EFFECTS OF MG DOPING ON THE SUPERCONDUCTING PROPERTIES OF YBA2CU3O7-DELTA AND LA1.85SR0.15CUO4 SYSTEMS

The superconducting properties of YBa2(Cu1-xMg(x)3O7-delta (0 less-tha n-or-equal-to x less-than-or-equal-to 0.1; delta less-than-or-equal-to 0.08) and La1.85Sr0.15(Cu1-xMg(x))O4 (0 less-than-or-equal-to x less- than-or-equal-to 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 v ery similar to those observed in Zn-doped samples. An onset temperatur e of 79 K is found for YBCO at the maximum Mg solubility (x congruent- to 0.025). In LASCO, superconductivity is suppressed at x = 0.03. Our results support the hypothesis that the detrimental effects on superco nductivity of all non-magnetic dopants are similar, and are linked to the formation of a net magnetic moment in the CuO2 plane. An estimatio n 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 applicat ion of an external DC magnetic field.