Layer-Dependent Magnon Mode Merging  in Synthetic Antiferromagnets

We report broadband magnetic resonance studies of hexalayer, octalayer, and decalayer synthetic antiferromagnets (SAFs) composed of permalloy layers antiferromagnetically coupled through ruthenium spacers and capped with platinum. In the hexalayer SAF, the magnon spectra reveal multiple dispersive branches that merge symmetrically near zero field, indicating strong interlayer exchange coupling and dissipative magnon hybridization. This mode merging reflects tunable level attraction governed by spin-pumping–induced fieldlike and dampinglike torques. In contrast, the octalayer and decalayer SAFs exhibit spectral features resembling those of the tetralayer and bilayer, respectively, indicating a layer-number-dependent evolution of magnon spectra. These findings demonstrate the potential for engineering magnon band structures through multilayer design and spin-pumping control. Ongoing angular-dependent resonance studies are expected to further elucidate the underlying coupling and damping mechanisms.