Center for Spintronics Research and Development

The Center for Spintronics Research and Development aims for Keio University to play a central role in spintronics research in Japan by disseminating world-leading research results in a wide range of fields, from basic to applied science. Spintronics is a new academic field that discovers new physical phenomena obtained by controlling both the electrical and magnetic properties of materials and links these results to innovation in the electronics and information and communication industries. Researchers and alumni of Keio University have contributed significantly to its creation and development, and the Center for Spintronics Research and Development was established to lead its further development as a basic academic discipline and its application in industry.

Spin quantum computers, nitrogen-vacancy centers, isotope engineering, spin current, magnons, spin-torque devices

■ Regarding activities continuing from FY2019 (SU): Background, rationale, and goals for continuation

The Center for Spintronics Research and Development aims for Keio University to play a central role in spintronics research in Japan by disseminating world-leading research results in a wide range of fields, from basic physics to engineering applications. Spintronics is a new academic field that discovers new physical phenomena obtained by controlling both the electrical properties (charge) and magnetic properties (spin angular momentum) of electrons, one of the elementary particles that make up matter, and links these results to innovation in the electronics and information and communication industries. Researchers and alumni of Keio University have contributed significantly to its creation and development, and the Center for Spintronics Research and Development will be established to lead its further development as a basic academic discipline and its application in industry.

This center serves as a base for spintronics researchers within Keio and also fulfills the mission of being the core of the Spintronics Research Network, which promotes collaboration among spintronics researchers in Japan and abroad. To this end, as representatives of the spintronics research community in Japan, five hub universities—the University of Tokyo, Tohoku University, Osaka University, Kyoto University, and Keio University—submitted a proposal for the "Spintronics Research Network of Japan (Spin-RNJ)" hub formation plan to the "Large-Scale Academic Research Projects—Roadmap 2020" and it was selected with the highest evaluation rank. This program evaluates and certifies research fields that require focused national funding (MEXT's Large-Scale Academic Frontier Promotion Project), which not only greatly expands the opportunities for Keio University's spintronics research to acquire large-scale budgets but also leads to the development of the academic field and enhancement of international competitiveness through personnel exchange among the hub universities. Furthermore, by utilizing these budgets to establish a center office and a shared research space, we will create new breakthroughs in spintronics research by enabling organic collaboration among center members.

■ New activity goals, content, and implementation background for FY2020

The main activities are as follows. This fiscal year, despite the constraints of the COVID-19 pandemic, we have already achieved sufficient results as a startup center, including holding the Spin-RNJ Young Researchers' Online Research Presentation Meeting (June 3–4, 2020, 267 participants).

■ Implementation details for the fiscal year's business plan, research results, and degree of achievement (as an SU)

This fiscal year, we conducted research leading to completely new device applications using electron spin and obtained the following results.

(1) "Development of a new method for generating magnetism using sound waves in materials": The magneto-rotational effect is a universal physical phenomenon indicating that the origin of magnetism in materials is the rotational motion of electrons, and it was anticipated as a completely new method for magnetic control. However, the magnitude of this effect, even when rotated with the latest centrifuges, was weaker than the Earth's magnetic field and could not be used for controlling the magnetism of materials. The research group discovered for the first time in the world that sound waves, in which atoms rotate locally at a speed of more than one billion times per second, generate a huge magneto-rotational effect (more than 100,000 times the Earth's magnetic field) in a nickel-iron alloy magnet.

(2) "Development of a spin-wave diode with giant non-reciprocity": In a composite material combining a magnet and a semiconductor, we discovered that the amplitude of "spin waves," which are waves of magnetism, can be greatly modulated depending on the injection direction of sound waves and the direction of magnetism. With conventional methods, when the magnet was made to a nanometer-scale thickness, the amplitudes of spin waves propagating in the forward and reverse directions became equal, making it difficult to achieve spin-wave rectification. Our research group fabricated a nickel/silicon composite material combining a 20-nanometer-thick thin-film nickel magnet and a 400-nanometer semiconductor silicon, and clarified that the reverse spin-wave amplitude can be reduced to less than one-twelfth of the forward amplitude.

In particular, the result of (1) is the outcome of international joint research with the Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, and is a product of the close research exchange facilitated by this center, thus achieving our original objective.

(1) Number of published papers: 10

(2) Number of conference presentations: (Domestic) 17, (International) 7

(3) Achievements in social contribution, such as events: 12

Among our research achievements, those with potential for practical application include:

(1) "The emergence of giant non-reciprocity in spin waves," a byproduct of research investigating the spatial distribution of spin current sources in surface acoustic waves, and

(2) "The magneto-rotational effect using sound waves," discovered in experiments quantitatively evaluating the lattice rotation of surface acoustic waves.

(1) In this research, we experimentally verified that in a Si/Al composite material, the non-reciprocity of spin-wave excitation using surface acoustic waves is dramatically enhanced by the thickness of the Si film. This is a research result that can dramatically improve the performance of spin-wave diodes, which are essential for the operation of spin-wave devices that realize high-speed, low-power information communication and logic operations using spin waves. Also, since the reverse spin-wave amplitude can be reduced to almost zero, it can be implemented as a spin-wave switch that turns the signal conversion from microwaves to spin waves on and off depending on the direction of the magnet's magnetism. Furthermore, the non-reciprocity of spin waves in composite materials also creates non-reciprocity in the surface acoustic waves coupled with them. In conventional SAW filter devices using surface acoustic waves, interference between incident and reflected waves between transmitting and receiving antennas caused device performance degradation and failure. By creating non-reciprocity in surface acoustic waves with the newly developed composite material, interference between incident and reflected waves propagating in opposite directions can be reduced. This is expected to improve the performance of SAW filter devices widely used in wireless communication terminals that perform advanced information processing, such as smartphones, and has significant ripple effects on the communication industry. Furthermore, it is expected to lead to the creation of completely new devices, such as elastic wave diodes that rectify sound waves propagating through materials.

(2) This is a universal effect based on the law of conservation of rotational motion and is independent of the properties of the magnet, so it can be applied to all cutting-edge magnetic devices. It opens up a major path for the operation of spin devices using sound waves, which have less energy loss compared to electric currents that involve Joule heating, and will lead to a significant reduction in power consumption for spin devices (basic components of artificial intelligence circuits that require low-power, high-speed operation, such as MRAM and other spin memories, and logic operation devices using spin waves). This matter was press-released (joint release by JST and Keio University) on April 1, 2020, and May 28, 2020.