Effective electric field in dc magnetization measurements: Comparing magnetization to transport critical currents

We report values of critical current (J_c) for Tl₂Ba₂CaCu₂O_x(Tl-2212) epitaxial thin films in magnetic fields up to 5 T and at temperatures of 5, 40, and 77 K determined from dc magnetization and using a standard transport technique. Although we verify the applicability of Bean's model [Rev. Mod. Phys. 36, 31 (1964)], these values only agree at small applied magnetic fields and at low temperatures. Our results indicate that the differences observed in J_c's determined from the two techniques may be understood by considering only the field-dependent, nonlinear E-J characteristic and the ``effective'' electric-field criterion used in determining J_c.