Magnetization Studies of Mercury-Cuprates and Its Precursors

The mercury-cuprates such as (Hg, Cr)/Sr-1201 and (Hg, Re)/Sr-1201 were synthesized by encapsulation-technique at 880–920 °C for 12–16 h, and characterized by power XRD, M(T) and M(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 with T_c^onset ~ 58T_{\rm c}^{{\rm onset}} \sim 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 the M(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.