Asymmetric ferromagnet-superconductor-ferromagnet switch

In layered ferromagnet-superconductor-ferromagnet (F-1/S/F-2) structures, the critical temperature T-c of the superconductors depends on the magnetic orientation of the ferromagnetic layers F-1 and F-2 relative to each other. So far, the experimentally observed magnitude of change in T-c for structures utilizing weak ferromagnets has been 2 orders of magnitude smaller than is expected from calculations. We theoretically show that such a discrepancy can result from the asymmetry of F/S boundaries, and we test this possibility by performing experiments on structures where F-1 and F-2 are independently varied. Our experimental results indicate that asymmetric boundaries are not the source of the discrepancy. If boundary asymmetry is causing the suppressed magnitude of T-c changes, it may only be possible to detect in structures with thinner ferromagnetic layers.