Enhancement of ultrafast demagnetization rate and Gilbert damping driven by femtosecond laser-induced spin currents in F e81 G a19/ i r20 M n80 bilayers

In spintronics applications, ultrafast spin dynamics have to be controlled at femtosecond timescales via femtosecond laser radiation. At such ultrafast timescales, the effect of the Gilbert damping factor α on ultrafast demagnetization time τM should be considered. In previous explorations for the relationship between these two parameters, it was found that the theoretical calculations based on the local spin-flip scattering model do not agree with the experimental results. Here, we find that in Fe81Ga19(FeGa)/Ir20Mn80(IrMn) bilayers, the unconventional IrMn thickness dependence of α results from the competition between spin currents pumped from the ferromagnetic (FM) FeGa layer to the antiferromagnetic (AFM) IrMn layer and those pumped from the AFM layer to the FM layer. More importantly, we establish a proportional relationship between the change of the ultrafast demagnetization rate and the enhancement of Gilbert damping induced by the spin currents via interfacial spin chemical potential μs. Our work builds a bridge to connect the ultrafast demagnetization time and Gilbert damping in ultrafast photoinduced spin-current-dominated systems, which not only explains the disagreement between experimental and theoretical results in the relation of τM with α but provides further insight into ultrafast spin dynamics as well.