We have investigated the angle and magnetic field dependence of the ac susceptibility in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$ and Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals at low external fields. The ac field was applied perpendicular to the Cu${\mathrm{O}}_2$ planes. The first and third harmonics of the ac susceptibility exhibit remarkably sharp features when the dc field component perpendicular to the Cu${\mathrm{O}}_2$ planes passes a threshold field $H_{\mathrm{th}}$. $H_{\mathrm{th}}$ is strongly temperature dependent, but is independent of the parallel field component. We propose a simple model which excellently explains the data. Within this model the peak structures are related to the irreversibility line. We discuss the implications of the model for the interpretation of the ac susceptibility.

• Received 8 July 1996

DOI:https://doi.org/10.1103/PhysRevB.54.15478