Effect of the spin-twist structure on the spin-wave dynamics in Fe55Pt45/Ni80Fe20 exchange coupled bi-layers with varying Ni80Fe20 thickness

We have investigated optically induced ultrafast magnetization dynamics of a series of Fe55Pt45/Ni80Fe20 exchange spring bi-layers with varying Ni80Fe20 thickness. Rich spin-wave spectra are observed; whose frequency shows a strong dependence on the Ni80Fe20 layer thickness. Micromagnetic simulations based on a simplified magnetic microstructure were able to reproduce the experimental data qualitatively. The spin twist structure introduced in the Ni80Fe20 layer gives rise to new modes in the composite system as opposed to the bare Ni80Fe20 films.