Superconducting transition in Nb/Gd/Nb trilayers

We report the observation of oscillations in the superconducting transition temperature $T_c$ of Nb/Gd/Nb trilayers as the Gd layer thickness $d_{\mathrm{Gd}}$ is varied. The $T_c$ minimum occurs at $d_{\mathrm{Gd}}\approx 13$ Å. The results are consistent with our previous study of Nb/Gd multilayers and are in support of the theoretical prediction of a $\pi$-phase junction in superconductor/ferromagnet/superconductor systems.