Clean-limit pair breaking and two-dimensional vortex dynamics in ferromagnet-superconductor heterostructures

Superconductivity in YBa2Cu3O7 films of thickness dY lt;500 λ sandwiched between ferromagnetic and nonmagnetic epitaxial layers has been probed. The irreversibility temperatureTirr(H) in the vortex state maps onto a clean-limit pair-breaking temperature Tc(H), and a universal relation exists between the pair-breaking field Hpb and an effective thickness deff(lt;dY) of the superconductor. The vortex response in these two-dimensional (2D) films indicates a crossover temperature T*, which separates two regimes of flux motion controlled by formation of dislocations (Tgt;T*), and migration of preexisting dislocations and dislocation pairs (Tlt;T*) in the 2D flux line lattice