Optimizing the Activation of Acoustic and Optical Magnons in Synthetic Antiferromagnets and Ferrimagnets through Field-like and Damping-like Torques*

Synthetic magnetic materials, constructed via thin-film deposition, serve as a robust platform for exploring antiferromagnetic and ferrimagnetic behaviors. In such systems, both acoustic and optical magnon modes can be excited at GHz frequencies using ferromagnetic resonance-based (FMR) techniques. Typically, by employing a system of coupled Landau-Lifschitz-Gilbert (LLG) equations, one can predict magnon frequencies and interactions across a myriad of structures. Despite the general agreement between spectroscopy data and the eigenmodes predicted by the LLG model, a particular mode (or modes) may remain quiescent in the spectroscopy data across a range of external field strengths—an observation that has yet to be adequately explained. We develop a theoretical framework that models the coupling strength of a polarized driving field (as well as polarized damping-like torques) to a magnon mode over a range of external field strengths. We show that our model of coupling strength aligns with previously reported spectroscopy data for various synthetic antiferromagnetic and ferrimagnetic systems. Our work provides a predictive guide for the ability of a spectroscopic method to drive a particular magnon branch.