CONICET | Buscador de Institutos y Recursos Humanos

FeCo alloys have been widely studied for decades because of their unique metallic and magnetic properties. In recent years the search of new systems adequate for spintronic devices promoted the study of bilayers constituted by a ferromagnetic (FM) film in contact with a nonmagnetic conductive metal (NM) (Ta, W, Os, Pt, Au, etc.). In these bilayers when the FM is exposed to microwave radiation it can be driven to its ferromagnetic resonance (FMR) condition to produce the injection of a pure spin current through the FM/NM interface. In this phenomenon, known as spin pumping, a pure spin current is the result of an additional relaxation mechanism that appears in the bilayer mediated by the NM conduction electrons.For applications in spintronics it is desirable that the injected spin current (Js) be as high as possible. The spin current intensity depends on intrinsic parameters of both the FM and the NM layers. The damping constant (α), the effective spin mixing conductance (g↑↓), the spin Hall angle (ΘSH) and the spin diffusion length (λsd) are among the most critical parameters that determine the values of Js. In recent years, great efforts have been made in the search for new materials that considerably improve these parameters [1,2]. Yttrium iron garnet (YIG) and permalloy (Py) are among the most studied as a FM layer. YIG thin films have a very low damping constant (α ~ 10-4) [2] which is a desired property for an efficient spin current injection. However, when applications require conductive materials, the relatively large damping constant of most ferromagnets (α ~ 10-2) [3] is detrimental to the spin current efficiency due to the Js ~ 1⁄α2 dependence.In this work 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 FM/NM 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.