Influence of electromagnetic anisotropy on the flux-pinning strength of columnar defects in Bi2.2-xPbxSr1.8CaCu2.0Oy

Magnetization measurements have been performed on Bi(Pb)2212 single crystals with various electromagnetic anisotropies (gamma(2)=1.2X10(3)similar to 4.4X10(4)) before and after introducing columnar defects by Ta ion irradiation. The irreversibility fields of these crystals increased systematically with decreasing gamma(2) at various reduced temperatures t = T/T-c, indicating that the smaller anisotropy makes the columnar defects substantially more effective in flux pinning. The record-high irreversibility field and critical current density for single crystals of the Bi2212 phase was observed in our oxygen-overdoped Bi(Pb)2212 with columnar defects. This study experimentally reveals the reduction of the electromagnetic anisotropy and the introduction of pinning centers work cooperatively in high-T-c superconductor compounds.