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

Ultrafast spin dynamics in Tbx(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.