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The oscillation behavior of the superconducting transition temperature Tc as a function of the ferromagnetic Co layer thickness (dCo) has been examined for Nb/Co superconductor(S)/ferromagnetic(F) trilayer series (F/S/F) and pentalayer series (F/S/F/S/F). Tc of the pentalayer series takes a local maximum between dCo=2.0-3.2 nm, where Tc of the trilayer shows a local minimum. This difference in the Tc versus dCo curves provides a clear evidence for the occurrence of the pi phase in the pentalayers, which has been theoretically predicted by Buzdin et al., Radovic et al., and Tagirov.

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