Laser fluence tunable spin transport and ultrafast demagnetization in BiSbTe1.5Se1.5/Co20Fe60B20 bilayers

Topological insulator (TI)/ferromagnet heterostructures hold immense application potential for spin-orbitronic memory technologies owing to strong spin-orbit coupling of TIs combined with ultrahigh spin-charge interconversion efficiency. Here, we use all-optical time-resolved magneto-optical Kerr effect magnetometry to demonstrate fluence-modulated spin pumping and ultrafast demagnetization in $\mathrm{Sub}/\mathrm{BiSbT}{\mathrm{e}}_{1.5}{\mathrm{Se}}_{1.5}(\mathrm{BSTS})/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}(\mathrm{CoFeB})/\mathrm{Si}{\mathrm{O}}_{2}$ thin films. The effective spin-mixing conductance of the BSTS/CoFeB interface is extracted, and the two-magnon scattering (TMS) effect is isolated to reveal the true spin-pumping contribution to the damping enhancement at various laser pump fluences. The demagnetization time and Gilbert damping are found to be inversely correlated in BSTS/CoFeB due to spin-pumping-driven pure spin-current transport and interfacial spin accumulations. The fluence-dependent enhancement of spin pumping at the BSTS/CoFeB interface, surpassing TMS losses, reveals enhanced spin absorption at higher pump fluences, resulting in a factor-of-two enhancement of the spin-mixing conductance. This systematic study of femtosecond to nanosecond magnetization dynamics and pump-fluence-tunable spin pumping in a BSTS/CoFeB heterostructure can aid in the functionalization of TI-based spin-orbitronic devices and the integration of topological spintronics with femtomagnetism.