Crystallographic direction related spin current transmission in MgO(001)/Fe0.79Si0.21(001)/Pt(111) epitaxial bilayers

Spin-pumping and spin-orbit torque efficiencies in ferromagnetic/heavy metal bilayers, which usually consist of the polycrystalline grains, are related to the spin current transmission from ferromagnets to heavy metals and vice versa, respectively. In this work, epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$ films were fabricated to investigate the spin-pumping efficiency and spin-orbit torque efficiency by ferromagnetic resonance and harmonic Hall resistance measurement techniques, respectively. Ferromagnetic resonance results show that the incremental magnetic damping constants in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$, compared with those of the epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Al}{\mathrm{O}}_{x}$ reference sample, depend on whether the external magnetic field is applied along the in-plane easy or hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$. The Pt thickness-dependent anisotropic damping constant was ascribed to the anisotropic spin current absorption in epitaxial Pt(111) layer. When electric currents were applied along the easy and hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$ in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$ bilayers, a large difference between spin-orbit torques generated from Pt(111) was observed by a harmonic Hall resistance measurement method. Both of the results suggest that spin current transmission efficiency is related to the anisotropic spin Hall effect of epitaxial Pt(111) layer due to the different spin-orbit interaction energies along the different crystallographic directions of Pt.