Modulation of Spin and Lattice Dynamics by Thickness Control of Tb in Si/CoFe/Tb/Al Thin Films

As a significant area of fundamental research for designing and developing spintronic devices, spin pumping has gained great interest for efficient control of spin current relaxation and magnetic damping in the ferromagnet/rare earth bilayer system. In this study, laser-induced ultrafast spin and lattice dynamics were comprehensively investigated in samples with a structure of Si substrate/CoFe (10 nm)/Tb (t_Tb = 0, 2, 5 nm)/Al (5 nm) using optical pump-probe experiments. It is found that the Gilbert damping factor of CoFe can be increased from 0.0096 to 0.0196 after inserting a 5 nm Tb neighbor layer. This increase is attributed to the pumping of magnetization precession energy from CoFe to the highly damped ferromagnetic Tb atoms via the interfacial antiferromagnetic exchange interaction. The extracted effective spin-mixing conductance g_eff^↑↓ in CoFe/Tb is nearly two times that in CoFeB/Pt, validating Tb as an excellent spin sink. As the measurement temperature (T) is reduced below 150 K, magnetization precession almost vanishes for the CoFe/Tb (5 nm) sample due to significantly enhanced damping. Interestingly, accompanied by the diminished magnetization precession at low T, a long-lived coherent lattice vibration mode of acoustic phonon is excited, showing an increased amplitude with t_Tb because of the increased heat accumulation. This study demonstrates that Tb serves not only as a good spin sink layer, but also as a heat sink material for efficient modulation of the ultrafast spin and lattice dynamics.