  Synthetic antiferromagnet, comprised of two ferromagnetic layers separated by
a non-magnetic layer, possesses two uniform precession resonance modes:
in-phase acoustic mode and out-of-phase optic mode. In this work, we
theoretically and numerically demonstrated the strong coupling between acoustic
and optic magnon modes. The strong coupling is attributed to the symmetry
breaking of the system, which can be realized by tilting the bias field or
constructing an asymmetrical synthetic antiferromagnet. It is found that the
coupling strength can be highly adjusted by tuning the tilting angle of bias
field, the magnitude of antiferromagnetic interlayer exchange coupling, and the
thicknesses of ferromagnetic layers. Furthermore, the coupling between acoustic
and optic magnon modes can even reach the ultrastrong coupling regime. Our
findings show high promise for investigating quantum phenomenon with a magnonic
platform.
