The richness in both the dispersion and energy of antiferromagnetic magnons has spurred the magnetism community to consider antiferromagnets for future spintronic/magnonic applications. However, the excitation and control of antiferromagnetic magnons remains challenging, especially when compared to ferromagnetic counterparts. A middle ground is found with synthetic antiferromagnet metamaterials, where acoustic and optical magnons exist at GHz frequencies. In these materials, the magnon energy spectrum can be tuned by static symmetry-breaking external fields or dipolar interactions hybridizing optical and acoustic magnon branches. Here, we experimentally measure the magnon energy spectrum of synthetic antiferromagnetic tetralayers, and discover avoided energy level crossings in the energy spectrum that are unexplained by the antiferromagnetic interlayer coupling. We explain our experimental results using a phenomenological model incorporating both fieldlike and dampinglike torques generated by spin pumping in noncollinear magnetic multilayers separated by normal-metal spacers. We show that an asymmetry in the fieldlike torques acting on different magnetic layers can lift the spectral degeneracies of acoustic and optical magnon branches and yield symmetry-breaking induced magnon-magnon interactions. Our work extends the phenomenology of spin pumping to noncollinear magnetization configurations and significantly expands ways of engineering magnon-magnon interactions within antiferromagnets and quantum hybrid magnonic materials.
