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 local 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 presence of a geometrical barrier is derived and is shown to describe well the measured behavior. The dissipation peak and the onset of higher harmonics at elevated temperatures are due to geometrical barrier and do not reflect a change in the bulk resistivity.