REVIEW 2 major objections 4 minor 50 references
Only fourfold-connected altermagnets let orbital magnetization lead spin magnetization under spin-orbit coupling, producing a coaxial Hall effect that tracks the Néel vector.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-10 22:32 UTC pith:EDDYQJRR
load-bearing objection Clean classification of altermagnetic OSLGs by SOC order; fourfold case uniquely gives first-order MO vs second-order MS and a coaxial Hall geometry, with consistent DFT checks. the 2 major comments →
Spin-orbit magnetism in altermagnets
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
When opposite-spin sublattices of a collinear altermagnet are related by a fourfold rotation (type-III oriented spin Laue groups), the leading spin-orbit corrections give first-order orbital magnetization and second-order spin magnetization; the same symmetry class admits a coaxial Hall effect in which both magnetizations align parallel to the Néel vector, as verified by density-functional calculations on KV2Se2O.
What carries the argument
SOC tensor expansion inside oriented spin Laue groups: the 3× 3 tensor χ(θ,φ) is expanded in powers of spin-orbit strength, and the lowest non-vanishing symmetry-adapted coefficients ω(n) for orbital versus spin magnetization are fixed by the connecting rotation {- 1∥(2l)} together with the collinear spin-only group ∞m1.
Load-bearing premise
The lowest non-vanishing orders of the SOC-tensor coefficients stay the same for every Néel direction that allows net magnetization, and those orders are completely fixed by the fourfold connector and the collinear spin-only group; higher-order lattice or multi-orbital terms that mix the orders would erase the type-III distinction.
What would settle it
In a confirmed type-III altermagnet such as KV2Se2O, artificially scale the spin-orbit strength in a controlled DFT or tight-binding calculation and check whether orbital magnetization grows linearly while spin magnetization grows quadratically; any simultaneous first-order rise of both components would falsify the claimed order disparity.
If this is right
- Type-III altermagnets can be screened by the presence of a fourfold sublattice connector, yielding candidates that combine large anomalous Hall conductivity with net moments of order 10^{-3} µB or smaller.
- The coaxial Hall geometry produces a transverse voltage proportional to sin ψ rather than cos ψ, vanishing when current is parallel to the Néel vector and peaking when they are orthogonal.
- Because orbital and spin moments lock to the Néel vector, a weak external field can reverse the Néel order by 180 degrees in the same manner as ordinary ferromagnets, enabling low-power antiferromagnetic switching.
- The same oriented-spin-group expansion can be reused for magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and other SOC-driven responses, giving a uniform design language for spintronic materials.
Where Pith is reading between the lines
- If the order disparity survives multi-orbital corrections, type-III materials become the preferred platform for Hall-based memory that is simultaneously stray-field-free and electrically readable.
- Transport experiments on KV2Se2O should focus on the angular dependence of the Hall voltage under in-plane field rotation; a clean sin ψ pattern would confirm coaxial locking without requiring absolute magnetization measurements.
- The same symmetry filter can be applied retroactively to disputed candidates such as RuO2: absence of the predicted first-order orbital / second-order spin hierarchy would argue against a type-III magnetic ground state.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses oriented spin Laue groups (OSLGs) and a spin-orbit-coupling (SOC) tensor expansion (Eqs. 1–3) to classify the leading-order SOC dependence of orbital magnetization MO and spin magnetization MS in collinear altermagnets. Ten altermagnetic OSLGs are partitioned into three types according to the spatial rotation that connects opposite-spin sublattices (Table I). Only type-III groups (fourfold connection) produce an order disparity: MO appears at first order in SOC while MS is second-order. In these systems the authors identify a “coaxial Hall effect” in which MO, MS and the Néel vector N become mutually parallel for selected high-symmetry orientations of N. The predicted scalings are checked by DFT on LiFe2F6 (MO ∝ λ, MS ∝ λ²) and the coaxial geometry is demonstrated for the metallic altermagnet KV2Se2O (AHC ~100 S/cm, net moment <10^{-3} µB). A short high-throughput screen of MAGNDATA yields seven candidate materials possessing the requisite {−1∥4} symmetry.
Significance. If the type-III order disparity and the associated coaxial Hall effect survive experimental scrutiny, the work supplies a concrete symmetry filter for altermagnets that simultaneously host a sizable anomalous Hall conductivity and a negligible net magnetization—precisely the combination desired for stray-field-free spintronics. The classification is derived from the authors’ earlier, independently constructed OSLG and SOC-tensor formalisms rather than fitted to the present data, and the DFT checks on LiFe2F6 and KV2Se2O provide falsifiable numerical signatures (λ versus λ² scaling and coaxial alignment). The framework also generalizes, at least in principle, to other SOC-induced responses (anisotropy, DMI, polarization). These features make the paper a useful addition to the rapidly growing altermagnet literature.
major comments (2)
- [Distinct SOC scaling behaviors / Supplemental Material S2] The central claim that the lowest non-vanishing orders of MO and MS are invariant for every net-magnetization-allowed Néel direction rests on the joint action of the collinear spin-only group ∞m1 and the connecting operation {−1∥(2l)} (Supplemental Material S2). The main text asserts this invariance without showing that multi-orbital or higher-order lattice corrections cannot mix the orders. A short explicit argument or numerical counter-check (e.g., varying Hubbard U or including higher multipoles in the LiFe2F6 expansion) would make the type-III distinction robust rather than conditional on the S2 derivation alone.
