PGI-1 Talk: Dr. Shun Okumura

Quantum Theory of Materials Seminar

Odd-parity spin splitting and anomalous Hall effect in spiral magnets

Off-diagonal electrical transport phenomena reflecting characteristic magnetic structures have been actively studied. The topological Hall effect, for instance, originates from the noncoplanar magnetic texture of skyrmions, imprinting a spin Berry phase on conduction electrons even in the absence of spin-orbit coupling (SOC) [1]. In contrast, coplanar 120° magnetic structures exhibit a pronounced anomalous Hall effect (AHE) driven by the synergy between magnetic octupoles and SOC [2]. More recently, altermagnetism with collinear antiferromagnetic order has revealed an SOC-mediated AHE [3]. However, in the case of spiral magnetism, a prototypical example of a non-collinear yet coplanar magnetic structure, the conditions required to realize AHE have yet to be thoroughly explored.

In this study, we investigate the AHE in metallic systems coupled to commensurate spiral magnetic textures, as exemplified in Fig. 1(a). The electronic bands exhibit odd-parity spin splitting with spin polarization perpendicular to the helical plane [Fig. 1(b)], leading to a characteristic AHE in the presence of SOC. This spiral-induced AHE depends sensitively on the SOC type, helical plane, and magnetization direction [4]. We also demonstrate that “p-wave” magnets with a commensurate spin spiral, recently identified in an f-electron material, can host this enormous AHE [5].

Fig. 1: (a) Schematic picture of the spiral-induced anomalous Hall effect on the yz spin helix.
(b) Odd-parity spin splitting in the low-energy electronic bands with spin polarization 〈𝑠𝐤x〉.

[1] A. Neubauer et al., Phys. Rev. Lett. 102, 186602 (2009).
[2] S. Nakatsuji et al., Nature 527, 212 (2015).
[3] L. Šmejkal et al., Sci. Adv. 6, eaaz8809 (2020).
[4] S. Okumura, M. M. Hirschmann, and Y. Motome, in preparation.
[5] R. Yamada, M. T. Birch, P. R. Baral, S. Okumura et al., Nature 646, 837 (2025).

Contact

Prof. Dr. Stefan Blügel
Phone: +49 2461 61-4249
Email: s.bluegel@fz-juelich.de

Shun Okumura (Research Associate, Prof. Yukitoshi Motome Group)
Department of Applied Physics, The University of Tokyo, Japan
Email: okumura@ap.t.u-tokyo.ac.jp