The Rigakubu News

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Spin flows in atomic vibrations!

Takuya Kawada(2nd Year Doctoral Student, Department of Physics)

Shinji Kawaguchi (Assistant Professor, Department of Physics )

Masamitsu Hayashi, Associate Professor , Department of Physics

Electrons have "charge" and angular momentum "spin" corresponding to their rotation. By possessing spin, electrons behave as tiny magnets. When the coupling between spins is strong, a material becomes ferromagnetic, with many electron spins pointing in the same direction, forming a magnet that sticks to refrigerators.

It is well known that when electrons move, the transfer of charge generates an "electric current. Since electrons have spin, research over the past decade has shown that the movement of electrons generates a "spin current. Spin current is a flow of electrons with opposite spin in opposite directions (e.g., electrons with upward spin move to the right and those with downward spin move to the left), and while current carries electric charge, spin current carries angular momentum. It has been demonstrated both theoretically and experimentally that injecting a spin current into a magnet can flip the N and S poles. It has also been shown that an electric current is generated when a spin current is injected into a specific material.

When a battery is connected to a copper wire, an electric current flows. On the other hand, one of the main themes of advanced spintronics research is the elucidation of the mechanism by which spin currents are generated. There are two major mechanisms that have been elucidated so far, and they are called the "spin Hall effect" and the "spin pumping effect," respectively. The spin Hall effect is a phenomenon in which a spin current is generated in a direction orthogonal to the electric current flowing in a material. The spin Hall effect reflects the characteristics of electrons in a material as waves, and therefore, a large effect is observed in certain materials. In the spin-pumping effect, the spin current is generated by a time-dependent change in the direction of the N-pole of a magnet.

In this study, we found that the spin current is induced by the high-speed vibration of atoms in a material as a new mechanism. We investigated the spin currents generated in materials using surface acoustic waves, in which atoms are vibrated and rotated at extremely high speeds of more than 100 million times per second. As a result, they observed the generation of spin currents by high-speed vibration of atoms in materials with a large spin-orbit interaction (e.g., tungsten and platinum), which is a value that expresses the coupling between the spin direction and momentum of electrons. Furthermore, it was found that an electric current is generated from the generated spin current, and a kind of vibrational power generation via the spin current was discovered. Although the coupling of electron spin and rotational motion has been known for a long time, the generation of spin currents from high-speed vibration of atoms via spin-orbit interaction has not been predicted at all, suggesting the existence of a new interaction between mechanical motion and spin. With the development of MEMS and NEM technologies for controlling micro- and nano-machinery (very small electronic devices integrating micro- and nano-scale electrical circuits and mechanical components on a single substrate, a technology mainly applied to sensors), the control of mechanical motion in nanostructures has been attracting increasing attention every year. This study opens up new developments in the mutual control of electron spin and mechanical motion, and contributes to the formation and development of a new research field called spin mechatronics.

Figure: Conceptual diagram showing how "spin currents," in which electron spins are oriented by atomic vibrations, are generated in materials with large "spin-orbit interaction. The colored spheres and the arrows through them represent electrons and their spin orientations. The black spheres are atoms constituting the crystal. The positions of the atoms are shifted from their equilibrium state due to atomic vibrations. In reality, the spin current is generated by the movement of many electrons in the same way as the colored spheres.

This study is based on T. Kawada et al . , Science Advances 7, 9697 (2021).

(Press release on January 7, 2021)

Published in Faculty of Science News, May 2021

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