High spin pumping efficiency in Fe 80 Co 20 /Ta bilayers

We have investigated the spin pumping phenomenon in Fe 80 Co 20 /Ta bilayers of different thicknesses in which the ferromagnet grows epitaxially on MgO substrates. We have obtained a Gilbert damping constant for Fe 80 Co 20 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 Fe 80 Co 20 thickness, we extracted the value of the effective spin mixing conductance g ↑↓ = 5.7(3) × 10 14 cm −2 . Analyzing the Ta and Fe 80 Co 20 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 Fe 80 Co 20 /Ta system is up to an order of magnitude more efficient than other metallic ferromagnetic bilayers reported in the literature.