When spin and sound coexist: Physicists at RPTU generate hybrid spin-sound waves
Acoustic frequency filters, which convert electrical signals into miniaturized sound waves, separate the different frequency bands for mobile communications, Wi-Fi, and GPS in smartphones. Physicists at RPTU University Kaiserslautern-Landau have now been able to show that such miniaturized sound waves can couple strongly with spin waves in yttrium iron garnet. This results in novel hybrid spin-sound waves in the gigahertz frequency range. The use of such nanoscale hybrid spin-sound waves opens up potential for agile frequency filters for the upcoming 6G mobile communications generation. The fundamental study by the RPTU researchers has been published in the journal Nature Communications.
Surface acoustic waves (SAWs) are ubiquitous. They unleash destructive power in the form of earthquake waves and, at the same time, perform their service completely unnoticed, but used billions of times over, in the form of miniaturized frequency filters in the gigahertz frequency range in smartphones.
The RPTU research team led by Professor Mathias Weiler is working on opening up new fields of application for such miniaturized sound-based microwave components. The key to this is the interconnection of established SAW technology with spin phenomena. "Sound waves can propagate not only in air, but also in matter. This causes the lattice atoms of the material to oscillate," explains the physicist. Since the electrons of the lattice atoms have a quantum mechanical angular momentum, known as spin, this can also be excited to oscillate. The sound waves then generate spin waves in magnetically ordered materials.
Sound waves and spin waves coexisting
The research team investigated such collective acoustic excitations of spins in the ferrimagnetic insulator yttrium iron garnet (YIG). YIG exhibits an extremely long spin wave lifetime, making it an ideal object of study. The recently published paper shows that hybrid excitations—known as magnon-polaron excitations—can be formed in a nanostructured acoustic surface wave resonator. "We have observed that the quantum mechanical coupling of spin and sound can lead to the formation of a novel chimeric wave that is neither a sound wave nor a spin wave. Spin and sound can no longer be separated in this excitation, but coexist," explains Kevin Künstle, first author of the study.
Acoustic filters and ferrimagnetic insulators combined
In particular, the researchers were able to show that this chimeric wave oscillates periodically between the sound and spin states. The characteristic transition frequency of this oscillation—the so-called Rabi frequency—is significantly greater than all loss rates in the system. This is clear evidence that the system is in a strong coupling regime.
To explain these phenomena, a theoretical model was developed in collaboration with colleagues from the RPTU working group led by Professor Akashdeep Kamra, which can quantitatively predict the observed coupling strength.
The quantitative understanding of the coupling phenomena and the control over the strength of the spin-sound coupling, also demonstrated in the work, opens up new perspectives for the technological use of hybrid states of sound and spin waves. “Our hybrid spin-sound excitations combine two pillars of microwave technology: acoustic filters and ferrimagnetic insulators,” adds Professor Weiler. “In the future, such systems could be used to expand the functionality of miniaturized microwave components. For example, agile frequency filters that can be adjusted during operation could be realized. This opens up new concepts for the implementation of 6G communication networks, the mobile communications standard of the future.”
This research is funded by the European Research Council through the ERC Consolidator Grant “MAWiCS – magnetoacoustic waves in complex spin systems” and by the German Research Foundation as part of the Collaborative Research Center “Spin+X.”
The recent study:
K. Künstle, Y. Kunz, T. Moussa, K. Lasinger, K. Yamamoto, P. Pirro, J. F. Gregg, A. Kamra, and M. Weiler, Magnon-polaron control in a surface magnetoacoustic wave resonator, Nat Commun 16, 10116 (2025). https://doi.org/10.1038/s41467-025-66301-x
Scientific contacts:
Kevin Künstle
Phone: 0631 205-4616
E-Mail: kuenstle(at)rptu.de
Prof. Dr. Mathias Weiler
Phone: 0631 205-4099
E-Mail: mweiler(at)rptu.de