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

Abstract

The ultrafast demagnetization processes (UDP) in Ta ([Formula: see text] 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 ([Formula: see text][Formula: see text]nm), we observed the UDP showing a two-step decay behavior, with a relatively longer decay time ([Formula: see text] around 3.0 ps in the second step due to the equilibrium of spin-lattice relaxation following the 4[Formula: see text] occupation. As a 10[Formula: see text]nm Ta layer is deposited, the two-step demagnetization still exists while [Formula: see text] decreases to [Formula: see text]1.9[Formula: see text]ps. Nevertheless, the second-step decay ([Formula: see text]) 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.