Symmetry breaking induced magnon-magnon coupling in synthetic antiferromagnets

Abstract

We propose a general theory of ferromagnetic resonance (FMR) spectroscopy for symmetry-breaking synthetic antiferromagnets (SAFs). Generally, both inhomogeneity and different thickness of the two ferromagnetic sublayers of a SAF result in the intrinsic symmetry breaking, thus excludes the 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, hence generate an indirect gap in FMR frequencies. The gap widths for several typical cases are presented and compared with existing data. In particular, for the inhomogeneity case, the indirect gap width is linearly proportional to the asymmetry degree and takes a square (rather than linear) dependence on the magnon-magnon coupling strength, indicating a case-by-case recalibration of the measuring method of the latter. Our theory provides a simple but physical understanding of the rich structure of FMR spectra for asymmetric SAFs.