- [Coaxial Hall effect / high-throughput screening] The coaxial Hall effect is demonstrated only for one metallic candidate (KV2Se2O) and one insulating example (LiFe2F6). Given that five of the seven screened materials are large-gap insulators, it remains unclear whether the first-order MO (and therefore a sizable AHC) survives when the Fermi level lies inside a gap. A brief calculation of the Berry-curvature integral for at least one additional insulator, or an explicit statement that the effect is expected only in metals/semimetals, is needed to support the claim of a general materials-design principle.
minor comments (4)
- [Fig. 2 caption] Figure 2(b) caption refers to “the actual SOC value for Mn3Sn (λ0)” while the material under study is LiFe2F6; the reference compound should be corrected or clarified.
- [Decoupled scaling hierarchy in LiFe2F6] The oriented spin-space-group symbols (e.g., P−142/1m−1n1m∞100m1) are dense; a short glossary or a pointer to the earlier OSLG paper would help non-specialist readers.
- [Throughout] Typographical inconsistencies appear in several places (N´eel vs Néel, “seeting” for “setting”, missing spaces around µB). A careful proof-reading pass is recommended.
- [Distinct SOC scaling behaviors] The claim that RuO2 is “the only reported type-III candidate” should be qualified by the ongoing experimental controversy over its magnetic order; a brief note that the present classification is independent of that debate would avoid confusion.
Circularity Check
Mild self-citation of the authors' own oriented-spin-group and SOC-tensor formalisms; the type-III order disparity and coaxial-Hall claims are derived applications checked by independent DFT, not forced by construction.
specific steps
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self citation load bearing
[Introduction, paragraph beginning 'To address this issue…'; also SOC tensor expansion section and Table I]
"To address this issue, in this Letter we employ the newly developed oriented spin space group theory[32] to systematically investigate the dependence of spin magnetization and orbital magnetization on the SOC effect in altermagnets. … By treating SOC as a perturbation, the emergent MO and MS can be expanded as a power series of χ(θ,ϕ): Ma= au(0)a+ au(1)a,ij au ij( heta, au)+ au2 au(2)a,ij,kl au ij( heta, au) au kl( heta, au)+ au au."
The entire classification of the ten OSLGs into Types I–III (and therefore the claim that only fourfold-connected sublattices produce distinct MO/MS orders) rests on the transformation properties and tensor expansion introduced in the authors' own prior papers [31,32]. Those citations are load-bearing for the symmetry algebra used here. However, [31,32] are general formalisms, not data-fitted constructions whose output is being re-labeled as a prediction; the present results are applications of that algebra plus independent DFT verification. The circularity is therefore mild and does not force the central claim by definition.
full rationale
The derivation chain begins from the SOC Hamiltonian rewritten as a 3 imes3 tensor χ (Eq. 1), expanded as a power series in χ(θ,ϕ) (Eq. 3), then classified under the ten altermagnetic oriented spin Laue groups by evaluating the lowest non-vanishing ω(n) coefficients. Those groups and the tensor method are taken from the authors' prior works ([31], [32]), which supply the transformation rules for MS and MO under {U||R}. This is ordinary self-citation of a general mathematical framework; the framework itself is not fitted to the present materials or to the claimed order disparity. The new content—the partition into Types I–III according to the connecting rotation (twofold / sixfold / fourfold), the survival of the MO-first / MS-second hierarchy only for even l under the joint action of ∞m1 and {−1∥(2l)}, and the coaxial alignment MO ∥ MS ∥ N for the −14/1m−1m1m∞m1 case—is obtained by applying those rules, not by redefining the inputs. The two DFT examples (λ-scaling of MO ~ λ, MS ~ λ² in LiFe2F6; ~100 S cm−1 AHE with |M| < 10−3 µB and coaxial geometry in KV2Se2O) are parameter-free first-principles checks external to the symmetry algebra. No quantity is fitted and then re-predicted, no uniqueness theorem is imported to forbid alternatives, and no known empirical pattern is merely renamed. The single soft assumption (invariance of leading orders across all net-magnetization-allowed directions) is stated and deferred to SM S2; it does not render the central claim tautological. Hence only a minor self-citation burden exists, scoring 2.
