Eigenmodes of synthetic antiferromagnetic skyrmions

Abstract: We investigate the excitation modes of confined synthetic-antiferromagnetic (SAF) skyrmions using micromagnetic eigenvalue and ringdown simulations. Starting from a single skyrmion in a ferromagnetic layer, where the lowest-frequency modes are a gyrotropic and a breathing mode, we study how antiferromagnetic interlayer coupling modifies the dynamics in SAF bilayers. We consider several geometries: single SAF skyrmions in square and rectangular confinement, unequal layer thicknesses, and strips containing multiple skyrmions. The antiferromagnetic coupling strongly modifies the low-frequency dynamics. The square geometry exhibits two nearly degenerate gyrotropic modes, where in each both layers have the same rotation sense. In rectangular geometries, we instead find nearly linear SAF skyrmion translation emerging from opposite gyration sense in the two layers. These translational modes become the characteristic low-frequency excitations of SAF skyrmion chains. For skyrmion chains, we identify collective translational and breathing modes with standing-wave-like spatial profiles. Beyond ferromagnetic-like breathing modes, the SAF geometry supports breathing oscillations in which the two layers oscillate out of phase. We further demonstrate signal propagation along extended SAF skyrmion chains with propagation velocities comparable to ferromagnetic skyrmion chains. These results provide a systematic description of the collective dynamics of SAF skyrmions arising from the interplay of geometric confinement, intralayer, and interlayer coupling.