Symmetry breaking induced magnon-magnon coupling in synthetic antiferromagnets

TL;DR

This paper develops a theory explaining how symmetry-breaking in synthetic antiferromagnets leads to magnon-magnon coupling, resulting in hybridized modes and frequency gaps observable in ferromagnetic resonance spectra.

Contribution

It introduces a comprehensive theory linking symmetry-breaking mechanisms to magnon-magnon coupling and spectral features in synthetic antiferromagnets.

Findings

Symmetry breaking causes hybridization of resonance modes.

An indirect gap appears in ferromagnetic resonance frequencies.

The theory matches experimental observations of frequency gaps.

Abstract

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.