Coupling phenomena and vortex transitions in superconducting Ni/Nb multilayers

We have studied some structural and superconducting properties of Ni/Nb multilayered films prepared by magnetron sputtering. Magnetization measurements allowed us to establish a rich H · T diagram that indicates a 3D-2D dimensional crossover of the vortex system, as revealed by the parallel upper critical field behavior. Consistently, it was identified a possible decoupling line associated with the transition of Abrikosov vortex lines into vortex pancakes, starting around the same crossover region. An irreversibility line was also determined and it suggests different regimes of the vortex matter, where the role played by the Ni ferromagnetic layers might be relevant