Ultrastrong magnon-magnon coupling in synthetic antiferromagnets induced by interlayer Dzyaloshinskii-Moriya interaction

Recently, there has been significant interest in magnon-magnon hybrid systems in the field of spintronics due to their potential for quantum engineering. Achieving strong coupling, and even ultrastrong coupling (USC), is crucial for enhancing the efficiency of quantum information processing. While the strong coupling has been widely observed in magnon-magnon hybrid systems, the USC has been rarely reported. Therefore, the current focus of research is to explore more possible mechanisms for realizing magnon-magnon USC. Here, we highlight that the interlayer Dzyaloshinskii-Moriya interaction (DMI) can break the rotation symmetry in synthetic antiferromagnets (SAFs) when the DMI vector D is nonorthogonal to the external dc magnetic field. As a result, magnon-magnon coupling can be induced by the interlayer DMI, as confirmed by both macrospin model and micromagnetic simulations. We suggest that the strength of magnon-magnon coupling can be tuned to a considerable degree by properly selecting the direction of D and adjusting the material parameters of SAF, such as the magnetic anisotropy field, the effective interlayer DMI field, and the effective Ruderman-Kittel-Kasuya-Yosida interaction field. Surprisingly, a sudden increase of coupling strength takes place when the total magnetic anisotropy field changes the sign. We emphasize that within this region, the coupling properties exhibit high sensitivity to anisotropy, and even an extremely small interlayer DMI intensity can induce the USC. The maximum normalized coupling strength in this region can surpass 0.5, which is far in excess of the commonly used value 0.1 that is considered as a requirement for establishing USC. These results place SAFs with interlayer DMI in focus for the future study of magnon-magnon USC, which can be of considerable interest for the experimental research.