Role of grains and weak links in the nonlinear magnetic response of Y-Ba-Cu-O

We present a study of the field dependence of the remanent nonlinear magnetic response induced in sintered Y-Ba-Cu-O samples by momentary application of a dc magnetic field H. The results show that, contrary to theoretical predictions, the remanent nonlinear response depends strongly on H. We attribute this behavior to weak links subjected to internal fields induced by flux trapped within the grains. To support this view, we present an extensive comparison between ac and dc data in the same samples. This comparison clarifies the role of grains and of weak links in various field regimes, and demonstrates the relationship between the remanent nonlinear response and the trapped flux.