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We have deposited polycrystalline Re doped $(Fe_{65}Co_{35})_{100-x}Re_{x}$ (0 $\leq$ x $\leq$ 12.6 at\%) thin films grown under identical conditions and sandwiched between thin layers of Ru in order to study the phenomenon of spin pumping as a function of Re concentration. In-plane and out-of-plane ferromagnetic resonance spectroscopy results show an enhancement of the Gilbert damping with an increase in Re doping. We found evidence of an increase in the real part of effective spin mixing conductance [Re($g^{\uparrow\downarrow}_{eff}$)] with the increase in Re doping of 6.6 at\%, while a decrease is evident at higher Re doping. The increase in Re($g^{\uparrow\downarrow}_{eff}$) can be linked to the Re doping induced change of the interface electronic structure in the non-magnetic Ru layer and the effect interfacial spin-orbit coupling has on the effective spin-mixing conductance. The lowest and highest values of Re($g^{\uparrow\downarrow}_{eff}$) are found to be 9.883(02) $nm^{-2}$ and 19.697(02) $nm^{-2}$ for 0 at\% and 6.6 at\% Re doping, respectively. The saturation magnetization decreases with increasing Re doping, from 2.362(13) T for the undoped film to 1.740(03) T for 12.6 at\% Re doping. This study opens a new direction of tuning the spin-mixing conductance in magnetic heterostructures by doping of the ferromagnetic layerr, which is essential for the realization of energy efficient operation of spintronic devices.

The prime requirements for the spin transfer torque based ferromagnetic (FM)/nonmagnetic (NM) bilayer spin devices are (i) the absence of two-magnon scattering (TMS) noise, (ii) minimum energy dissipation and (iii) fast switching. To realize these objectives we have studied the thickness, Py (permalloy) thicknesses (t(Py)) and beta-Ta thicknesses (t(Ta)), dependent magnetization dynamics behaviour of the epitaxial Py (t(Py) = 3-20 nm)/beta-Ta (t(Ta) = 1.5-15 nm) system. The t(Py) dependence of TMS in epitaxial Py nanolayers (t(Py) = 3-20 nm) grown on a Si(400)/TiN(200) (8 nm) substrate is explored in terms of uniform and non-uniform magnetization precession regimes by employing ferromagnetic resonance field (H-r), linewidth (Delta H), and Gilbert damping constant (alpha) behaviour. It is found that in Py, t(Py) < 10 nm, layers TMS is dominating due to non-uniform precession of the magnetization. However in Py, t(Py) >= 10 nm, layers the uniform magnetization precession dominates, therefore Py layers, t(Py) >= 10 nm, are almost free from TMS. Furthermore, a nearly TMS free 12 nm epitaxial Py(200) layer is capped with beta-Ta (t(Ta) = 1.5-15 nm) layers to explore the t(Ta) dependent magnetization precession of epitaxial Py (12 nm) in terms of change in effective Gilbert damping constant (alpha(eff)). An anomalous decrease in alpha(eff) from 0.0087 at t(Ta) = 0 to a minimum value of 0.0077 at t(Ta) = 6 nm, and its subsequent increase for t(Ta) > 6 nm are observed in the epitaxial Py (12 nm)/beta-Ta(t(Ta)) system. Therefore the Si(400)/TiN(200) (8 nm)/Py(200) (12 nm)/beta-Ta(200) (6 nm) epitaxial system with nearly uniform magnetic precession and minimum effective Gilbert damping is suitable for low energy loss and ultrafast switching applications in spin transfer torque devices.