3d-metal doping of the high-temperature superconducting perovskites La-Sr-Cu-O and Y-Ba-Cu-O

The structural, magnetic, and superconducting properties of the mixed compounds La1.85Sr0.15Cu1-xMxO4 (0<x<0.3) (M=Ni or Zn) and YBa2Cu3-xNixO7-y (x=0.25 or 0.5) were studied. dc resistance, meissner, and shielding measurements have shown that both magnetic Ni and diamagnetic Zn ions destroy the superconductivity in the La-Sr-Cu-O system. Within the Y-Ba-Cu-O system, the replacement of copper by nickel depresses Tc from 93 K (x=0) to 50 K (x=0.5). At higher nickel contents, the phase does not form. The upper critical field Bc2(T) of the nickel-doped samples was measured near Tc in dc fields to 23 T. Resistive midpoint transitions yield dBc2dT=4.5 and 1.77 T/K for the Ni-doped La-Sr-Cu-O (x=0.025) and Y-Ba-Cu-O (x=0.5) systems, respectively. We compare doping results for magnetic and nonmagnetic ions, and discuss the origin of the depression in Tc.