Institute for Materials Research, Tohoku University

Graduate School of Science, Tohoku University

International Synchrotron Radiation Innovation Smart Research Center, Tohoku University

Japan Synchrotron Radiation Research Institute

High Energy Accelerator Research Organization

Institute for Composite Nuclear Science, Kyoto University

Graduate School of Science, The University of Tokyo

Summary

High thermoelectric conversion efficiency and large anomalous Hall effect, which originate from the spin of electrons in materials, have been thought to occur only when the electron spins are aligned. On the other hand, large effects have also been reported in a state called antiferromagnetism, in which the spins are aligned in such a way that they cancel each other out, and it has been believed that some states are oriented such that the spins do not cancel each other out even though they do. This was theoretically predicted as a "magnetic octupole" but had not been detected experimentally.

Associate Professor Motoi Kimata and Professor Hiroyuki Nojiri of the Institute for Materials Research, Tohoku University, Dr. Norimasa Jakube, Senior Researcher Yoshinori Kotani, and Postdoctoral Researcher Yuichi Yokoyama of the Japan Synchrotron Radiation Research Institute (JASRI), Graduate Student Kensuke Kurita and Associate Professor Takashi Koretsune of the Graduate School of Science, Tohoku University, and Senior Researcher Yuichi Yamazaki of the National Institute for Materials Science, Associate Professor Hironori Nakao and Professor Kenta Amemiya of the Institute for Materials Structure Science at the High Energy Accelerator Research Organization (KEK), Assistant Professor Chihiro Tabata of the Institute for Complex Nuclear Science, Kyoto University, Professor Satoshi Nakatsuji of the Graduate School of Science, The University of Tokyo, and Professor Tetsuya Nakamura of the International Center for Synchrotron Radiation Innovation and Smart Research, Tohoku University, among other research groups, The research group led by Assistant Professor Chihiro Tabata and Professor Satoshi Nakatsuji of the Graduate School of Science at the University of Tokyo and Professor Tetsuya Nakamura of the International Center for Synchrotron Radiation Innovation and Smart Research at Tohoku University has revealed a hidden "magnetic octupole" in a state called antiferromagnetism, in which electron spins cancel each other, which is the microscopic origin of magnets, through synchrotron X-ray experiments.

The magnetic octupole detected in this study can be controlled faster than conventional spins, and is expected to contribute to significantly faster spintronics devices. The results of this study propose a new method to investigate the origin of novel spintronics and thermoelectric conversion functions, and also open up new possibilities for X-ray magnetic spectroscopy and resonant X-ray scattering using synchrotron radiation.

Figure: Schematic diagram of the arrangement pattern of spins (magnetic dipoles) and magnetic octupoles in _Mn3Sn
The upper magnetic structure is realized in actual _Mn3Sn, while the lower magnetic structure is different from _{that of Mn3Sn} used for comparison. The red arrows represent spin, and the light blue and orange gourd-shaped illustrations represent the shape of the d-electron orbitals at each atomic position. The pink and green electron orbitals represent magnetic octupoles. In the above magnetic structure, the relative arrangement (orientation) of the spin and orbital of each Mn atom at the apex of the triangle is different for each position. Therefore, it was theoretically suggested that when the contributions from each Mn atom position are added together, the spins cancel each other out, but the symmetric degrees of freedom of the magnetic octupole remain. As a result, the physical property response originating from the localized giant magnetic field becomes finite and observable. The emergence of a finite number of magnetic octupoles in a group (cluster) of three Mn atoms is the origin of this physical property, which is called clustered magnetic octupole. On the other hand, in the magnetic structure below, the relative arrangement of spins and orbitals at each Mn atom position is the same, and all the degrees of freedom cancel each other and disappear when the whole is added together.

The results were published online in Nature Communications on September 22, 2021 at 10:00 (UK time).

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