Magnetization Studies of Mercury-Cuprates and Its Precursors

The mercury-cuprates such as (Hg, Cr)/Sr-1201 and (Hg, Re)/Sr-1201 were synthesized byencapsulation-technique at 880–920°C for 12–16 h, and characterized by power XRD,M(T) andM(H). All the precursors (Sr₂CuO₃, Cr_0.30Sr₂CuO _y or Re_0.10Sr₂CuO_y) were found to be non-superconducting. The samples with optimal composition (Hg_0.70Cr_0.30)Sr₂CuO_4+δ [(Hg, Cr)/Sr-1201] and (Hg_0.90Re_0.10)Sr₂CuO_4+δ [(Hg, Re)/Sr-1201] are superconducting withT _c ^onset ∼ 58 and ∼ 54 K, respectively. At low temperatures (below 10 K), on cooling in D.C.-magnetic field of 4 kOe, the sample (Hg, Re)/Sr-1201 shows a dramatic decrease in diamagnetic signal. Further, this effect is also faintly reflected in theM(H) curve. Such a behaviour was not observed in the (Hg, Cr)/Sr-1201 cuprates. We ascribe this effect to a field enhanced paramagnetic contribution possibly arising from mixed valent (6+, 7+ dominant) Re ions in the (Hg, Re)/Sr-1201 cuprates.