Generic positive effects of low level impurity doping on flux pinning properties of HTSC and MgB2

Low level impurity doping effects on flux pinning properties have systematically studied for Bi(Pb)2212 single crystals, Y123 melt-solidified bulk and MgB₂ polycrystalline bulks. In all these systems, moderately dilute impurity doping was found to be effective for improving critical current properties without losing their original T_c. For Bi(Pb)2212, iron, cobalt, nickel and yttrium were effective dopant for enhancement of J_c particularly under low fields. Second peak effect in J_c-H properties of Y123 were observed in Sr doped Y123 bulk. These positive effects were happened only when mean distance of impurity ions was longer than the vortex core size, 2ξ_ab. In the case of MgB₂, both aluminum doping for magnesium and carbon doping for boron improved J_c with their low doping levels less than 1%.