Magnetic superstructures resonate with global 6G developers

Researchers at Osaka Metropolitan University observed unprecedented collective resonance movement in chiral helimagnets that allow a boost in current frequency bands.

When will 6G become a reality? The race to realize sixth generation (6G) wireless communication systems requires the development of suitable magnetic materials. Scientists from Osaka Metropolitan University and their colleagues discovered an unprecedented collective resonance at high frequencies in a magnetic superstructure called a chiral spin-soliton lattice (CSL), revealing CSL-hosting chiral helimagnets as a promising material for 6G technology. The study was published in Physical Review Letters.

Future communication technologies require an extension of the frequency band from the current few gigahertz (GHz) to more than 100 GHz. Such high frequencies are not yet possible, as existing magnetic materials used in communication equipment can only resonate and absorb microwaves up to about 70 GHz with a practically strong magnetic field. To address this gap in knowledge and technology, the research team led by Professor Yoshihiko Togawa of Osaka Metropolitan University delved into the spiral spin superstructure CSL. “CSL has a tunable structure in periodicity, meaning it can be continuously modulated by changing the external magnetic field strength,” explains Professor Togawa. “The CSL phonon mode, or collective resonance mode — when the CSL’s kinks collectively oscillate around their equilibrium position — allows for frequency ranges wider than those for conventional ferromagnetic materials.” This CSL phonon mode is theoretically understood, but never observed in experiments.

Looking for the CSL phonon mode, the team experimented on CrNb3s6, a typical chiral magnetic crystal harboring CSL. They first generated CSL in CrNb3s6 and then observed its resonance behavior under changing external magnetic field strengths. A specially designed microwave circuit was used to detect the magnetic resonance signals.

The researchers observed resonance in three modes, namely the “Kittel mode”, the “asymmetric mode” and the “multiple resonance mode”. In Kittel mode, similar to what is observed in conventional ferromagnetic materials, the resonance frequency only increases as the magnetic field strength increases, meaning that creating the high frequencies necessary for 6G would require an impractically strong magnetic field. The CSL phonon was also not found in the asymmetric mode.

In the multiple resonance mode, the CSL phonon was detected; Contrary to what is observed with magnetic materials currently in use, the frequency increases spontaneously when the magnetic field strength decreases. This is an unprecedented phenomenon that will potentially enable a boost to over 100 GHz with a relatively weak magnetic field – this boost is a much needed mechanism to achieve 6G operability.

“We managed to observe this resonance movement for the first time,” noted first author Dr. Yusuke Shimamoto op. “Thanks to the excellent structural controllability, the resonant frequency can be controlled over a wide band up to the sub-terahertz band. This broadband and variable frequency characteristic exceeds 5G and is expected to be used in research and development of next-generation communication technologies.”

Reference:

  1. 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 Assessment Letters, 2022; 128 (24) DOI: 10.1103/PhysRevLett.128.247203
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