Enhancement of ultrafast demagnetization rate and Gilbert damping driven by femtosecond laser-induced spin currents in Fe81Ga19/Ir20Mn80 bilayers

TL;DR

This study demonstrates how femtosecond laser-induced spin currents in FeGa/IrMn bilayers enhance ultrafast demagnetization rates and Gilbert damping, revealing a proportional relationship and resolving previous theoretical-experimental discrepancies.

Contribution

It uncovers the unconventional IrMn thickness dependence of Gilbert damping and establishes a direct link between ultrafast demagnetization rate and damping enhancement via spin currents.

Findings

IrMn thickness affects Gilbert damping due to competing spin currents.

Ultrafast demagnetization rate is proportional to Gilbert damping enhancement.

The work explains previous theoretical and experimental discrepancies.

Abstract

In spintronics applications, ultrafast spin dynamics have to be controlled at femtosecond (fs) timescales via fs-laser radiation. At such ultrafast timescales, the effect of the Gilbert damping factor {\alpha} on ultrafast demagnetization time 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 {\alpha} 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 . Our work builds a bridge to connect the ultrafast demagnetization time and Gilbert damping in ultrafast photo-induced spin currents dominated systems, which not only explains the disagreement between experimental and theoretical results in the relation of {\tau}_M with {\alpha}, but provides further insight into ultrafast spin dynamics as well.