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Experimental Verification of a New Magnetic Concept — Observation of Altermagnetic Materials via Neutron Scattering Experiments —

Masuda Group

Background of the Research: Magnetic materials have traditionally been classified into two categories based on their microscopic spin structure: ferromagnetic materials, where spins align parallel (Fig. 1(a), top), and antiferromagnetic materials, where spins align antiparallel (Fig. 1(b), top). Recently, a third type of magnetic material, known as "altermagnetic materials," has been proposed [1]. This new classification is based on the symmetry of not only the spins but also the surrounding crystal structure. In antiferromagnetic materials, the crystal structure around adjacent spins is identical. However, in altermagnetic materials, as shown in Fig. 1(c) (top), the crystal structures around up-spins (indicated by red arrows) and down-spins (blue arrows) do not overlap without a 90-degree rotation. Thus, these materials are classified as altermagnetic when the spins are arranged antiparallel with distinct crystal symmetries.

masuda-a1-fig1.jpg
Fig. 1. (a): Ferromagnetic, (b): Antiferromagnetic, (c): Altermagnetic spin structures (top), and the dispersion relations of magnons (bottom). M indicates magnetization. The red and blue rotating circles represent opposite chiralities (right- and left-handed).

The altermagnetic materials predicted the existence of chiral magnons, which are quasiparticles that can carry spin current [2]. While the chiral magnons in ferromagnetic materials have been studied, their application in spintronics devices is limited due to low-frequency (GHz) operation and undesirable stray magnetic fields due to non-zero magnetization. On the other hand, antiferromagnetic materials promise high-frequency (THz) operation, but their magnons' chirality cancels out (Fig. 1(b), bottom), preventing spin current. Altermagnetic materials combine the advantages of both. Their magnons are theoretically predicted to exhibit large chiral splitting at high frequencies (Fig. 1(c), bottom), capable of generating spin current. Despite having zero magnetization like antiferromagnetic materials (eliminating stray fields), they possess chiral magnons similar to ferromagnetic materials, making them ideal for device applications. Observing the magnons in altermagnetic materials is crucial both for verifying their properties and exploring their application potential. Until now, no successful observation had been made.

Content of the Research: The research group synthesized high-quality large single crystals of MnTe, a candidate material for alternating magnetism, in order to observe the magnon dispersion of alternating magnets. This material contains Mn ions, which are well-suited for observing magnetic properties, and spin splitting in its electronic band, a characteristic of alternating magnetism, had been reported in photoemission spectroscopy experiments. Therefore, it was predicted that this material would also be suitable for observing the chiral splitting of magnons. Furthermore, the research group conducted inelastic neutron scattering experiments using the High-Resolution Chopper Spectrometer (HRC) [3] at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). The HODACA spectrometer [4] at the Research Reactor JRR-3 was also used for crystal evaluation.

The observed neutron spectra are shown in Figs. 2(a) and 2(c). In Fig. 2(a), at high energies above E = 30 meV, a magnon splitting of approximately 2 meV, indicated by white circles, was observed. On the other hand, the magnon dispersion in the low-energy, small-momentum region rises linearly, similar to that of an antiferromagnet. These are crucial pieces of evidence for the existence of alternating magnets.

masuda-a1-fig2.jpg
Fig. 2. (a) & (c) Neutron spectra of MnTe. These show different momentum regions, but h = 1.33 in (a) and l = -1.33 in (c) represent the same momentum (-1.33, 0, -1.33). At this momentum, a magnon splitting of approximately 2 meV was observed. (b) & (d) Calculated magnon chirality. Red and blue represent magnons with different chiralities. The gray solid and dashed lines indicate the calculated magnon dispersion.

Figure 2(c) shows the high-energy spectrum in a different momentum region, where the alternating propagation of the split magnon dispersion along the momentum axis was clearly observed. The calculated magnon dispersion is shown in Figs. 2(b) and 2(d) by black solid and dashed lines. The calculations perfectly reproduced the observed neutron spectra. Furthermore, when counterclockwise chirality is represented in red and clockwise chirality in blue, at low energies, the two chiralities cancel each other out, resulting in a colorless region. However, at higher energies, the two magnons have different chiralities, clearly appearing in blue and red. In Fig. 2(d), it was confirmed that the chirality alternates. From these results, it became clear that the observed magnons are chiral magnons that carry spin current.

Future Prospects: Altermagnetic materials represent a new concept in magnetism. The experimental verification of chiral magnons in this study reveals their potential to generate spin current. This discovery paves the way for future advancements in high-speed, efficient electronic devices, potentially revolutionizing everyday life.


References
  • [1] L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X 12, 040501 (2022).
  • [2] M. Naka, S. Hayami, H. Kusunose, Y. Yanagi, Y. Motome, and H. Seo, Nat. Commun. 10, 4305 (2019).
  • [3] S. Itoh, T. Yokoo, S. Satoh, S. ichiro Yano, D. Kawana, J. Suzuki, and T. J. Sato, Nucl. Instrum. Methods Phys. Res., Sect. A 631, 90 (2011).
  • [4] H. Kikuchi, S. Asai, T. J. Sato, T. Nakajima, L. Harriger, I. Zaliznyak, and T. Masuda, J. Phys. Soc. Jpn. 93, 091004 (2024).
Authors
  • L. Zheyuan, M. Ozeki, S. Asai, S. Itoha, and  T. Masuda
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