Asymmetric ferromagnet-superconductor-ferromagnet switch

In layered ferromagnet-superconductor-ferromagnet (F1 /S/ F2) structures, the critical temperature Tc of the superconductors depends on the magnetic orientation of the ferromagnetic layers F1 and F2 relative to each other. So far, the experimentally observed magnitude of change in Tc 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 F1 and F2 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 Tc changes, it may only be possible to detect in structures with thinner ferromagnetic layers.