Research conducted by scientists at Tohoku University and affiliated institutions has revealed a significant phenomenon known as Rabi-like splitting in synthetic antiferromagnets. This finding, published on July 20, 2025, in the journal Physical Review Letters, demonstrates how nonlinear interactions between magnons—quasiparticles associated with magnetic excitations—can influence the behavior of these materials.
Synthetic antiferromagnets are engineered materials composed of alternating ferromagnetic layers, which have opposing magnetic moments. These layers are separated by a non-magnetic spacer. When subjected to external forces, such as radio frequency (RF) currents, the magnetic moments within these layers exhibit rapid changes, leading to complex magnetization patterns.
The researchers focused on two main collective spin oscillation modes in synthetic antiferromagnets: the acoustic mode, where the ferromagnetic layers rotate in synchronization, and the optical mode, characterized by the layers rotating in opposite directions. Previous studies have primarily examined these modes separately, but the current research aims to explore their interaction.
Shigemi Mizukami, co-senior author of the study, explained how the investigation originated from two different research paths. “Dr. Aakanksha Sud had previously studied the Rabi-like splitting due to linear mode coupling using electrical methods under broken symmetry. In parallel, my colleagues and I were exploring nonlinear dynamics in similar systems through all-optical techniques,” Mizukami noted. This approach led to a pivotal question: Could nonlinear coupling occur without breaking symmetry?
To investigate this, the team combined electrical excitation techniques with nonlinear dynamics. They collaborated with theoretical physicist Dr. K. Yamamoto, who helped confirm that the observed Rabi-like splitting resulted from three-magnon interactions without the need for symmetry breaking.
The experimental setup included applying an RF current to a synthetic antiferromagnet made of two ferromagnetic layers coupled antiferromagnetically. This current induced oscillations in the magnetic layers. Notably, the researchers discovered that setting the driving frequency of the RF current to half of the resonance frequency of the optical mode facilitated nonlinear interactions between the modes. Under these conditions, they observed that the spectral peak of the acoustic mode split into two distinct peaks, indicative of Rabi-like splitting.
Mizukami emphasized the significance of their findings: “Our key discovery is that the Rabi-like splitting due to nonlinear magnon coupling can take place in a symmetric system without breaking symmetry. This reveals that intrinsic nonlinearities can hybridize magnon modes.”
The implications of this research extend beyond theoretical interest. The results provide a foundation for further exploration of nonlinear interactions and multi-mode coupling in synthetic antiferromagnets and similar magnetic materials. Future studies could lead to advancements in tunable magnetic and spintronic devices, enhancing the capabilities of these technologies.
Looking ahead, Mizukami stated, “We are now considering how nonlinear coupling influences propagating magnons, in addition to standing magnetic resonances demonstrated in this study.” Aakanksha Sud added, “We aim to develop new device architectures to control magnon propagation through material design and nanofabrication, creating scalable, low-power platforms for spintronic and neuromorphic computing based on nonlinear magnon dynamics.”
This research not only advances the understanding of nonlinear dynamics in condensed matter systems but also opens new avenues for practical applications in the field of magnetism and spintronics.
