Press Release

March 17, 2022
Keio University

Discovery of a Mechanism for Producing Electromotive Forces from
Magnetic Materials via the Gyromagnetic Effect:
Toward Applications of Spintronic Devices by Sound Waves
Takumi Funato, a Project Assistant Professor at the Global Research Institute at Keio
University (Spintronics Research and Development Center), and Mamoru Matsuo, anAssociate Professor at the Kavli Institute for Theoretical Science at the Chinese Academy ofSciences, have theoretically demonstrated that the gyromagnetic effect induces electromotiveforce when sound waves are injected into magnets. The gyromagnetic effect, discovered byEinstein and de Haas and Barnett, is a historically significant phenomenon indicating thatthe origin of magnetism in matter is the rotation of electrons called "spin."
The gyromagnetic effect, however, is so small that it was considered impossible to apply to
spintronics devices, where magnetic control of materials is essential. In a recent experiment,
researchers demonstrated that it is possible to generate spin current via the gyromagnetic
effect by utilizing sound waves to rotate lattices more than a billion times per second.
Researchers have also discovered methods to convert the spin currents into charge currents;
however, they require sophisticated device structures and precious metals. To overcome theselimitations, the research group in this study theoretically clarified the interplay ofmagnetization, electron spin, and lattice rotation in ferromagnetic metal monolayers, whereinjected sound waves induce lattice rotational motion.
The researchers demonstrated that the gyromagnetic effect could induce electromotive force
with a simple device structure. The mechanism discovered in this study is expected tosignificantly contribute to applications of the gyromagnetic effect in spin devices, which has
been difficult so far. The results of this research were published online in the American
physics journal Physical Review Letters on February 18, 2022 (Eastern Standard Time).
1. Main Points of Research The gyromagnetic effect, or the interconversion between macroscopic rotation and magnetism,
is a universal effect based on the conservation of rotating motion.
 According to previous studies, inducing gyromagnetic phenomena, realized by sound waves
accompanied by mechanical rotations of atoms at a rate of more than one billion times per
second, requires precious metals and sophisticated device structures.
 This study proposed that a sound wave induces electromotive force via the gyromagnetic effectin a simple device structure, such as a single ferromagnetic metal layer.
 The results are expected to resolve the difficulties of applying the gyromagnetic effect in spin
devices, as the methods proposed do not require rare metals or complicated device structures.

