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

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We report the superconducting transition temperature Tc vs. thickness dF of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni₀.₈₁Fe₀.₁₉ (Permalloy), Co₀.₅Fe₀.₅) 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 dF where Tc vs. dF begins to flatten out. Using this rearranged theory we conclude: 1) the effective exchange energy, Eex, is about 15 times smaller than measured by ARPES and 5 times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξF, for Cooper pairs in F is larger than the electron mean free path, ℓF; 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.