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 magnetization 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 elements, typically below 1 at.%, definitely increase the pinning force density and shift the magnetic irrversibility lines towards higher fields. In contrast, higher concentrations strongly suppress the critical current density and the irreversibility fields. A special situation occurs in the regime of low temperatures below about 10 K where we find that even very low concentrations of the doping elements are deteriorative for the critical current density.