Axiom & Free-Parameter Ledger
axioms (3)
- domain assumption Collinear magnets are fully classified by the ten oriented spin Laue groups (OSLGs) obtained by adjoining an SO(2) freedom between spin and lattice frames to the ordinary spin Laue groups.
- domain assumption The SOC Hamiltonian can be written as λ L̂ᵀ χ σ̂ and the induced magnetizations expanded as a power series in the components of χ(θ,ϕ); the leading non-zero order is fixed solely by the residual magnetic Laue group after SOC is turned on.
- ad hoc to paper The collinear spin-only group ∞m1 together with a connecting operation {−1∥(2l)} forces MO to order l−1 and MS to order l when l is even, while both are pushed to order l when l is odd.
invented entities (2)
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coaxial Hall effect
no independent evidence
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type-I / type-II / type-III altermagnetic OSLGs
no independent evidence
read the original abstract
The mechanism enabling antiferromagnets, including altermagnets, to exhibit a prominent anomalous Hall effect despite a vanishingly small net magnetization has long remained elusive. Here, by employing oriented spin group theory and spin-orbit-coupling tensor expansion, we systematically disentangle the perturbative behaviors of orbital and spin magnetizations with respect to spin-orbit coupling. Remarkably, we find that only if the opposite-spin sublattices are connected through a fourfold rotation, the orbital and spin magnetizations exhibit distinct perturbative orders. In these altermagnets, we further discover a coaxial Hall effect characterized by the induced spin and orbital magnetizations aligning parallel to the N\'eel vector, which we further demonstrate by first-principles calculations in the altermagnet KV$_{2}$Se$_{2}$O. This effect holds great promise for achieving deterministic switching of the N\'eel order under weak external fields. Our work provides a systematic symmetry approach to identify potential altermagnetic candidates combining a large anomalous Hall effect with minimal net magnetization, paving the way for high-performance, stray-field-free spintronic applications.
Figures
Reference graph
Works this paper leans on
-
[1]
axis. This effect fundamentally alters the opera- tional principles of an antiferromagnetic anomalous Hall device [Fig. 3(b)]. While most altermagnets exhibit an anomalous Hall vector perpendicular toN, yielding a (a) (b) (c) S Γ Γ a b c E J N FIG. 3. (a) Magnetic structure of KV 2Se2O; (b) Schematic of coaxial Hall effect.; (c)S y projection band structu...
-
[2]
Q. Liu, X. Dai, and S. Bl¨ ugel, Different facets of uncon- ventional magnetism, Nat. Phys.21, 329 (2025)
work page 2025
- [3]
-
[4]
L. ˇSmejkal, J. Sinova, and T. Jungwirth, Emerging Re- search landscape of altermagnetism, Phys. Rev. X12, 040501 (2022)
work page 2022
-
[5]
L. M. Sandratskii, R. F. Egorov, and A. A. Berdyshev, Energy band structure and electronic properties of nias type compounds. ii. antiferromagnetic manganese tel- luride, Phys. Status Solidi B104, 103 (1981)
work page 1981
- [6]
-
[7]
L.-D. Yuan, Z. Wang, J.-W. Luo, E. I. Rashba, and A. Zunger, Giant momentum-dependent spin splitting in centrosymmetric low-zantiferromagnets, Phys. Rev. B 102, 014422 (2020)
work page 2020
-
[8]
H.-Y. Ma, M. Hu, N. Li, J. Liu, J. Liu, W. Yao, J.-F. Jia, J. Liu, and J. Liu, Multifunctional antiferromagnetic ma- terials with giant piezomagnetism and noncollinear spin current, Nat. Commun.12, 2846 (2021)
work page 2021
-
[9]
I. I. Mazin, K. Koepernik, M. D. Johannes, R. Gonz´ alez- Hern´ andez, and L.ˇSmejkal, Prediction of unconventional magnetism in doped FeSb 2, Proc. Natl. Acad. Sci.118, e2108924118 (2021)
work page 2021
- [10]
-
[11]
I. Solovyev, S. Nikolaev, and A. Tanaka, Altermagnetism and weak ferromagnetism, npj Quantum Materials11, 54 (2026)
work page 2026
-
[12]
N. Nagaosa, J. Sinova, S. Onoda, A. H. MacDonald, and N. P. Ong, Anomalous Hall effect, Rev. Mod. Phys.82, 1539 (2010)
work page 2010
-
[13]
L. ˇSmejkal, R. Gonz´ alez-Hern´ andez, T. Jungwirth, and J. Sinova, Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets, Sci. Adv.6, eaaz8809 (2020)
work page 2020
-
[14]
R. D. Gonzalez Betancourt, J. Zub´ aˇ c, R. Gonzalez- Hernandez, K. Geishendorf, Z. ˇSob´ aˇ n, G. Springholz, K. Olejn´ ık, L.ˇSmejkal, J. Sinova, T. Jungwirth, S. T. B. Goennenwein, A. Thomas, H. Reichlov´ a, J.ˇZelezn´ y, and D. Kriegner, Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor, Phy...
