High Spin Pumping Efficiency in Fe80Co20/Ta Bilayers

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 λsd = 1.1(3) nm and ΘSH = –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.