Takuya Kawada (Department of Physics, 1st Year Doctoral Student )

Shinji Kawaguchi (Assistant Professor, Department of Physics)

Masamitsu Hayashi, Associate Professor, Department of Physics

Takumi Funado (Doctoral Student, Nagoya University)

Hiroshi Kono (Professor, Nagoya University)

Points of the presentation

• It was discovered by DC electrical measurements that spin currents ^{(Note 2)}, which are magnetic flows, are generated when high-speed oscillations, called surface acoustic waves ^{(Note 1)}, propagate through heavy metals.
• This phenomenon is attributed to an unknown vibration-spin current conversion mechanism via the spin-orbit interaction ^{(Note 3} ), which couples the orbital motion of electrons to spin.
• In addition to contributing to the construction of fundamental science on the interconversion between spin and mechanical motion, this research is expected to be applied to the control of fine devices and novel vibrational power generation devices using spin.

Summary of presentation

In spintronics research, which aims to realize next-generation high-performance devices by utilizing spin, the microscopic magnetism of electrons, it is an important theme to elucidate the connection between spin and other physical quantities. In recent years, the interconversion between mechanical motion of an object such as rotation, vibration, and deformation and spin has attracted much attention, but the difficulty in detecting spin changes originating from mechanical motion has led to a need for further experimental studies.

A research group led by Graduate Student Takuya Kawada, Assistant Professor Masashi Kawaguchi, and Associate Professor Masamitsu Hayashi of the Department of Physics, Graduate School of Science, The University of Tokyo, in collaboration with Professor Hiroshi Kono and Graduate Student Takumi Funato of the Graduate School of Science, Nagoya University, has revealed the existence of a completely new oscillation-spin current conversion mechanism that occurs in heavy metals with large spin-orbit interaction, by simple The research team, in collaboration with Professor Hiroshi Kono and Graduate Student Takumi Funato of the Department of Chemistry, Materials and Bioengineering, has revealed the existence of a completely new vibrational-spin current conversion mechanism in heavy metals with large spin-orbit interaction by simple electrical measurements. In this study, the DC EMF generated by propagating atomic-scale high-speed vibrations called surface acoustic waves in a heavy metal/ferromagnet heterostructure was investigated in detail, and it was shown that the experimental results can be explained by a new interaction-based model in which lattice vibrations are coupled to electron spins via spin-orbit coupling. The model is based on a new interaction in which lattice vibrations are coupled to electron spins via spin-orbit coupling. This result, which bridges atomic vibration, spin, and electricity, is a stepping stone to explore mechanical motion and the function of spin in various materials, and is expected to lead to the control of microscopic devices using spin and the realization of novel power generation devices that can extract electricity from any vibration in our surroundings.

Contents of presentation

Background of the research and problems in previous studies
Electrons, which govern the properties of matter, have two degrees of freedom: charge and spin. Electronic devices around us represent 0s and 1s by the presence or absence of electric charge, and their operation is controlled by electric current, which is the flow of electric charge. On the other hand, it is also possible to represent 0s and 1s by the direction of spin (minute magnetism), which is another degree of freedom of electrons, and this is used in practical applications such as magnetic storage devices. In recent years, it has also become possible to handle spin currents, which are flows of spin, and it has become clear that spin currents can be used to control the direction of magnets and to convert various physical quantities, such as electric current, light, and heat, to and from spin currents. Since spin currents essentially do not involve charge flow and energy loss due to heat generation can be suppressed, they are being vigorously studied with the aim of realizing next-generation power-saving devices.

In order to realize various functions in spin-based devices, it is necessary to elucidate the connection between the various physical degrees of freedom and spin, and to freely interconvert them. Recently, mechanical motions such as vibration, rotation, and deformation of objects have attracted attention as new targets for coupling with spins. The phenomena in which the direction of magnetization changes due to macroscopic rotation of an object and vice versa have been known since the early 1900s as the Barnett effect and the Einstein-de Haas effect, respectively. However, in macroscopic systems, the rotation speed cannot be very large, and the spin change caused by the rotation is very small. Recently, a method was proposed to study the interaction between spin and mechanical motion using surface acoustic waves generated when radio frequency power is applied to comb-shaped electrodes fabricated on a piezoelectric substrate. Using this method, atoms can be made to vibrate and rotate at extremely high speeds of over 100 million times per second, and it is expected that the effect of the mechanical motion of an object on its spin can be investigated in more detail. However, methods for detecting spin changes originating from surface acoustic waves are limited, and new experimental approaches are needed to elucidate the coupling between spin and mechanical motion in a variety of materials.

Research Activities and Results
In this study, a few-nm-thick film of heavy metal and ferromagnet (cobalt iron boron) was deposited between a pair of comb-shaped electrodes fabricated on a piezoelectric substrate, and the DC electromotive force generated by the propagation of surface acoustic waves was investigated in detail (Figure 1).

Figure 1: Schematic of the experimental system used in this study. Surface acoustic waves propagate in a heavy metal/ferromagnetic heterostructure with an in-plane magnetic field applied, and the DC electromotive force generated during the propagation is measured.

