Magnetic behavior of thin superconducting disks with B-dependent J(c)

The ac susceptibility, chi = chi'+i chi " Of a superconducting thin disk in a perpendicular magnetic field is calculated in the critical-state model a ssuming a field-dependent critical current, J(c)(B). We find both analytica lly and numerically that the asymptotic behavior at large ac-field amplitud es B-am changes from chi' proportional to B-am(-3/2) and chi " proportional to B-am(-1) for the Bean model, to chi' proportional to B-am(-3) and chi " proportional to B-am(-2) for J(c) decreasing with \B \ as \B \ (-1) or fas ter. In the parametric chi "(chi') plot the peak of chi " increases in magn itude and shifts toward chi' = 0. This allows an easy experimental discrimi nation between a Bean model behavior, one with J(c)(B), and one where flux creep is an ingredient. Account of the B-dependence of J(c) improves substa ntially agreement with available experimental chi "(chi') data.