MAGNETIC IRREVERSIBILITY LINES AND CRITICAL CURRENTS OF BI(2212) SINGLE-CRYSTALS DOPED BY FE, NI, CO AND ZN

We report measurements of the AC-susceptibility and the irreversible m agnetization of Bi(2212) single crystals doped with up to 2 at.% of Fe , Ni, Co and Zn on the Cu position. Low concentrations of the doping e lements, typically below 1 at.%, definitely increase the pinning force density and shift the magnetic irrversibility lines towards higher fi elds. In contrast, higher concentrations strongly suppress the critica l current density and the irreversibility fields. A special situation occurs in the regime of low temperatures below about 10 K where we fin d that even very low concentrations of the doping elements are deterio rative for the critical current density.