AC and DC magnetic measurements

Ž . Ž . w Single crystals of Bi, Pb Sr Ca Cu O T s110 K , prepared by fused-salt reaction technique S.-Y. Chu, M.E. 2 2 2 3 x c Ž . Ž . McHenry, J. Mater. Res. 13 1998 589; S.-Y. Chu, M.E. McHenry, IEEE Trans. Appl. Supercond. 7 1997 1150; S.-Y. Ž . x Chu, M.E. McHenry, Electrochem. Soc. Proc. 97-2 1997 9. , have been characterized by both DC and AC magnetic measurements between 5 K and 108 K with a magnetic field perpendicular to the ab plane of the Bi-2223 crystal lattice. A numerical calculation indicates that the induced fields from the superconducting current loops in the separated neighboring Ž 3. crystals are very small and consequently can be neglected. Treating the individual crystals 73=73=1 mm as bar-shaped superconductors, we used a modified Bean model to determine the critical current density. For the first time, a strong temperature dependence of the current density at zero applied field is observed in these pristine single crystals with weak pinning. These then offer an excellent model system to probe the enhancements of persistent currents by means of proton irradiation. q 2000 Published by Elsevier Science B.V.