When will the sixth generation 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 have detected an unprecedented collective resonance at high frequencies in a magnetic superstructure called a spin-helical soliton (CSL) network, revealing the existence of CSL-hosting helical magnets as a promising material for 6G technology. The study was published in Physical Review Letters.
Future communication technologies require frequency bandwidth expansion from the current few gigahertz (GHz) to more than 100 GHz. Such high frequencies are not yet possible, given that the current magnetic materials used in communication equipment can only resonate and absorb microwaves up to about 70 GHz with a magnetic field of practical strength. To address this gap in knowledge and technology, the research team led by Professor Yoshihiko Togawa of Osaka Metropolitan University delved into the superstructure of the CSL helical spindle.
Professor Togawa explained that “CSL has a tunable structure in periodicity, which means that it can be continuously modified by changing the intensity of the external magnetic field.” “CSL’s phonon mode, or collective resonance mode—when the kinks of a CSL oscillate collectively about their equilibrium position—allows for wider frequency ranges than those of conventional magnetic materials.” This CSL phonon mode is understood in theory, but has never been observed in experiments.
In search of CSL phonon mode, the team experimented with CrNb3s6, a typical chiral magnetic crystal hosts a CSL. They first create a CSL in CrNb3s6 Then he observed its resonant behavior under changing external magnetic field strength. A specially designed microwave circuit was used to detect the magnetic resonance signals.
The researchers observed resonances in three modes, namely ‘kettle mode’, ‘asymmetric mode’ and ‘multiple resonance mode’. In the Kittel mode, similar to what is observed in conventional magnetic materials, the resonance frequency increases only if the magnetic field strength increases, which means that creating the high frequencies needed for 6G requires an impractically strong magnetic field. No CSL phonon was found in the asymmetric mode either.
In the multi-resonance mode, a CSL phonon is detected; Contrary to what is observed with magnetic materials currently in use, the frequency increases automatically when the magnetic field strength decreases. This is an unprecedented phenomenon that would enable boosting above 100 GHz with a relatively weak magnetic field – this boosting is a much needed mechanism to achieve 6 GHz operability.
“We have succeeded in observing this resonance movement for the first time,” first author Dr. Yosuke Shimamoto noted. “Due to its excellent structural controllability, the resonant frequency can be controlled over a broad band of up to sub terahertz. The broadband and variable frequency characteristic goes beyond 5G and is expected to be used in the research and development of next-generation communication technologies.”
Y. Shimamoto et al, Observation of collective resonance modes in a Chiral Spin Soliton Lattice with tunable Magnon dispersion, Physical Review Letters (2022). DOI: 10.1103/ PhysRevLett.128.247203
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