We have investigated the spin pumping phenomenon in Fe80Co20/Ta bilayers of different thicknesses in which the ferromagnet grows epitaxially on MgO substrates. We have obtained a Gilbert damping constant for Fe80Co20 as low as α = 1.3(1) × 10−3, which is smaller than values generally reported in metallic ferromagnets. From the dependence of α with the Fe80Co20 thickness, we extracted the value of the effective spin mixing conductance
g
↑↓
= 5.7(3) × 1014 cm−2. Analyzing the Ta and Fe80Co20 thickness dependence of the inverse spin Hall effect voltage, we obtained the transport parameters
λ
sd
= 1.1(3) nm and
Θ
SH
= –0.024(3). We have developed an expression to quantify the efficiency of the spin pumping phenomenon in ferromagnetic/nonmagnetic metal bilayers. The calculated efficiency shows that the Fe80Co20/Ta system is up to an order of magnitude more efficient than other metallic ferromagnetic bilayers reported in the literature.
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