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Discovery of a new kind of magnet: 'Weyl magnet' A route toward realization of magnetic device driven by magnetic Weyl fermions

December 13, 2018

Evidence for Weyl fermions in Mn<sub>3</sub>Sn
Evidence for Weyl fermions in Mn3Sn
(a)Weyl fermions in condensed matter systems show an unconventional positive longitudinal magnetoconductivity (negative longitudinal magneto resistivity), called chiral anomaly. (b) This appears only when a magnetic field is applied parallel to an electric field (B//I, θ = 0 deg.). With an increasing magnetic field, the breaking of the imbalanced charge conservation between the Weyl points with opposite chirality makes the materials more conductive. This novel phenomenon is known as one of the experimental evidence of the Weyl fermions. (c) ARPES band mapping near the Fermi level is compared with DFT band calculations. Weyl (band crossing) points with opposite chirality are denoted by blue and red closed circles, respectively. (d) These Weyl points are found along the K–M–K line in the bands on the kz = 0 plane near EF.
© 2018 Takahiro Tomita, Kenta Kuroda, Satoru Nakatsuji.

Weyl fermions were discovered in 2015 near the Fermi level in the nonmagnetic semimetal TaAs. Weyl points in the momentum space serve as a pair of magnetic monopoles through the topological aspects of the wave functions for electrons. Moreover, the fictitious magnetic fields due to the monopoles may induce novel electric transports, and could be useful for low energy consumption electronics. In contrast to the nonmagnetic Weyl fermions in TaAs, magnetic Weyl fermions are known to appear in magnets, thus would enable us to control Weyl fermions by external magnetic fields. This functionality will be necessary for device applications, and many efforts have been made to search for magnetic Weyl fermions. However, they have remained hypothetical until now.

Recently, an antiferromagnetic manganese-tin alloy, Mn3Sn, was found to exhibit large anomalous Hall and Nernst effects, even at room temperature. These anomalous Hall and Nernst effects are usually known to be proportional to magnetization and thus have only been observed in ferromagnets. The spontaneous Hall resistivity in the antiferromagnet with vanishingly small magnetization indicates that the large fictitious field equivalent to a few hundred T must exist in the momentum space. Recent density functional theory (DFT) calculations predict that the large fictitious field, or Berry curvature, may well appear due to the formation of Weyl points near the Fermi energy EF .

The research groups of Professor Satoru Nakatsuji, Associate Professor Takeshi Kondo and Professor Shik Shin at the Institute for Solid State Physics at the University of Tokyo and their collaborators at RIKEN demonstrated the realization of magnetic Weyl fermions in Mn3Sn for the first time. Their study revealed the existence of a “Weyl magnet,” a new magnet with tunable magnetic Weyl fermions by magnetic fields at room temperature. They found strong experimental evidence for the Weyl fermions in Mn3Sn, namely, that the band structure revealed by angle resolved photoemission spectroscopy is found roughly consistent with DFT and the chiral anomaly is clarified in the magnetotransport measurements. Thus, these experiments demonstrate that the large anomalous Nernst signals arise from the Berry curvature associated with the Weyl points near the Fermi energy.

The research groups revealed extremely large magnetic transports and thermoelectric effects in the Mn3Sn magnet. By their new discovery of Weyl magnets, the mystery of these novel properties can be solved. They anticipate that further new phenomena will emerge through the interplay between electron correlation and topology in Weyl magnets.

"Our groups so far revealed extremely large magnetic transports and thermoelectric effects in the Mn3Sn magnet," said Nakatsuji. He continues, "By our new discovery of a Weyl magnet, the mystery of these novel properties has been solved. We anticipate that further new phenomena will emerge through the interplay between electron correlation and topology in Weyl magnets."

Papers

K. Kuroda, T. Tomita, M.-T. Suzuki, M.-T. Suzuki, C. Bareille, A. A. Nugroho, P. Goswami, M. Ochi, M. Ikhlas, M. Nakayama, S. Akebi, R. Noguchi, R. Ishii, N. Inami, K. Ono, K. Kumigashira, A. Varykhalov, T. Muro, T. Koretsune, R. Arita, S. Shin, Takeshi Kondo, and S. Nakatsuji, "Evidence for magnetic Weyl fermions in a correlated metal," Nature Materials: September 25, 2017, doi:10.1038/NMAT4987.
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