YbFe₆Ge₆ is a kagome-lattice intermetallic whose Fe³⁺ moments order antiferromagnetically below $T_{\mathrm{N}}\approx$ 500 K. Neutron diffraction shows an A-type structure with spins along the $c$ axis above the spin-reorientation temperature $T_{\mathrm{S}\mathrm{R}}\approx$ 63 K; on cooling the moments rotate into the kagome plane while the propagation vector remains $𝒌=(0,0,0)$, preserving inversion–time-reversal symmetry and eliminating static scalar chirality [1]. Magnetotransport reveals that this reorientation generates an anomalous Hall conductivity $\Delta {\sigma }_{\mathrm{x}\mathrm{y}}\approx$ 30 Ω^-1 cm^-1 at 10 K for in-plane fields, whereas no signal appears for out-of-plane fields or $T>T_{\mathrm{S}\mathrm{R}}$, confirming its anisotropy dependence rather than any coupling to net magnetization [1].

Low-energy spin dynamics were resolved by inelastic neutron scattering. As shown in Fig. 1(a) [1], magnetic intensity at $𝑸=(0,0,1)$ in the spin-reoriented phase is continuous up to $\approx$ 0.6 meV, defining a gapless magnon branch above the 0.14 meV resolution. The temperature evolution of intensities at 0.55 meV and 2.55 meV, plotted in Fig. 1(b) [1], demonstrates that the gap remains closed throughout the easy-plane phase and reopens abruptly at TSR, growing to 1.34 meV by 80 K. The concomitant loss of $\Delta {\sigma }_{\mathrm{x}\mathrm{y}}$, reproduced in Fig. 1(c) [1], shows a one-to-one correspondence between gapless magnons and Hall response: whenever the sub-meV continuum vanishes, the transverse conductivity collapses. A 7 T field also quenches $\Delta {\sigma }_{\mathrm{x}\mathrm{y}}$; its Zeeman energy for the 1.5 ${\mu }_{\mathrm{B}}$ Fe moment is 0.6 meV, matching the upper edge of the gapless band and reinforcing the causal link.

Because the combined space inversion and time-reversal (IT) symmetry eliminates equilibrium Berry curvature and the magnetization never exceeds 0.3 ${\mu }_{\mathrm{B}}$/Fe even at 50 T, conventional intrinsic or skew-scattering mechanisms are excluded. Instead, propagating magnons transiently cant neighboring Fe moments; coupling to partially polarized Yb³⁺ spins biases the distribution of local scalar chiralities, creating a net dynamic Berry phase that deflects itinerant electrons. When the magnon gap opens thermally or via Zeeman splitting these fluctuations are suppressed and the Hall signal disappears [2]. Comparable fluctuation-driven Hall effects in kagome ferromagnets $A$Mn₆Sn₆ [3] and in Fe₃Sn₂ [4] require non-collinear ground states, yet YbFe₆Ge₆ shows that collinear antiferromagnets can host the same physics provided that low-energy magnons are gapless. The absence of any Hall signal in FeSn, whose gap stays near 2 meV at all temperatures, underscores the necessity of near-zero-energy modes [5]. Only the quasi-acoustic branch softens across TSR; higher-energy magnons up to 40 meV remain unchanged, indicating that anisotropy rather than exchange drives the soft mode. Integrating the inelastic intensity yields a fluctuating Fe moment of ≈ 1.5 ${\mu }_{\mathrm{B}}$, consistent with diffraction and validating the local-moment picture. These results demonstrate that centrosymmetric, magnetically compensated antiferromagnets can exhibit field-controllable topological transport when anisotropy collapses the magnon gap to zero, extending antiferromagnetic spintronics beyond systems with static chirality and suggesting that engineered soft-mode transitions could enable chirality-mediated charge–spin conversion at terahertz frequencies [6].

References

• [1] W. Yao et al., Phys. Rev. Lett. 134, 186501 (2025).
• [2] W. Wang et al., Nat. Mater. 18, 1054 (2019).
• [3] N. Ghimire et al., Sci. Adv. 6, eabe2680 (2020).
• [4] M. Kang et al., Nat. Mater. 19, 163 (2020).
• [5] S.-H. Do et al., Phys. Rev. B 105, L180403 (2022).
• [6] Y. Fujishiro et al., Nat. Commun. 12, 317 (2021).

Authors

• W. Yao^a,b, S. Liu^b H. Kikuchi, H. Ishikawa, Ø. S. Fjellvåg^c,d, D. W. Tam^c, F. Ye^e, D. L. Abernathy^e, G. D. A. Wood^f, D. Adroja^f,g, C-M. Wu^h, C-L. Huangⁱ, B. Gao^a, Y. Xie^a, Y. Gao^a, K. Rao^a, E. Morosan^a, K. Kindo, T. Masuda, K. Hashimoto^b, T. Shibauchi^b, and P. Dai^a
• ^aRice University
• ^bThe University of Tokyo
• ^cPaul Scherrer Institut
• ^dInstitute for Energy Technology
• ^eOak Ridge National Laboratory
• ^fRutherford Appleton Laboratory
• ^gUniversity of Johannesburg
• ^hNational Synchrotron Radiation Research Center
• ⁱNational Cheng Kung University