<p>Spin-transfer torque (STT) in magnetoresistance devices has enabled key applications such as STT-magnetoresistive random access memory, spin torque oscillators, and energy-assisted magnetic recording. In the device structures, where a free layer (FL) magnetization is manipulated by spin injection from a spin injection layer (SIL), the critical current density required for operation is directly proportional to the damping (<i>α</i>) constant of FL and inversely proportional to the STT efficiency, which depends on the spin polarization (<i>P</i>) of the materials. Here, we investigate the effect of low <i>α</i> and high <i>P</i> of Co₂FeGa_0.5Ge_0.5 (CFGG) Heusler alloy on the operation current required for STT-induced magnetization reversal in current perpendicular-to-plane giant magnetoresistance devices. Devices with CFGG as a FL material achieved a large reduction in the operation current, as compared to those with conventional NiFe-FL owing to the very low <i>α</i> of CFGG, demonstrating the advantage of CFGG as a FL material. As the advantage of high spin polarization CFGG for SIL, we analyzed the effect of bilayer SIL consisting of CoFe and thin CFGG layers, focusing on utilizing the spin scattering asymmetry at the CoFe/CFGG interface. Devices with the CoFe/CFGG-SIL exhibited the lowest critical current, demonstrating enhanced STT efficiency. In addition, the correlation of STT efficiency with magnetoresistance ratio was comprehensively investigated, showing that device-to-device distribution in STT-efficiency was smaller in CoFe/CFGG-SIL. These findings highlight the potential of CFGG Heusler alloy and CoFe/CFGG bilayer structures as key components for the development of efficient and stable STT-based spintronic devices.</p>