Role of the Heat Sink Layer Ta for Ultrafast Spin Dynamic Process in Amorphous TbFeCo Thin Films

The ultrafast demagnetization processes (UDP) in Ta (t nm)/TbFeCo (20 nm) films have been studied using the time-resolved magneto-optical Kerr effect (TRMOKE). With a fixed pump fluence of 2 mJ/cm², for the sample without a Ta underlayer (t=0nm), we observed the UDP showing a two-step decay behavior, with a relatively longer decay time (τ₂) around 3.0 ps in the second step due to the equilibrium of spin-lattice relaxation following the 4f occupation. As a 10nm Ta layer is deposited, the two-step demagnetization still exists while τ₂ decreases to ∼1.9ps. Nevertheless, the second-step decay (τ₂=0ps) disappears as the Ta layer thickness is increased up to 20 nm, only the first-step UDP occurs within 500 fs, followed by a fast recovery process. The rapid magnetization recovery rate strongly depends on the pump fluence. We infer that the Ta layer provides conduction electrons involving the thermal equilibrium of spin-lattice interaction and serves as heat bath taking away energy from spins of TbFeCo alloy film in UDP.