Anomalously Weak Cooper Pair-breaking by Exchange Energy in Ferromagnet/Superconductor Bilayers

We report the superconducting transition temperature T _c versus thickness d _F of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni _0.81 Fe _0.19 (Permalloy), Co _0.5Fe _0.5) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness d _F where T _c versus d _F begins to flatten out. Using this rearranged theory, we conclude 1) the effective exchange energy, E _{e x} , is about 15 times smaller than measured by ARPES and five times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξ _F , for Cooper pairs in F is larger than the electron mean free path, ? _F ; and 3) the 3d-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.