12. Background of ResearchElectrons have magnetic properties that originate in their rotation (microscopic angularmomentum1), called "spin," and electric properties. In spintronics, which simultaneously utilizes
these two properties, researchers have studied the control of spin by magnetic fields and charge
currents, and realized applications for these phenomena. The interconversion between electron
spin and mechanical rotation (macro angular momentum) was experimentally demonstrated byBarnett2, Einstein and de Haas3 about a century ago. This important phenomenon, discovered in
the course of fundamental scientific questioning into the origin of magnetism, is called the“gyromagnetic effect.” The faster the rotation of the material, the more significant the
gyromagnetic effect; however, even at high-speed rotations, which are achievable with state-ofthe-art technology capable of rates of around 10,000 times per second, the effect is extremely
weak—inferior to even the Earth's magnetic field. Due to this, the gyromagnetic effect has notreceived much attention in spintronics. In 2013, however, a theoretical prediction made by
Associate Professor Mamoru Matsuo and colleagues renewed interest in the interaction between
magnetism and mechanical rotations [1]. Subsequently, a research group led by Professor YukioNozaki experimentally demonstrated that the gyromagnetic effect induced by mechanical rotationof atoms at a rate of more than one billion times per second, accompanied by a sound wave,generates spin currents [2,3]. Professor Nozaki and colleagues generated an ac spin current incopper and ferromagnetic metal composites by injecting a surface acoustic wave traveling along
the surface of materials. Through this experimentation, it was confirmed that magnetic waves
were excited in ferromagnetic metals by the ac spin current flowing from copper. The researchgroup also electrically detected the spin current generated via the gyromagnetic effect by joining
platinum, a highly efficient spin-to-charge conversion material, to composites [4]. However, thesemethods carried certain limitations, such as the requirement of precious metals including
platinum and multilayer device structures.
3. Content of Research and Results
The present research group theoretically proposed that electromotive force can be induced via thegyromagnetic effect in a simple device structure consisting of a ferromagnetic metal monolayer.
When sound acoustic waves (SAWs) are injected into ferromagnetic metals, free-electron spins are
subjected to the gyromagnetic effect by the rotational motion of lattices. Magnetic waves are
simultaneously induced by the magnetoelastic effect4 that changes the direction of the magnetism
associated with the elastic deformation [5,6]. Electromotive force is generated by coupling
magnetic waves and free electron spins. The present research group discovered that electromotive
force originated from the gyromagnetic effect, circumventing conventional restrictions ofmaterials and device structures, by combining the gyromagnetic effect and magnetoelastic effect.
In this study, they analyzed the electromotive force generated by Rayleigh waves5, one type ofSAW. The results suggest that electromotive force is induced in the direction of SAW propagationand in the direction of the thickness of the material. In addition, the results suggested thatelectromotive force has nonreciprocity6 in the directions of SAW propagation and magnetism.
According to the researchers’ estimations, an experimentally observable magnitude ofelectromotive force can be induced in a polycrystalline nickel system.
24. Future Developments
The mechanism discovered in this study is expected to significantly contribute to the applicationof the gyromagnetic effect, a feat which has been difficult so far. Previously, microfabrication andprecious metals were necessary for utilizing spin currents induced by the gyromagnetic effect.
This study enables the application of the gyromagnetic effect for a variety of spin devices without
any limitations other than the ability to utilize SAWs. The results of this study are expected makemagnetic devices more energy efficient as they do not rely on Joule heat, unlike electric devices,and contribute to the development of rare-metal-free technologies.
Acknowledgements
This research was sponsored by the Japan Science and Technology Agency’s (JST) Strategic
Research for Evolutional Science and Technology initiative CREST: “Development of functionally
graded materials and application to spin devices by combining nano-structure control andcomputational science” (Research Director: Yukio Nozaki. Project Number: JPMJCR19J4).
Additional support came from the JSPS Grants-in-Aid for Scientific Research Program(KAKENHI) 21K20356, 20H01863, and 21H04565.
References
1 M. Matsuo, J. Ieda, K. Harii, E. Saitoh, and S. Maekawa: Mechanical Generation of Spin Current
by Spin-Rotation Coupling, Phys. Rev. B 87, 180402(R) (2013).
2 D. Kobayashi, T. Yoshikawa, M. Matsuo, R. Iguchi, S. Maekawa, E. Saitoh, and Y. Nozaki: Spin
Current Generation Using a Surface Acoustic Wave Generated via Spin-Rotation Coupling, Phys.
Rev. Lett. 119, 077202 (2017).
3 Y. Kurimune, M. Matsuo, and Y. Nozaki: Observation of Gyromagnetic Spin Wave Resonance in
NiFe Films, Phys. Rev. Lett. 124, 217205 (2020).
4 S. Tateno, G. Okano, M. Matsuo, and Y. Nozaki: Electrical evaluation of the alternating spincurrent generated via spin-vorticity coupling, Phys. Rev. B 102, 104406 (2020).
5 M. Weiler, H. Huebl, F.S. Goerg, F.D. Czeschka, R. Gross, and S.T.B. Goennenwein: Spin
Pumping with Coherent Elastic Waves, Phys. Rev. Lett. 108, 176601 (2012).
6 T. Kawada, M. Kawaguchi, T. Funato, H. Kohno, M. Hayashi: Acoustic spin Hall effect in strong
spin-orbit metals, Sci. Adv. 7 eabd9697 (2021).
Details of Journal Article
T. Funato and M. Matsuo: “Spin Elastodynamic Motive Force”, Physical Review Letters, (Vol. 128,No. 7) 077201 (2022)
DOI: 10.1103/PhysRevLett.128.077201
Glossary
1 Angular momentum: a vector quality that describes the direction and magnitude of rotation.
Electrons have microscopic angular momentum called spin, which is known to be the origin ofmagnetism.
2 Barnett effect: the effect in which the magnetism strength changes when a magnet is rotated. It3was discovered by Barnett in 1915 through an experiment.
3 Einstein-de Haas effect: the effect in which a magnet begins to rotate when its magnetic strength

is changed. It was experimentally verified by Einstein and de Haas in 1915. It is said to be theonly experiment conducted by Einstein.

4 Magnetoelastic effect: an effect different from the gyromagnetic effect, in which the direction ofmagnetism changes when the magnet is elastically deformed, or conversely, the magnet iselastically deformed depending on the direction of magnetism.

5 Rayleigh wave: a type of surface acoustic wave that propagates on the surface of a material, in

which the local elliptical motion of the atoms that make up the material propagates as a wave.

6Nonreciprocity: a phenomenon in which different magnitudes of electromotive forces aregenerated when the directions of Rayleigh waves are reversed.

Figure 1. Schematic diagram of the mechanism of electromotive force generation
derived from the gyromagnetic effect of sound waves
The magnetoelastic coupling driven by elastic deformation works on the magnetismof ferromagnetic materials, and the gyromagnetic effect driven by rotational
deformation works on the spin of free electrons. The combination of these two effects
generates an electromotive force derived from the gyromagnetic effect without theneed for precious metals or sophisticated device structures.

4*Please direct any requests or inquiries on press coverage to the contact information provided
below in advance.
・Inquiries about research
Project Assistant Professor Takumi Funato
Keio University Global Research Institute (KGRI) Project
TEL: +81 45-566-1677
FAX: +81 45-566-1677
Email: t_funato@keio.jp
・Inquiries about press release
Keio University Office of Communications and Public Relations (Mr. Sawano)
TEL: +81 3-5427-1541
FAX: +81 3-5441-7640
Email: m-pr@adst.keio.ac.jp
https://www.keio.ac.jp/en/

5