Magnetic Instabilities along the Superconducting Phase Boundary of Nb/Ni Multilayers

We report vibrating reed and SQUID magnetometer data that exhibit prominent cusps or oscillations of the SC onset temperature, $\vert \Delta $T$_{C}$(H)$\vert \quad \approx $ 0.01 to 0.7 K, for a [Nb(23nm)/Ni(5nm)]$_{5}$ multilayer (ML) in DC magnetic fields applied nearly parallel to the ML plane. The vibrating reed data exhibit additional structures below T$_{C}$ that may mark multiple SC transitions or vortex lattice rearrangements within the ML. This striking behavior would then pose new challenges for theoretical and experimental investigations of SC/FM interfaces that involve ``pi phase shifts'' of the SC order parameter and exotic (``LOFF'') pairing states. Alternatively, the anomalies may signal dynamical instabilities within a confined, strongly anisotropic Abrikosov vortex lattice.