Magnetic properties and microstructure of sintered high-Tc Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O oxides

AC complex susceptibility χ\´-iχ", DC magnetization, resistive transition curves in various magnetic fields, and the critical-current density were measured for sintered Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O samples whose major phase was the high- T_c phase of each system. The single peak observed in χ" vs. temperature curves was very sharp for both samples of each system. The hysteresis in DC magnetization curves at 77 K rapidly decreased with increasing magnetic field for both systems. This may be related to the layered morphology of the platelike grains. The resistive transition curves of both samples were broadened by the application of a magnetic field as in the case of Y₁Ba₂Cu₃ O₇. In magnetic fields, rather long tails to low temperatures were observed in resistive transition curves for Bi-Sr-Ca-Cu-O. J_cs at 77 K and zero field were 250 A/cm² and 600 A/cm² for Bi- and Tl-based samples, respectively. These J_cs rapidly decreased with increasing magnetic field