Magnetization measurements have been performed on Bi(Pb)2212 single crystals with various electromagnetic anisotropies $({\gamma }^2=1.2\mathrm{}\times {10}^3\sim 4.4\times {10}^4)$ 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 ${t=T/T}_{\mathrm{c}},$ 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.

• Received 17 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.1452