HODACA Research Highlight

Gapless Magnon‑Driven Anomalous Hall Conductivity in the Collinear Kagome Antiferromagnet YbFe6Ge6

 W. Yao et al., Phys. Rev. Lett. 134, 186501 (2025). 

YbFe6Ge6 is a kagome‑lattice intermetallic whose Fe3+ moments order antiferromagnetically below TN ≈ 500 K. Neutron diffraction shows an A‑type structure with spins along the c axis above the spin‑reorientation temperature TSR ≈ 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 Δσxy ≈ 30 Ω-1 cm-1 at 10 K for in‑plane fields, whereas no signal appears for out‑of‑plane fields or T > TSR, 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 ≈ 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 Δσxy, 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 Δσxy; its Zeeman energy for the 1.5 μB 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 μB/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 AMn6Sn6 [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 μB, 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].

Fig. 1. (a) Low-energy spin excitations at (0, 0, 1) at selected temperatures measured at HODACA, offset for clarity. Horizontal dashed lines indicate the zero intensity for the data above 2.5 K. Dashed curves are the fits to 65 K and 80 K data [36]. The light blue region shows energy resolution, and the arrow marks 0.6 meV, below which gapless excitations emerge. (b) Temperature dependence of the intensities at (0, 0, 1) at 0.55 meV and 2.55 meV. (c) Temperature dependence of the maximum magnitudes of Δσxy. The bold gray curve is a guide to the eyes. Inset illustrates electron scattering by the spin fluctuations.

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. Yaoa,b, S. Liub H. Kikuchi, H. Ishikawa, Ø. S. Fjellvågc,d,  D. W. Tamc, F. Yee, D. L. Abernathye, G. D. A. Woodf, D. Adrojaf,g, C-M. Wuh, C-L. Huangi, B. Gaoa, Y. Xiea, Y. Gaoa, K. Raoa, E. Morosana, K. Kindo, T. Masuda, K. Hashimotob, T. Shibauchib, and P. Daia

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

PI of Joint-use project: P. Dai
Host lab: Masuda Group and Kindo Group

Dirac Magnons in Honeycomb-Lattice NiTiO3

H. Kikuchi et al., J. Phys. Soc. Jpn. 94, 024703 (2025).  

Since the discovery of topological insulators, the concept of topology has become widely recognized as an important aspect of condensed‑matter physics. These materials host massless Dirac fermions on their edges or surfaces, giving rise to phenomena such as the quantum Hall effect. Recently, potential applications for highly efficient spintronic devices that exploit spin currents along these edges or surfaces have attracted significant attention. Moreover, the notion of topology has been extended from fermionic to magnonic systems, as evidenced by phenomena such as the thermal Hall effect.

Inelastic neutron scattering (INS) experiments have confirmed the existence of topological magnons in several materials. For example, in layered honeycomb-lattice ferromagnet CrI3 [1] and three-dimensional ferromagnet Mn5Ge3 [2], bulk magnon dispersions exhibit gaps at the K point, and theoretical calculations predict the presence of edge states—Dirac magnons—within these gaps. Dirac magnons have also been observed in the three‑dimensional antiferromagnet Cu3TeO6 [3] and in the ilmenite-type antiferromagnet CoTiO3 [4], where linear band crossings at the K point create Dirac cones for both bulk and edge modes. These findings underscore the rapid expansion of topological motifs in magnonic systems that mirror those long explored in electronic counterparts.

In this study, we investigate NiTiO3 [5], which has the same crystal structure and magnetic ordering as CoTiO3. Magnetic-susceptibility measurements reveal that NiTiO3 exhibits stronger interlayer interactions, whereas CoTiO3 is dominated by intralayer coupling. To determine the spin Hamiltonian and examine the presence of Dirac magnons in NiTiO3, we conducted single-crystal INS experiments.

