Chiral spin soliton lattice: Critical for global 6G developers

A research team from Osaka Metropolitan University has discovered that a magnetic structure called a chiral spin soliton lattice is vital for global 6G developers.

Osaka Metropolitan University (OMU) researchers detected collective resonance at remarkably high and broad frequency bands, for the first time. In a magnetic superstructure known as, ‘chiral spin soliton lattice’ (CSL), they discovered that resonance could occur at such frequencies with small changes in magnetic field strength. The results revealed that CSL-hosting chiral helimagnets are promising materials for future communication technologies, such as 6G.

The CSL phonon modes – or the collective resonance modes – are observed in the chiral magnetic crystal known as ‘CrNb3S6.’ By utilising broadband microwave spectroscopy, scientists detected unprecedented collective resonance modes at remarkably high and broad frequency bands. They discovered that resonance could potentially occur in CSL at beyond-5G frequencies with a small change in the strength of the magnetic field.

This study was recently published in Physical Review Letters.

Discovering the uses for CSL in 6G technology

The competition to recognise sixth-generation (6G) wireless communication systems requires the development of suitable magnetic materials. Scientists from OMU and their colleagues detected 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 research team has noted that future communication technologies require expanding the frequency band from the current few Gigahertz (GHz) to over 100 GHz. Such high frequencies are not currently feasible given that existing magnetic materials utilised in communication equipment can only resonate and absorb microwaves up to approximately 70 GHz with a practical-strength magnetic field. Addressing this gap in knowledge and technology, scientists led by Professor Yoshihiko Togawa from OMU delved into the helicoidal spin superstructure CSL.

“CSL has a tunable structure in periodicity, meaning it can be continuously modulated by changing the external magnetic field strength,” explained Professor Togawa. “The CSL phonon mode, or collective resonance mode ― when the CSL’s kinks oscillate collectively around their equilibrium position ― allows frequency ranges broader than those for conventional ferromagnetic materials.”

Pursuing the CSL phonon mode

The CSL phonon mode has been understood theoretically but never observed in experiments. With the intention of discovering the CSL phonon mode, the team experimented on CrNb3S6, which is a typical chiral magnetic crystal that hosts CSL. They first generated CSL in CrNb3S6 and then examined its resonance behaviour under altering external magnetic field strengths. A specially designed microwave circuit was utilised to detect the magnetic resonance signals.

The research team examined resonance in three modes, namely the Kittel mode, the asymmetric mode, and the multiple resonance mode. In the Kittel mode, similar to what is observed in conventional ferromagnetic materials, the resonance frequency increases only if the magnetic field strength increases, meaning that creating the high frequencies needed for 6G would require an impractically strong magnetic field. The CSL phonon was not discovered in the asymmetric mode, either.

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

“We succeeded in observing this resonance motion for the first time,” concluded first author Dr Yusuke Shimamoto. “Due to its excellent structural controllability, the resonance frequency can be controlled over a wide band up to the sub-terahertz band. This wideband and variable frequency characteristic exceeds 5G and is expected to be utilised in research and development of next-generation communication technologies.”

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