Layer-dependent magnon-magnon coupling in a synthetic antiferromagnet

The synthetic antiferromagnet (SAF) offers a versatile platform to couple with various quasiparticles that is an indispensable element for the realization of hybrid quantum computing systems. In this work, we found that the magnon-magnon coupling strength in layered SAFs, consisting of repeated layers of [Ni₈₀Fe₂₀/Ru]ₙ, is highly dependent on the finite layer number n. In particular, the self-hybridizations of individual acoustic or optical modes in the even-layers lead to the opening of local anticrossing gaps within the same type of mode in the resonance spectra. With the structural asymmetry of the odd-layers, a global gap between different modes can be generated whose size is found to decrease with increasing layer number. By combining the static linear chain model with the dynamically coupled Landau-Lifshitz equations, we further formulated a theoretical approach to qualitatively describe such highly layer-dependent behaviors of magnon-magnon coupling in SAFs. It is clear that the layer number is a significant factor that influences both the number of excited modes and the associated mode hybridizations. Our findings shed light on the magnon-magnon coupling in layered antiferromagnets that may invigorate the development of magnonics.