Converting Spin Waves to Vibrational Waves
A branch of electronic device engineering called spintronics uses electron spins to store and transmit information. A research team has now opened up new possibilities for information processing with spins by showing how spin signals can be translated into acoustic signals (phonons) that can be transmitted through materials [1]. Phonons can travel undisturbed for longer distances, so this conversion might extend the capabilities of spintronics, much as the conversion of electrical pulses into light is used for long-distance telecommunication.
In a spin current, electrons that are preferentially aligned in one spin state can be thought of as remaining stationary while a wave of spin reorientation passes through the material. Spin currents are already used in devices such as specialized magnetic memories and other computing elements, in which information is encoded and transferred using the spins.
Transmitting spin-encoded information across a chip is challenging, however, because the spin currents can easily dissipate. One solution is to convert spin currents into phonons, which can typically travel several millimeters without scattering or suffering from electric-field interference. Researchers have previously shown that a phenomenon called the acoustic spin Hall effect (ASHE) can turn acoustic vibrations of a crystal lattice into a spin current [2]. The ASHE is similar to the spin Hall effect (SHE), in which a charge current passing through a conductor can induce a spin current at right angles. The spin current of the SHE arises because of the way electron momentum can interact with electron spin in some materials, while the spin current of the ASHE results from interactions between phonon-induced distortions in the crystal lattice and the spin orientations of the electrons.
The ASHE offers a way to turn phonons into spin currents. But to use such vibrations to transmit spin signals, one would need to perform the conversion in both directions. A spin-to-phonon conversion, called the inverse ASHE, should be possible in principle but has not previously been observed.
Now Dezheng Yang, a specialist in magnetic materials at Lanzhou University in China, and co-workers have demonstrated the inverse ASHE. The key challenge was to identify the right materials for the job. Strong spin currents can be produced in heavy metals, and the researchers chose tantalum, which has an atomic mass 3 times that of iron. Yang and colleagues’ device is comprised of micrometer-wide bilayer strips in which a tantalum layer sits atop an iron-nickel magnetic layer, which controls the orientation of the spins.
The researchers generate a spin current in the metal, and this current is partially reflected at the interface of the two materials. “This reflected spin current couples efficiently to the metal lattice,” Yang explains, “exerting a periodic stress on the lattice and exciting coherent vibrations.”
To maximize the effect, the researchers placed many parallel bilayer strips side by side in a so-called superlattice to create a large interfacial area. They deposited the materials on a slab of lithium niobium oxide, which is piezoelectric, meaning that distortions like those produced by phonons generate an electric field, which allows the phonons to be tracked.
As they anticipated, a spin current in the tantalum strips generated acoustic waves via the inverse ASHE. The researchers saw the same effect, but a little weaker, when they used platinum instead of tantalum—as expected given platinum’s weaker coupling of spin to phonons. “We believe this could open new directions in ‘acoustic spintronics,’ where information is carried not only by electrons but also by phonons,” Yang says.
Sergio Machado Rezende, a specialist in magnetism and spintronics at the Federal University of Pernambuco in Brazil, agrees with that claim. “The observation of the inverse ASHE completes the framework of spin–phonon interconversion,” he says, “and uncovers a phonon-mediated pathway for long-range spin transport.”
Spintronics expert Matthias Althammer of the Walther Meissner Institute in Germany says that the results will allow further experimental studies of spin transport via phonons, but he doubts that the efficiency of the interconversion is currently high enough to lead to immediate applications in spintronic devices. Condensed-matter theorist Rembert Duine of the University of Utrecht in the Netherlands adds that transport of spin signals via acoustic waves should eventually be practically valuable because of the robustness of traveling phonons compared with spin waves.
Correction (15 December 2025): The description of the device geometry has been updated.
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
References
- Y. Cao et al., “Inverse acoustic spin Hall effect in heavy metal-ferromagnet bilayers,” Phys. Rev. Lett. 135, 246705 (2025).
- T. Kawada, “Acoustic spin Hall effect in strong spin-orbit metals,” Sci. Adv. 7, eabd9697 (2021).