To appreciate why this matters, it helps to understand what makes kagome lattices so special. The kagome pattern is a two-dimensional tiling of corner-sharing triangles, and when magnetic atoms such as manganese arrange themselves on such a lattice, the resulting electronic structure is anything but ordinary. Quantum mechanics dictates that electrons moving through this geometry encounter Dirac cones, where bands cross linearly like in graphene; flat bands, where electrons become effectively immobile; and van Hove singularities, where the density of available electronic states spikes dramatically. These features concentrate an abstract quantum-mechanical quantity called Berry curvature into sharp hotspots in momentum space, and Berry curvature is the engine that drives the anomalous Hall effect, the appearance of a transverse voltage in a magnetic material even without an external Lorentz force.

YMn6Sn6 has become a poster child for this class of materials. It crystallizes in a hexagonal structure in which planes of manganese atoms form the kagome lattice, sandwiched between layers of tin and mixed yttrium-tin. The compound orders antiferromagnetically below a remarkably high Néel temperature of about 345 kelvin, comfortably above room temperature, and its low-temperature ground state is an incommensurate distorted spiral of manganese moments. When researchers apply a magnetic field within the kagome plane, this spiral unwinds in stages: first into a transverse conical spiral, then a fan-like configuration, then an intermediate phase, and finally a forced ferromagnetic state in which all moments align with the field. Magnetization measurements on the team’s single crystals revealed sharp anomalies at fields of roughly 2.5, 7.0, 9.9, and 10.5 tesla, mapping out this rich sequence of magnetic phases with excellent agreement to earlier work.

The crystals themselves are a triumph of materials synthesis. Grown from a tin flux, they exhibit residual resistivity ratios of about 44 along the crystallographic c-axis and 30 within the kagome plane, placing them squarely in what physicists call the clean transport regime. At low temperatures the electrical conductivity approaches 10^6 siemens per centimeter, comparable to conventional ferromagnetic metals at their purest. This cleanliness turns out to be far more than a technical achievement; as the new study demonstrates, it is the enabling condition for the record-breaking physics that follows, because certain scattering mechanisms that amplify the Hall response only become dominant when impurity densities are vanishingly small.

To connect the magnetic phases to electronic structure, the team combined three complementary probes. Angle-resolved photoemission spectroscopy, performed at the Pohang Light Source and the Advanced Light Source, directly imaged the bands of electrons near the Fermi level, revealing Dirac-like dispersions near the K points of the Brillouin zone and a flat band roughly 0.3 to 0.4 electron-volts below the Fermi energy. These measured dispersions matched density functional theory calculations remarkably well, provided the theorists modeled the spiral order with a commensurate double-layer antiferromagnetic configuration in which manganese moments are ferromagnetically aligned within each kagome bilayer but antiferromagnetically coupled between bilayers. Orbital analysis showed that the Dirac bands near the Fermi level derive mainly from manganese dz2 orbitals, while the flat band is dominated by in-plane dxy and dx2-y2 orbitals, a signature of the multi-orbital character of kagome electronics.

The theoretical centerpiece of the study is the demonstration that spin canting systematically rewrites this electronic landscape. The researchers modeled the effect of an in-plane field by rotating the manganese moments away from their antiferromagnetic orientation by an angle theta, sweeping continuously from the antiferromagnetic state at zero degrees to full ferromagnetic alignment at ninety degrees. As theta increases, the kagome flat bands spin-split and slide downward toward the Fermi level, while the band folding associated with the doubled unit cell weakens and eventually disappears. Most strikingly, at canting angles near seventy degrees, a spin-orbit-coupling-induced gap opens along the M-L direction of the Brillouin zone, and enormous Berry curvature hotspots erupt near the M point. The calculated intrinsic anomalous Hall conductivity peaks at exactly this canting angle, a robust result that survives even if the Fermi level is shifted by fifty milli-electron-volts.

