Magnetic Ion Substitution and Peak Effect in YBCO: the Strange Case of Y1–xGdxBa2Cu3O7–δ

We present the results of a study of the superconducting and paramagnetic properties of polycrystalline Y 1– x Gd x Ba 2 Cu 3 O 7–δ samples. The critical current density and critical temperature of YBCO were weakly decreased by the Gd doping. A peak effect, which is a nonmonotonic dependence of the critical current density on magnetic field, was detected for all samples. The peak position shifted to higher magnetic fields with increasing Gd content. This behavior is opposite to the shift of the peak effect observed for other YBCO compounds doped by magnetic ions. This unusual behavior is apparently related to the realized granular structure instead of the type of doping ion. A correlation between the peak position and the granule size was found in the investigated samples and other polycrystalline YBCO compounds.