Researchers at Osaka Metropolitan University have observed an unprecedented collective resonance motion in chiral helimagnets that allow current frequency bands to be increased.
When will 6G be a reality? The race to achieve sixth-generation (6G) wireless communication systems requires the development of suitable magnetic materials. Scientists from Osaka Metropolitan University and their colleagues have detected an unprecedented high-frequency collective resonance in a magnetic superstructure called a chiral spin soliton (CSL) lattice, revealing that chiral helimagnets harboring CSL are a promising material for 6G technology. The study was published in Physical Review Letters.
Future communication technologies require extending the frequency band from the current few gigahertz (GHz) to over 100 GHz. Such high frequencies are not yet possible since existing magnetic materials used in communications equipment can only resonate and absorb microwaves up to about 70 GHz with a magnetic field of practical strength. To fill this gap in knowledge and technology, the research team led by Professor Yoshihiko Togawa of Osaka Metropolitan University looked into the CSL helical spin superstructure. “The CSL has a periodicity-tunable structure, which means that it can be continuously modulated by changing the strength of the external magnetic field,” Prof. Togawa explained. “The CSL phonon mode, or collective resonance mode – when the folds of the CSL collectively oscillate around their equilibrium position – allows for wider frequency ranges than those of conventional ferromagnetic materials.” This CSL phonon mode has been understood theoretically, but never observed experimentally.
In search of the CSL phonon mode, the team experimented on CrNb3S6, a typical chiral magnetic crystal that hosts CSL. They first generated CSL in CrNb3S6 then observed its resonance behavior under changing external magnetic field intensities. A specially designed microwave circuit was used to detect magnetic resonance signals.
The researchers observed resonance in three modes, namely “Kittel mode”, “asymmetric mode” and “multiple resonance mode”. In the Kittel mode, similar to what is observed in conventional ferromagnetic materials, the resonant frequency only increases as the magnetic field strength increases, meaning that creating the high frequencies needed for 6G would require a field weak magnetic. The CSL phonon was also not found in asymmetric mode.
In the multiple resonance mode, the CSL phonon was detected; contrary to what is observed with the magnetic materials currently used, the frequency increases spontaneously when the intensity of the magnetic field decreases. This is an unprecedented phenomenon that will eventually allow an increase to over 100 GHz with a relatively weak magnetic field – this increase is a necessary mechanism to achieve 6G operability.
“We were able to observe this resonance motion for the first time,” noted first author Dr. Yusuke Shimamoto. “Due to its excellent structural controllability, the resonant frequency can be controlled over a wide band down to the sub-terahertz band. This wideband, variable-frequency characteristic surpasses 5G and should be used in the research and development of next-generation communication technologies.
- Y. Shimamoto, Y. Matsushima, T. Hasegawa, Y. Kousaka, I. Proskurin, J. Kishine, AS Ovchinnikov, FJT Goncalves, Y. Togawa. Observation of collective resonance modes in a chiral spin soliton lattice with tunable Magnon dispersion. Physical Examination Letters, 2022; 128 (24) DOI: 10.1103/PhysRevLett.128.247203
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