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Synthetic antiferromagnets are carefully engineered magnetic materials made up of alternating ferromagnetic layers with oppositely aligned magnetic moments, separated by a non-magnetic spacer. These materials can display interesting magnetization patterns, characterized by swift changes in the behavior of magnetic moments in response to external forces, such as radio frequency (RF) currents.

When the magnetization of each layer in synthetic antiferromagnets is disturbed by an external force, its magnetic moments start to "precess," or in other words, to rotate around their equilibrium direction. Past studies have identified two primary collective spin oscillation modes in synthetic antiferromagnets, influencing how magnetic moments precess.

The first is the acoustic mode, characterized by the synchronized rotation of ferromagnetic layers in the same direction and phase. The second is the optical mode, in which ferromagnetic layers rotate in opposite directions (i.e., with one layer's magnetization tipping up and the other down).

Researchers at Tohoku University and other institutes recently carried out a study to further investigate the interaction between the acoustic and optical modes in synthetic antiferromagnets. Their paper, in Â鶹ÒùÔºical Review Letters, reports the observation of a phenomenon known as Rabi-like splitting that hints at an exchange of energy between the two modes, which appears to arise from nonlinear interactions between three quasiparticles known as magnons.

"This work emerged from the convergence of two distinct research directions," Shigemi Mizukami, co-senior author of the paper, told Â鶹ÒùÔº. "Dr. Aakanksha Sud had previously studied the Rabi-like splitting due to linear mode coupling in synthetic antiferromagnets using electrical methods under broken symmetry.

"In parallel, my colleagues and I had been investigating linear and nonlinear dynamics in similar systems using all-optical techniques. This raised a key question for us: can strong nonlinear coupling emerge without breaking symmetry?"

To answer this research question, the team performed an experiment that combined electrical excitation techniques with nonlinear dynamics. To better understand their observations, they collaborated with theoretical physicist Dr. K. Yamamoto, who helped them to confirm that the clear Rabi-like splitting they observed emerged from three-magnon interactions without breaking symmetry.

"We used an electrical technique called RF rectification, which allows us to excite magnetization dynamics in a non-linear regime," explained Mizukami. "By applying an RF current to a synthetic antiferromagnet—made of two antiferromagnetically coupled ferromagnetic layers—we induced magnetic resonances and monitored the voltage signal they produced."

As part of their experiment, researchers excited a synthetic antiferromagnet using an RF current, which induced oscillations in its magnetic layers. The RF they applied had a driving frequency equal to half of the resonance frequency of the optical mode, as this enabled nonlinear interactions between the acoustic and optical modes.

Under these specific conditions, they found that the spectral peak of the acoustic mode was split into two, a phenomenon referred to as Rabi-like splitting. This phenomenon hints at a coupling between the acoustic and optical modes.

"Our key finding is that the Rabi-like splitting due to the nonlinear magnon coupling can occur in a symmetric system, without relying on the breaking of symmetry," said Mizukami. "This unveiled that intrinsic nonlinearities alone can hybridize magnon modes. These results open new opportunities for contributing to the broader understanding of nonlinear dynamics in condensed matter systems and electrically tunable magnon-based applications."

The results of this recent study could pave the way for further research aimed at exploring nonlinear interactions and multi-mode coupling in synthetic antiferromagnets and other magnetic materials. In the future, they could also contribute to the development of new tunable magnetic and spintronic devices.

"We are now considering how nonlinear coupling affects propagating magnons, not just standing magnetic resonances demonstrated in this study, for further understanding and in view of potential applications based on magnons," added Mizukami.

"We plan to develop new device architectures to control magnon propagation through material design and nanofabrication, to create scalable, low-power platforms for spintronic and neuromorphic computing based on nonlinear magnon dynamics," added Sud.

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