Measurements were performed using the multiplex spectrometer HODACA at JRR-3 and the triple-axis spectrometer CTAX at ORNL/HFIR [6]. Figure 1(a) shows the INS spectrum measured by HODACA along the (0, 0, l) direction, revealing spin-wave excitations with a band energy of 3.7 meV. Using linear spin-wave theory (LSWT), experimental data were accurately reproduced (Fig. 1(b)) by a model incorporating exchange interactions and easy-plane anisotropy. This model confirmed NiTiO3 as a three‑dimensional magnet in which interlayer coupling outweighs intralayer coupling. Moreover, whereas nearest‑neighbor interactions suffice to describe CoTiO3, modeling NiTiO3 demands exchange paths extending to third‑nearest neighbors within the ab plane.

Figure 1(c) shows the INS spectra measured by CTAX along the high‑symmetry path Γ1-M-K-Γ2. Near the K point, considering the instrumental energy resolution, the spectra were fitted by single Gaussian at the K point and two Gaussians near the K point. The dispersion relations calculated using the best fit parameters, shown in Fig. 1(d), reveal two modes that intersect linearly at the K point, confirming the formation of Dirac cones in NiTiO₃, just as in CoTiO₃. Previous work [4] argued that Dirac magnons remain stable against anisotropy and further-neighbor interactions. Our study experimentally demonstrates that Dirac magnons persist even with significant interlayer coupling and second- and third-neighbor interactions within the honeycomb layer.

Fig. 1. (a, b) (a) INS spectra along the (0, 0, l) direction measured by HODACA, and (b) calculated INS spectra using linear spin-wave theory. (c) Pseudocolor plot of INS spectra measured by CTAX along the reciprocal lattice points Γ1-M-K-Γ2. White lines represent dispersion relations calculated using LSWT, yellow dots indicate peak positions obtained by Gaussian fitting, and error bars correspond to the full width at half maximum (FWHM) from the Gaussian fitting. (d) Enlarged view of the calculated dispersion relations near the K point.

References

[1] L. Chen et al., Phys. Rev. X 8, 041028 (2018).

[2] M. dos Santos Dias et al., Nat. Commun. 14, 7321 (2023).

[3] W. Yao et al., Nat. Phys. 14, 1011 (2018).

[4] B. Yuan et al., Phys. Rev. X 10, 011062 (2020).

[5] K. Dey et al., Phys. Rev. B 103, 134438 (2021).

[6] H. Kikuchi et al., J. Phys. Soc. Jpn. 94, 024703 (2025).

Authors

H. Kikuchia, M. Ozekia, N. Kuritab, S. Asaia, T. J. Williamsc, T. Hongc, and T. Masudaa

aThe University of Tokyo

bInstitute of Science Tokyo

cOak Ridge National Laboratory 

A New Inelastic Neutron Spectrometer HODACA

H. Kikuchi et al., J. Phys. Soc. Jpn. 93, 091004 (2024).  

Neutron scattering is an indispensable experimental technique in a wide range of fields including physics, chemistry, and engineering. In 1949, Nobel Laureate C.G. Shull demonstrated its usefulness by elucidating the magnetic structure of the antiferromagnet MnO using neutron scattering [1]. Particularly, inelastic neutron scattering (INS) has proven to be a powerful tool for observing collective excitations of atoms and spins in condensed matter. These collective modes are characterized by a wavevector Q and energy E, thus measuring them allows for the determination of the system’s Hamiltonian. The dynamics of crystals [2], magnetic materials [3], and other systems have been actively studied using this technique.

