Magnetoresistance hysteresis and critical current density in granular RuSr₂Gd_2-xCe_xCu₂O_{10- δ}

We report resistivity data on ceramic RuSr₂Gd_2-xCe_x Cu₂O_10-δ which is the first Cu-O based system in which superconductivity (SC) in the CuO₂ planes and weak-ferromagnetism (W-FM) in the Ru sublattice are considered to coexist. Due to the granular nature of the materials the critical current density at 5 K is extremely small as compared to other high-T_C superconducting materials. Below the superconducting transition (T_C), the magnetoresistance ΔR(H) = R(H)-R(0) is positive and unexpected hysteresis loops are observed. The hysteresis loop is completely different than that observed in conventional homogeneous superconductors. ΔR(H) on decreasing the applied field (H_ext) is much smaller than the ΔR(H) for increasing (H_ext). The width of the loops depends strongly on the weak-link properties. Similar hysteresis loops are observed in the reference nonmagnetic NbSr₂Gd_1.5Ce_0.5Cu₂O₁₀ which is SC at T_C=28 K, thus the possibility that the hysteresis phenomenom is caused by the coexistence of both SC and magnetic states is excluded.