Effects of Mg doping on the superconducting properties of YBa₂Cu₃O7- deltaand La1.85Sr0.15CuO₄systems

The superconducting properties of YBa 2 (Cu 1-x Mg x ) 3 O 7- delta (0<or=x<or=0.1; delta <or=0.08) and La 1.85 Sr 0.15 (Cu 1-x Mg x )O 4 (0<or=x<or=0.07) have been studied. In both systems, an increase of the dopant content induces a contraction of the crystallographic c axis and a reduction of the critical temperature T c , with effects very similar to those observed in Zn-doped samples. An onset temperature of 79 K is found for YBCO at the maximum Mg solubility (x approximately=0.025). In LASCO, superconductivity is suppressed at x=0.03. Our results support the hypothesis that the detrimental effects on superconductivity of all non-magnetic dopants are similar, and are linked to the formation of a net magnetic moment in the CuO 2 plane. An estimation of the intergranular shielding current density J s (T,H) in Mg:YBCO is also presented. Mg substitution affects the temperature dependence of J s , but it does not change the variation induced by the application of an external DC magnetic field.