Realizing ultrastrong magnon-magnon coupling between ferromagnetic resonances in ferromagnetic coupled bilayers

In this work, we studied the magnon-magnon coupling between ferromagnetic resonances in simple $\mathrm{Ni}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ (CFB) bilayers. The coupling efficiency can be enhanced to 0.214 by increasing the interlayer exchange through precise adjustment of the ferromagnetic (FM) layer thickness, enabling the system to reach the ultrastrong coupling (USC) regime. Moreover, the insertion of different thicknesses of a nonmagnetic Ta layer in the bilayer with the strongest coupling, the USC can also be modulated by changing interlayer exchange interaction through varying the thickness of the Ta interlayer in the Ni/Ta/CFB trilayers. Our work provides a path to achieve the ultrastrong magnon-magnon coupling in a very simple FM bilayer and exhibits its potential applications in magnonic devices.