Magnetization and Re LIII-Edge Studies of (Hg,Re)Sr2CuO4 + δ System

Chemically stable nominal compositions of (Hg1-xRe x )Sr2CuO4 + δ (x = 0.05, 0.10, and 0.20), designated as (Hg,Re)-1201 phase, were synthesized at ∼920°C in partial vacuum. The samples with x = 0.05 and 0.10 are superconducting (SC) with Tconset ∼40 and ∼54 K, respectively. On the other hand, for the composition with x = 0.20, the sample is nonsuperconducting down to 5 K. The sample with composition (Hg0.9Re0.1)Sr2CuO4 + δ on cooling, below 10 K in dc-field of 4 kOe, shows a dramatic decrease (reversal) in diamagnetic signal. Such a reduction in diamagnetic response is also reflected by the sample's supercurrent induced magnetic hysteresis curve generated at 9 K. In the light of the available reports and our Re LIII-edge measurements, we attribute this effect to a field enhanced paramagnetic contribution probably arising mainly from mixed valent (6+, 7+ dominant) Re ions. The critical current density (Jc), calculated using Bean's model, for (Hg0.9Re0.1)Sr2CuO4 + δ is ∼6 × 103 A/cm2 (4.5 K, 5 kOe).