The inphase ac magnetic susceptibility (X′) of Nb/Ni bilayers was measured using a small inplane magnetic field (the drive field). The sample's response changes considerably in the presence of perpendicular stray fields that arise during magnetization reversal of the Ni film. The interplay between stray fields and flux lines inside the superconductor is associated to an enhancement of vortex entrance and exit, suggesting an effective magnetic coupling of the ferromagnetic and superconducting components. As a DC field probes the different configurations of the Ni layer throughout its hysteresis loop, the X′ signal traces a curve with irreversible features that are analogous to those of wellknown magnetoresistance measurements. A modification of the measuring temperature provides access to various kinds of superconductor/ferromagnet interaction.

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