The oscillation behavior of the superconducting transition temperature $T_c$ as a function of the ferromagnetic Co layer thickness ($d_{\mathrm{C}\mathrm{o}}$) has been examined for $\mathrm{N}\mathrm{b}/\mathrm{C}\mathrm{o}$ superconductor(S)/ferromagnetic(F) trilayer series (F/S/F) and pentalayer series (F/S/F/S/F). $T_c$ of the pentalayer series takes a local maximum between $d_{\mathrm{C}\mathrm{o}}=2.0–3.2\mathrm{n}\mathrm{m}$, where $T_c$ of the trilayer shows a local minimum. This difference in the $T_c$ versus $d_{\mathrm{C}\mathrm{o}}$ curves provides a clear evidence for the occurrence of the $\pi$ phase in the pentalayers, which has been theoretically predicted by Buzdin et al., Radović et al., and Tagirov.

• Received 14 June 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.057008