Strong magnon-magnon coupling between ferromagnetic resonances in Co₉₀Zr₁₀/Ta/Fe₂₀Ni₈₀ multilayers

We investigate the magnon-magnon coupling between the ferromagnetic resonances (FMRs) from different ferromagnetic layers in Co90Zr10(100 nm)/Ta/Fe20Ni80(100 nm) multilayers by using broadband ferromagnetic resonance. The strong coupling between two FMRs through interlayer exchange interaction is realized due to the fact that the thicknesses of both the ferromagnetic layers far exceed the corresponding exchange length. Moreover, we find that the coupling strength can be tuned by changing the thickness of the nonmagnetic Ta layer, revealing inversely proportional relationship between the coupling strength and the thickness of the nonmagnetic Ta layer. The coupling field and coupling efficiency can be regulated by varying the anisotropy of the ferromagnetic layers via oblique deposition. Furthermore, the coupling between FMR and perpendicular standing spin waves can also be achieved with suitable anisotropy and external magnetic field. These observations demonstrate that the ground state magnonic system provides a simple ideal platform for strong magnon-magnon coupling.