Correlation Between Ultrafast Demagnetization Process and Gilbert Damping in Amorphous TbFeCo Films

Ultrafast spin dynamics in Tb_x(FeCo)_1-x films is studied using the time-resolved magneto-optical Kerr effect. At pump fluence of 1.0 mJ/cm² the ultrafast demagnetization processes are accomplished by one-step and two-step decays (the first fast and second slow decays) for low and high Tb contents, respectively. For the low Tb contents (x <; 0.08), the demagnetization time is almost a constant (~ 600 fs), which is similar with the demagnetization time (τ₁) of the first fast decay for the high Tb contents due to bleaching effect. However, the following slow demagnetization time (τ₂) for the high Tb contents strongly depends on the Tb contents. It increases with increasing Tb contents, and reaches a maximum of 3.0 ps at x=0.21, then decreases with Tb content further increasing. The distinguished slow decay process is suggested to ascribe to the Gilbert damping factor.