NEGATIVE LOCAL PERMEABILITY IN BI2SR2CACU2O8 CRYSTALS

The local ac magnetic response in Bi2Sr2CaCu2O8 crystals was measured using a microscopic array of Hall sensors. The ac component of the loc al induction is found to attain negative values in contradiction with the standard models that predict a positive in-phase variation of the induction with the applied field. A model of ac response in the presen ce of a geometrical barrier is derived and is shown to describe well t he measured behavior. The dissipation peak and the onset of higher har monics at elevated temperatures are due to geometrical barrier and do not reflect a change in the bulk resistivity.