Role of Spin Transport through the β-Ta/Co₂₀Fe₆₀B₂₀ Interface on its Ultrafast Demagnetization: Implications for Ultra-High-Speed Spin-Orbitronic Devices

Femtosecond laser-induced spin manipulation has fueled intense interest but understanding its underlying microscopic mechanisms in complex systems such as nonmagnet (NM)/ferromagnet (FM) heterostructures having strong spin-orbit effect is still at its infancy. Here, we report on the ultrafast demagnetization, remagnetization, and damping study in β-Ta(t nm)/Co20Fe60B20(d nm)/SiO2(2 nm) thin-film heterostructures by systematically varying both the Ta and Co20Fe60B20 thicknesses. Based on experimental results, we have established a direct relationship between the variation of the demagnetization rate and the Gilbert damping constant, indicating interface spin transport as the prevailing mechanism for both the ultrafast demagnetization and the damping. At higher thicknesses (t ≥ 7 nm) of β-Ta, the spin accumulation coefficient is found to be ∼0.24 eV, which is about 1.8 times less than its value in the lower thickness regime (t < 7 nm). These results will have important implications toward the evolution of ultra-high-speed spin-orbitronic devices.