  We propose a general theory of microwave absorption spectroscopy for
symmetry-breaking synthetic antiferromagnets (SAFs). Generally, inhomogeneity
or different thickness of the two ferromagnetic sublayers of a SAF results in
the intrinsic symmetry breaking, while out-of-plane components of dc magnetic
fields lead to the extrinsic one. The broken symmetry of SAFs excludes the
original symmetry-protected crossing between pure in-phase and out-of-phase
resonance modes with opposite parity. Alternatively, new frequency branches
become hybridization of original bare modes in terms of
symmetry-breaking-induced magnon-magnon coupling, which results in an indirect
gap in ferromagnetic resonance frequencies. Also, the dependence of gap width
on the degree of symmetry breaking for several typical cases are presented and
compared with existing experiments. Our theory provides a simple but physical
understanding on the rich structure of ferromagnetic resonance spectra for
asymmetric SAFs.
