Oscillatory Superconducting Transition Temperature in Nb/Gd Multilayers

The superconducting transition temperature $T_c$ in Nb/Gd multilayers shows a pronounced nonmonotonic dependence on the ferromagnetic Gd layer thickness $d_{\mathrm{Gd}}$ for fixed Nb thickness $d_{\mathrm{Nb}}$. For $d_{\mathrm{Nb}}=500\mathrm{} \mathrm{and} 600$ Å, the $T_c$ oscillation occurs in the range $10\le d_{\mathrm{Gd}}\le 40$ Å where the Gd ferromagnetism is well established. These results provide the first evidence for the predicted $\pi$ phase in superconductor/ferromagnet multilayers where the order parameter in neighboring superconducting layers may have an intrinsic phase difference.