The neutron triple-axis spectrometer (TAS) has been widely used in both inelastic and elastic scattering experiments since its development in the 1950s, establishing its position as a versatile and important spectrometer [4]. By using a single analyzer and detector along with a focused neutron beam, TAS allows for high signal-to-noise (S/N) ratio measurements at specific Q-E points. On the other hand, chopper spectrometers employ an array of detectors surrounding the sample, combined with time-of-flight energy analysis, enabling efficient measurements across a wide Q-E space. Recently, there has been a global trend in the design, construction, and operation of multiplex-type spectrometers, which combine high S/N measurements with efficient Q-E space coverage [5]. In this study, we constructed a multiplex spectrometer called HODACA (HOrizontally Defocusing Analyzer Concurrent data Acquisition) based on the inverse Rowland inelastic spectrometer (IRIS) concept proposed by Harriger and Zaliznyak [6]. The instrument was installed at the C11 beam port of the research reactor JRR-3 [7].

As shown in Fig. 1, the HODACA spectrometer employs an array of analyzers arranged on a Rowland circle to refocus scattered neutrons. Due to the inscribed angle theorem, the reflection angles of all analyzers remain constant, and the trajectories of the neutrons form a pattern as if they are diffused from a sample image. Neutrons are then detected by an array of detectors positioned on a circle centered on the sample image. The use of radial collimators before and after the analyzers is expected to effectively reduce background noise. This spectrometer enables efficient and high-S/N measurements across a wide Q-space at constant energy. As a result, HODACA became a spectrometer capable of measuring spectra from −1 meV to 7 meV by fixing the scattered neutron energy Ef at 3.635 meV. It consists of 24 analyzers and 24 detectors spaced at 2◦ intervals, covering a scattering angle A2 of 46◦. Each analyzer is composed of 3 to 7 PG crystals mounted in a vertically focusing configuration. The vertical size of each analyzer (number of PG crystals) is determined to ensure that the solid angles spanned by the analyzer viewed from the sample position are the same. Radial collimators with divergence angle of 2◦ are installed between the sample-analyzer and analyzer-detector to minimize cross-talk of scattered neutrons from neighboring analyzers.

For the standard sample to measure INS spectra, we selected the frustrated magnetic compound CsFeCl3. Its dispersion relation at ambient pressure is well described by the Extended Spin Wave Theory (ESWT) [8]. The INS spectrum measured by the HODACA spectrometer is shown in Fig. 1(b), where Q = (1/3, 1/3, l) direction. The white lines in the figure represent the dispersion curves calculated using ESWT parameters from previous studies. The experimental results are well reproduced by the calculations using the previously reported parameters. An ideal spectrometer for measuring dynamics in the energy range of cold neutrons is now ready for users.

Fig. 1. (a) Representative figure of HODACA spectrometer. Green lines indicate the neutron paths. (b) False color plots for the (1/3, 1/3, l) direction. The integration ranges are (1/3 ± 0.05, 1/3 ± 0.05, l). The white curves represent the dispersion curves obtained from ESWT.

References

[1] C. G. Shull and J. S. Smart, Phys. Rev. 76, 1256 (1949).

[2] R. Pynn and G. L. Squires, Proc. R. Soc. A 326, 347 (1972).

[3] M. F. Collins et al., Phys. Rev. 179, 417 (1969).

[4] G. Shirane, S. M. Shapiro, and J. M. Tranquada, Neutron Scattering with a Triple-Axis Spectrometer (Cambridge University Press, Cambridge, U.K., 2008).

[5] F. Groitl et al., Rev. Sci. Instrum. 87, 035109 (2016).

[6] L. Harriger and I. Zaliznyak, 2015 NCNR Annual Report (2015) p. 46

[7] H. Kikuchi et al., J. Phys. Soc. Jpn. 93, 091004 (2024).

[8] S. Hayashida, M. Matsumoto, M. Hagihala, N. Kurita, H. Tanaka et al., Sci. Adv. 5, eaaw5639 (2019).

Authors

H. Kikuchia, S. Asaia, T. J. Satob, T. Nakajimaa, L. Harrigerc, I. Zaliznyakd and T. Masudaa

aThe University of Tokyo

bTohoku University

cNational Institute of Standards and Technology

dBrookhaven National Laboratory