Magnetic studies on the high temperature oxide superconductor Eu_0.4Gd_0.6Ba₂Cu₃O_{7- δ}

The oxide Eu_0.4Gd_0.6Ba₂Cu₃O_{7- δ} is superconducting with a transition temperature above liquid nitrogen temperature. We have carried out detailed magnetic studies on this compound. Low field dc susceptibility measurements and four probe electrical resistivity measurements yield a superconducting transition temperature of about 78 K. Measurements of the shielding effect give a value of 82% of the corresponding value for perfect diamagnetism. The lower critical field is found to be 600 Oe at 4.2 K. The hysteresis loop has been obtained at 4.2 K in fields up to 90 kOe from which the magnetic critical current density has been estimated. Above the superconducting transition temperature, the magnetic susceptibility of this compound is close to that of an assemblage of Gd³⁺ ions. In the superconducting state it is observed that, because of field penetration, the paramagnetism of the Gd³⁺ ion overtakes the diamagnetism of the material in an external field of about 5 kOe.