The effect of adding impurities in melt-processed bulk superconductors

It was recently reported that the critical current density (J(c)) of melt-processed YBa2CU3Oy based bulk superconductors increases by adding a small amount of Zn because its substitution for Cu depresses locally the superconductivity. Here we report on the effect of adding Zn to melt-processed DyBa2CU3Oy, and SmBa2Cu3Oy, the former is a system that exhibits a similar field dependence of J(c) as the Y system and the latter possesses a peak effect in the field dependence of J(c) We observed that the addition of a slight amount of Zn is effective for increasing J(c) and the trapped field in both systems. By measuring the local properties of the SmBa2Cu3Oy based bulk superconductor, we found that the increase in J(c) is particularly large in the region near the seed crystal, where the amount of pinning centers is deficient before adding Zn due to the segregation of the Sm2BaCuO5 phase. This enhancement in J(c) in the region near the seed crystal is probably most important for the increase of the trapped field of bulk samples.