Extinction and Recovery of Superconductivity by Interference in Superconductor/Ferromagnet Bilayers

In superconductor–ferromagnet (S/F) metallic contacts, the superconducting condensate penetrates through the S/F interface into a ferromagnetic layer. In contrast to the conventional S/N proximity effect, the pairing wave function not only decays deep into the F metal, but simultaneously oscillates. Interference of the oscillating pairing function in a ferromagnetic film gives rise to a modulation of the pairing function flux crossing the S/F interface, which results in oscillations of superconducting transition temperature of the adjacent S layer. In this work, we report on the experimental observation of the superconductivity reentrance phenomenon with double suppression of the superconductivity in Nb ∕ Cu1 − x Ni x bilayers as a function of the ferromagnetic layer thickness, d CuNi. The superconducting T c drops sharply with increasing d CuNi till total suppression of superconductivity at d CuNi ≈ 2. 5 nm. At a further increase of the Nb ∕ Cu1 − x Ni x layer thickness, the superconductivity restores at d CuNi ≥ 24 nm. Then, with the subsequent increase of d CuNi, the superconductivity vanishes again at d CuNi ≈ 38 nm.