Oscillatory Superconducting Transition Temperature in Nb/Gd Multilayers

Abstract

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 ${d}_{\mathrm{Nb}}=500 \mathrm{and} 600$ \AA{}, the ${T}_{c}$ oscillation occurs in the range $10\ensuremath{\le}{d}_{\mathrm{Gd}}\ensuremath{\le}40$ \AA{} where the Gd ferromagnetism is well established. These results provide the first evidence for the predicted $\ensuremath{\pi}$ phase in superconductor/ferromagnet multilayers where the order parameter in neighboring superconducting layers may have an intrinsic phase difference.