Central peak position in magnetization loops of high-T_csuperconductors

Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior J_c(B) of the critical current density. We show that within the critical-state model the magnetization as a function of applied field, B_a has an extremum located exactly at B_a= 0. This result is in excellent agreement with presented experimental data for a YBa₂Cu₃O_7-δthin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields, B_cp< 0, on the descending field branch of the loop. Our results show that a positive B_cpis a definite signature of granularity in superconductors.