In heavy metals such as tungsten, the electron motion associated with the surface acoustic wave is expected to be converted to spin because of the large spin-orbit interaction that links the orbital motion of the electrons to the spin. We also expected that the spin change generated could be detected by measuring the electromotive force while changing the direction of the magnetization of the ferromagnetic layer by a magnetic field. In fact, we found that a DC electromotive force with a period of 90° with respect to the direction of the magnetic field is generated during the propagation of surface acoustic waves in a tungsten/ferromagnetic heterostructure (Fig. 2 left). In heavy metals such as tungsten, the spin-orbit interaction, which links the orbital motion of electrons to spin, is large, so the electron motion associated with surface acoustic waves is expected to be converted to spin. We also expected that the spin change generated could be detected by measuring the electromotive force while changing the direction of the magnetization of the ferromagnetic layer by a magnetic field. In fact, we found that during the propagation of surface acoustic waves, a DC electromotive force is generated that changes with a period of 90° with respect to the direction of the magnetic field (Figure 2 left).

Figure 2: Angular dependence of the DC EMF generated by the propagation of surface acoustic waves in a heavy metal/ferromagnetic heterostructure. In the case of tungsten with a large spin-orbit interaction (left panel), the electromotive force changes with a period of 90° with respect to the magnetic field angle, while in the case of copper with a very small spin-orbit interaction (right panel), no magnetic field-dependent electromotive force is generated.

When tungsten is replaced by copper, a material with small spin-orbit interaction, no such electromotive force is observed in the measurement (Figure 2, right). This result implies that surface acoustic waves generate spin currents in heavy metals via spin-orbit interactions and that the heavy metal/ferromagnetic heterostructure can be used to electrically detect spin changes originating from surface acoustic waves. Furthermore, in this study, we worked on the construction of a model to explain the obtained experimental results. The results showed that the experimental results can be explained by assuming a novel interaction in which atomic vibrations are coupled to electron spins via spin-orbit coupling.

Significance of this study and future prospects
The oscillation-spin current conversion phenomenon discovered in this study is completely new both theoretically and experimentally, and is expected to make a significant contribution to the development of spin mechatronics ^{(Note 4), which} aims to freely convert spin and mechanical motion. The spin-orbit coupling, which is a key element of this phenomenon, is closely related to phenomena of both fundamental and applied importance, ^{such as} topological insulators ^{(Note 5) and} the Rashba effect ^{(Note 6)}, and these results provide a stepping stone to explore the mechanical motion and the function of spin in these interesting groups of materials. This research is expected not only to contribute to the development of basic science, but also to lead to the creation of new functional materials and device structures that link spin and mechanical motion. As a result, there is a possibility that this research will lead to new technologies such as energy harvesting ^{(Note 7)}, which is a new power generation device that extracts electricity from all kinds of vibrations around us, and MEMS and NEMS ^{(Note 8} ), which control machines using spins, which will be of great industrial value.

Figure 3: Conceptual diagram of spin currents generated by the coupling of atomic vibrations and electron spins via spin-orbit interactions.

Journals

Journal name	Science Advances
Title of paper	Acoustic spin Hall effect in strong spin-orbit metals
Author(s)	Takuya Kawada, Masashi Kawaguchi*, Takumi Funato, Hiroshi Kohno, Masamitsu Hayashi*, Takumi Funato
DOI Number	10.1126/sciadv.abd9697
Abstract URL	https://advances.sciencemag.org/content/7/2/eabd9697

Terminology

Note 1. Surface Acoustic Wave

This is a general term for waves propagating on a solid surface, like seismic waves. In particular, surface acoustic waves called Rayleigh waves, which can be generated using a piezoelectric substrate and comb-shaped electrodes, induce rotational motion of a lattice by coupling longitudinal and transverse waves. ↑ up

Note 2 Spin current

Corresponding to the flow of electric charge, which is called current, the flow of upward and downward spin in opposite directions is called spin current.　 ↑up

Note 3 Spin-orbit interaction

This is an interaction that couples the orbital angular momentum and spin angular momentum of an electron and works to relate the spin to the direction of electron motion. The spin-orbit interaction generally tends to be larger in heavier elements.　 ↑up

Note 4 Spin mechatronics

This is a research field that aims to control the spin in an object through the mechanical motion of the object such as rotation, deformation, and vibration, and conversely to control the mechanical behavior of objects such as ferromagnetic materials using spin. ↑up

Note 5 Topological insulator

Topological insulators are special materials in which electric currents and spin currents flow on the surface despite the fact that the interior of the material is an insulator, and are so named because they can be understood using the mathematical concept of topology, which describes how things are connected. It is said that low power consumption and ultrafast devices can be realized by utilizing the unique properties of electrons on the surface of topological insulators. ↑up

Note 6 Rashba effect

In systems with broken spatial inversion symmetry, such as interfaces and surfaces of different materials, there is a phenomenon in which the spin direction is fixed with respect to the direction of electron motion. This phenomenon is called the Rashba effect, after its proponent, and has attracted considerable interest due to its potential application to spintronics. ↑up

Note 7 Energy harvesting

This is a technology to extract electric power from universally existing minute energy sources such as light, heat, and vibration. It is being vigorously researched as an essential technology for realizing key concepts in next-generation society, such as environmental power generation and IoT devices that connect sensors and devices via the Internet to realize a better life. ↑up

Note 8 MEMS/NEMS

MEMS and NEMS are extremely small electronic devices that integrate micro- and nano-scale electric circuits and mechanical components on a single substrate using microfabrication technology. ↑up