5Pb0.5)Sr2(Ca1-xGdx)Cu2Oz bulk superconductors

Bulk (Tl0.5Pb0.5)Sr-2(Ca1-xGdx)Cu2Oz superconductors with x = 0.1, 0.2 and 0.3 were prepared by a wet chemical gel technique followed by a special heat-treatment procedure. X-ray diffraction analysis revealed practically phase-pure samples with tetragonal structure (space group P/4mmm). The temperature dependence of the real and imaginary parts of the ac susceptibility and the dependence on the ac applied magnetic field were investigated to learn about the superconducting and magnetic properties of bulk (Tl0.5Pb0.5)Sr-2(Ca1-xGdx)Cu2Oz superconductors. The critical current densities of the samples were derived from Bean's model. The temperature dependence of the critical current was successfully fitted based on a power law from the thermally activated magnetic flux creep model. From magneto-resistance measurements the temperature dependences of the transport critical currents and the irreversibility fields were found. The dc magnetic field dependence of the width of superconducting transitions and the critical currents was also obtained. These properties of the superconductors were analyzed within available theoretical models.