Improving the spin mixing conductance at the ferromagnet/heavy metal (FM/HM) interface with a low Gilbert damping constant in the FM layer is a key requirement for developing efficient spintronic devices. To evaluate the potential of ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}$ as the FM layers in such structures, epitaxial ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}$ single-layer and ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}/\mathrm{Pt}$ bilayer films were deposited on MgO substrates using magnetron sputtering, and their Gilbert damping constants were evaluated from ferromagnetic resonance spectra. While the single-layer samples exhibit a low damping constant of $\sim 1.07\times {10}^{-3}$, an enhancement in the damping constant was observed in the bilayer samples due to spin pumping across the ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}/\mathrm{Pt}$ interface, from which relatively large spin mixing conductance ($g_{\mathrm{eff}}^{\uparrow \downarrow }$) of $\left(2.78\pm 0.09\right)\times {10}^{19}{\mathrm{m}}^{-2}$ was evaluated. Further, to investigate the effect of interface property on $g_{\mathrm{eff}}^{\uparrow \downarrow }$, ultrathin ($\sim 0.2$ nm) Cu, Ni, Ru, Ta, or Cr insertion layers were introduced between the ${\mathrm{Co}}_2\mathrm{Fe}{\mathrm{Ga}}_{0.5}{\mathrm{Ge}}_{0.5}$ and Pt. Inserting a nonmagnetic (NM) Cu layer results in a pronounced decrease of $g_{\text{eff}}^{\uparrow \downarrow }$, whereas the FM Ni layer leaves $g_{\text{eff}}^{\uparrow \downarrow }$ nearly unchanged. In the case of Ru and Ta insertion layers, a modest reduction in $g_{\text{eff}}^{\uparrow \downarrow }$ is observed despite their NM nature. The antiferromagnetic Cr insertion layer also causes a small decrease in $g_{\text{eff}}^{\uparrow \downarrow }$. The observed change in $g_{\mathrm{eff}}^{\uparrow \downarrow }$ depends on the insertion material, indicating that their physical properties, such as magnetic, nonmagnetic, antiferromagnetic, light or heavy metal, reflect on $g_{\mathrm{eff}}^{\uparrow \downarrow }$.

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• Magnetization dynamics
• Spin current
• Spin pumping
• Half-metals
• Heusler alloy
• Magnetic thin films
• Ferromagnetic resonance