The experimental transport data bear this prediction out with uncanny fidelity. After carefully subtracting the ordinary Hall contribution, which arises from the Lorentz force on moving charges, the team isolated the anomalous Hall conductivity as a function of field. For fields applied within the kagome plane, the anomalous component σxzA climbs through the sequence of magnetic phases and reaches a staggering maximum of roughly 2.5 x 10^4 siemens per centimeter at 9.5 tesla, right in the fan-like phase and precisely where the calculations place the Berry curvature maximum. Above the saturation field of about 13.5 tesla, in the fully polarized ferromagnetic state, the value settles near 5.0 x 10^3 siemens per centimeter, still far above the best ferromagnets on record, including nickel at about 1.6 x 10^3, Co2MnGa at about 2.0 x 10^3, iron-doped CoS2 at about 2.5 x 10^3, and NdCrSb3 at about 2.9 x 10^3 siemens per centimeter.

What makes these numbers truly extraordinary is that they cannot be explained by the standard intrinsic mechanism alone. The intrinsic Berry-curvature contribution to anomalous Hall conductivity is bounded by a fundamental quantum scale of roughly e2 per hectare, about 10^3 siemens per centimeter, and the calculated intrinsic value for the ferromagnetic phase of YMn6Sn6 falls well short of what was measured. The measured conductivity exceeds the calculation by a factor of about twenty-three, an enhancement ratio an order of magnitude larger than the factor of one to four seen in other colossal anomalous Hall materials. Conventional skew scattering, in which spin-orbit coupling at impurity sites bends electron trajectories asymmetrically, also falls short of explaining such a large amplification.

The resolution, the authors argue, lies in a more powerful mechanism: Berry-curvature-induced skew scattering. Recent theoretical work has shown that finite Berry curvature renders electron wavefunctions noncommutative in real space, producing asymmetric scattering probabilities even when the impurities themselves carry no spin-orbit coupling. The asymmetric scattering rate in this framework scales with the impurity potential cubed times the Berry curvature at the Fermi wavevector, whereas conventional impurity skew scattering scales only linearly with impurity spin-orbit strength. Plugging in realistic numbers, the Berry-curvature-driven channel can outperform the conventional one by one to two orders of magnitude, exactly the amplification needed to account for the colossal values observed in the clean crystals of YMn6Sn6. Below saturation, the same mechanism, fed by the strongly field-tunable Berry curvature of the canted spin phases, reproduces the observed peak near 9.5 tesla when the same enhancement factor is applied to the calculated curve.

Perhaps the most consequential implication is conceptual. For decades, physicists have assumed that the anomalous Hall conductivity should simply track the magnetization, so that any deviation is attributed to a topological Hall effect arising from scalar spin chirality in real space. Earlier work on YMn6Sn6 had indeed invoked such a topological contribution from its spiral textures. But the new data argue against that interpretation: the Hall response peaks in the fan-like phase, where the spin configuration subtends negligible solid angle and chirality should vanish, while the transverse conical phase, where neutron experiments confirm sizable chirality, shows no clear topological Hall signature at low temperatures. Moreover, for fields along the c-axis, where moments cant continuously without phase transitions, the anomalous Hall conductivity grows monotonically all the way into the chirality-free ferromagnetic state. The message is clear: in kagome antiferromagnets, flat bands and Dirac crossings near the Fermi level act as such potent reservoirs of momentum-space Berry curvature that even gentle field-induced spin canting can dramatically reshape the Hall response, and in clean crystals this tunable curvature is amplified to record heights. For engineers dreaming of electrically reconfigurable topological devices, YMn6Sn6 just became the most exciting material on the bench.

Subject of Research: Magnetic-field-induced Berry curvature modulation and colossal anomalous Hall conductivity in the kagome antiferromagnet YMn6Sn6

Article Title: Colossal Magnetic‐Field‐Induced Anomalous Hall Conductivity in a Kagome Antiferromagnet YMn6Sn6

Article References: Choi, M., Kim, H., Sung, M., Seo, H., Choi, J., Kim, J., Jo, Y., Yeom, H. W., Jin, K.-H., & Kim, J. S. (2026). Colossal Magnetic‐Field‐Induced Anomalous Hall Conductivity in a Kagome Antiferromagnet YMn 6 Sn 6. Advanced Science, Article e77738. https://doi.org/10.1002/advs.77738

Image Credits: AI Generated

DOI: 10.1002/advs.77738

Keywords: kagome lattice, YMn6Sn6, antiferromagnetism, anomalous Hall effect, Berry curvature, spin canting, skew scattering, Dirac bands, flat bands, spintronics, ARPES, density functional theory