Giant topological magneto-optical effect in noncoplanar antiferromagnet

Article https://doi.org/10.1038/s41467-026-72889-5 Giant topological magneto-optical effect in noncoplanar antiferromagnet Received: 27 October 2025 Accepted: 24 April 2026 1234567890():,; 1234567890():,; Check for updates Y. Okamura Y. Takahashi 1,2,5 , Y. Hayashi1,5, N. D. Khanh 1 , Y. Tokura 1,3,4 , S. Seki 1,2 & 1 Geometrical frustration on triangular lattice is expected to exhibit diverse quantum spin and electronic states endowed with emergent electromagnetic phenomena. The all-in-all-out (AIAO)-type antiferromagnetic spin structure is one such example, possessing the scalar spin chirality that generates giant emergent magnetic field with vanishingly small magnetization. Here, we report on the large spontaneous magneto-optical Kerr effect (MOKE) caused by the AIAO/AOAI state in quasi-two-dimensional triangular-lattice compound CoNb3S6. Over the entire measured energy region from 55 to 2000 meV, the MOKE is found to be dominated only by the spin chirality. Essential role of momentum-space Berry curvature for both MOKE and dc Hall effect is demonstrated by the spectral analysis of optical Hall conductivity derived from MOKE. The figure of merit of observed topological MOKE, lightpolarization rotation angle divided by magnetization, largely exceeds other magnets including time-reversal-symmetry broken antiferromagnet Mn3Sn. Our findings demonstrate the strong light-spin coupling through the spin chirality, paving the way for antiferromagnetic spintronics and future optospintronic devices. Nontrivial spin ordering provides a fertile ground to explore unconventional electromagnetic properties and functionalities in quantum materials. Recent extensive research on time-reversal-symmetry (TRS) broken antiferromagnets has made the important advance in the field of contemporary materials science1, potentially overcoming the limitation of spintronics based on ferromagnets. This class of materials is expected to exhibit versatile electromagnetic phenomena allowed by TRS breaking2–7, such as anomalous Hall effect despite vanishingly small magnetization (M), which is promising for the next-generation information medium. However, the TRS-broken antiferromagnets are still very rare and their electromagnetic responses are often rooted in the relativistic spin-orbit coupling similar to that of ferromagnets. Recently, triangular lattice compounds CoMe3S6 (Me = Nb, Ta) are revealed to be a new member of TRS-broken antiferromagnets (Fig. 1a), which show the large spontaneous Hall effect despite vanishingly small net M8–12. The polarized neutron scattering experiments reveal that this unconventional Hall effect is caused by formation of the all-in-all-out (AIAO)-type antiferromagnetic spin structure on the slightly elongated Co-tetrahedron along the c axis (Fig. 1b)10,11, which can be viewed as the short wavelength limit of magnetic skyrmion lattice13. The noncoplanar spin arrangement hosts the scalar spin chirality defined for three neighboring localized spins, χijk = Si・(Sj×Sk) (Si: spin at i-th site), which corresponds to the solid angle subtended by those spins. The spin chirality acts as the fictitious magnetic field on the charge carriers moving in the background of noncoplanar spin texture even without spin-orbit coupling, resulting in the topological Hall effect (THE). Therefore, the spontaneous Hall effect in CoMe3S6 is essentially different from the anomalous Hall effect in the ferromagnets and in the representative TRS-broken antiferromagnet Mn3Sn, which is governed by the spin-orbit coupling2,4–6,14. The AIAO spin texture in CoMe3S6 thus provides the new research arena for the TRS-broken antiferromagnets and attracts much 1 Department of Applied Physics and Quantum Phase Electronics Centre, University of Tokyo, Tokyo, Japan. 2Research Centre for Advanced Science and Technology, University of Tokyo, Tokyo, Japan. 3RIKEN Centre for Emergent Matter Science (CEMS), Wako, Japan. 4Tokyo College, University of Tokyo, e-mail: ; Tokyo, Japan. 5These authors contributed equally: Y. Okamura, Y. Hayashi. Nature Communications | (2026)17:4409 1 Article https://doi.org/10.1038/s41467-026-72889-5 S a Nb b Co c a b +Beff c -Beff 0.04 T (μΩ cm) ρyx Timereversal 4 0 0.00 -2 B||[001] 25 K -0.02 -4 -3 All-in-all-out f ZFC [001] g -1.5 0 1.5 -0.04 3 Magnetic field (T) All-out-all-in e 0.02 2 Magnetization (μB/f.u.) d FC with +B h FC with −B 4 LED θ K (mrad) 0 -4 CMOS camera Fig. 1 | Spontaneous time-reversal symmetry breaking probed by MOKE in AIAO-type antiferromagnet CoNb3S6. a Crystal structure of CoNb3S6. b The magnetic unit cell and schematic illustration of spin structure. The red lines highlight the crystallographic unit cell shown in a. The blue shaded region highlights the tetrahedral Co unit. c Schematic illustration of all-in-all-out (AIAO) and all-out-all-in (AOAI) type antiferromagnetic spin structures. This time-reversal pair is characterized by the opposite sign of emergent magnetic field Beff represented by green arrows. d Magnetic-field dependence of magnetization M (light blue curve) and topological Hall resistivity ρTyx after subtracting the normal Hall component (red curve). e Schematic illustration of MOKE imaging. f-h Magneto-optical Kerr imaging on (001) plane after zero-field cooling (ZFC) (f), field cooling (FC) under positive field (g), and FC under negative field (h). The black scale bar represents the length of 20 μm. These images were taken at zero magnetic field. attention as the possible long-sought experimental realization of pioneering theoretical predictions11,15–18. In the Kondo-lattice model on triangular lattice, the AIAO state can emerge through the Fermi surface nesting, which generates the THE even without magnetic field, as observed experimentally. Notably, in addition to such the topological transport phenomenon, this scalar spin chiral state exhibits the strong coupling to the light through the interband electronic transitions19–24. In particular, it is predicted to show the magneto-optical effect induced by the fictitious magnetic field of spin chirality, so-called topological magneto-optical effect19. This mechanism does not necessarily require the spin-orbit coupling and net M, which is contrasted to the conventional magneto-optical effect in ferromagnets that is proportional to M and governed by the interplay between band exchange splitting and spin-orbit coupling25,26. Followed by the prediction, this intriguing optical phenomenon has also been studied in skyrmion systems with finite scalar spin chirality20,21; however, the observed response is relatively weak and confined to a limited optical frequency range. We note that the materials proposed in ref. 19, such as γ-Fe1-xMnx and KxRhO2, are also the important candidates showing the topological MOKE caused by the AIAO structure in addition to CoMe3S6. In this work, we show the giant topological magneto-optical Kerr effect (MOKE) induced by the (...truncated)