Real time evidence of two-magnon scattering in exchange coupled bilayers

Coherent spin waves in exchange biased bilayers have been excited by ultrafast photomodulation of the exchange bias anisotropy. Vector and time resolved Kerr effect was employed to trace the trajectory of the magnetization vector of the ferromagnetic layer in real time on the picosecond timescale. Photoinduced easy axis precession in CoFe/IrMn samples with different IrMn thicknesses, thus, different exchange bias fields, has been studied. The antiferromagnetic thickness dependence of the decay rate of the induced precession was investigated as a measure of the effective magnetic damping in the system.
The observed increase of the extracted effective damping parameter is proportional to the square of the exchange bias field. Two-magnon scattering of the coherent spin precession of the ferromagnetic layer at local interfacial fluctuations of the exchange bias field can account for an additional dissipation mechanism. The two-magnon damping field in exchange bias systems is proportional to the square of the exchange bias field. Hence, there is time-domain evidence of two-magnon damping involved in magnetic relaxation processes of photoexcited exchange coupled bilayers.