work page 2023
-
[15]
A. Fakhredine, R. M. Sattigeri, G. Cuono, and C. Autieri, Interplay between altermagnetism and nonsymmorphic symmetries generating large anomalous Hall conductivity by semi-Dirac points induced anticrossings, Phys. Rev. B 108, 115138 (2023)
work page 2023
-
[16]
L. Han, X. Fu, R. Peng, X. Cheng, J. Dai, L. Liu, Y. Li, Y. Zhang, W. Zhu, H. Bai, Y. Zhou, S. Liang, C. Chen, Q. Wang, X. Chen, L. Yang, Y. Zhang, C. Song, J. Liu, and F. Pan, Electrical 180°switching of N´ eel vector in spin-splitting antiferromagnet, Sci. Adv.10, eadn0479 (2024)
work page 2024
- [17]
-
[18]
R. Takagi, R. Hirakida, Y. Settai, R. Oiwa, H. Tak- agi, A. Kitaori, K. Yamauchi, H. Inoue, J.-I. Yamaura, D. Nishio-Hamane, S. Itoh, S. Aji, H. Saito, T. Nakajima, T. Nomoto, R. Arita, and S. Seki, Spontaneous Hall ef- fect induced by collinear antiferromagnetic order at room temperature, Nat. Mater.24, 63 (2024)
work page 2024
-
[19]
Y. Liu, J. Li, and Q. Liu, Chern-Insulator phase in anti- ferromagnets, Nano Lett.23, 8650 (2023)
work page 2023
-
[20]
R.-C. Xiao, H. Li, H. Han, W. Gan, M. Yang, D.-F. Shao, S.-H. Zhang, Y. Gao, M. Tian, and J. Zhou, Anomalous- Hall N´ eel textures in altermagnetic materials, Sci. China: Phys. Mech. Astron.69, 217511 (2025)
work page 2025
-
[21]
W. F. Brinkman and R. J. Elliott, Theory of spin-space groups, Proc. R. Soc. A294, 343 (1966)
work page 1966
-
[22]
D. B. Litvin, Spin point groups, Acta Crystallogr. Sect. A33, 279 (1977)
work page 1977
-
[23]
L. M. Sandratskii, Classification of single-electron states in a crystal on the basis of spin space groups, Russ. Phys. J.22, 941 (1979)
work page 1979
-
[24]
P. Liu, J. Li, J. Han, X. Wan, and Q. Liu, Spin-group symmetry in magnetic materials with negligible spin- orbit coupling, Phys. Rev. X12, 021016 (2022)
work page 2022
-
[25]
X. Chen, J. Ren, Y. Zhu, Y. Yu, A. Zhang, P. Liu, J. Li, Y. Liu, C. Li, and Q. Liu, Enumeration and representa- tion theory of spin space groups, Phys. Rev. X14, 031038 (2024)
work page 2024
-
[26]
Z. Xiao, J. Zhao, Y. Li, R. Shindou, and Z.-D. Song, Spin space groups: Full classification and applications, Phys. 6 Rev. X14, 031037 (2024)
work page 2024
- [27]
-
[28]
H. Watanabe, K. Shinohara, T. Nomoto, A. Togo, and R. Arita, Symmetry analysis with spin crystallographic groups: Disentangling effects free of spin-orbit coupling in emergent electromagnetism, Phys. Rev. B109, 094438 (2024)
work page 2024
-
[29]
P. A. McClarty and J. G. Rau, Landau theory of alter- magnetism, Phys. Rev. Lett.132, 176702 (2024)
work page 2024
-
[30]
M. Roig, Y. Yu, R. C. Ekman, A. Kreisel, B. M. Ander- sen, and D. F. Agterberg, Quasisymmetry-constrained spin ferromagnetism in altermagnets, Phys. Rev. Lett. 135, 016703 (2025)
work page 2025
- [31]
-
[32]
Z. Liu, M. Wei, W. Peng, D. Hou, Y. Gao, and Q. Niu, Multipolar Anisotropy in Anomalous Hall Ef- fect from Spin-Group Symmetry Breaking, Phys. Rev. X 15, 031006 (2025)
work page 2025
-
[33]
Y. Liu, X. Chen, Y. Yu, J. Etxebarria, J. M. Perez-Mato, and Q. Liu, Symmetry classification of magnetic orders using oriented spin space groups, Nature652, 869 (2026)
work page 2026
-
[34]
D. Xiao, M.-C. Chang, and Q. Niu, Berry phase effects on electronic properties, Rev. Mod. Phys.82, 1959 (2010)
work page 1959
-
[35]
See Supplemental Material for details of SO-tensor ex- pansion, derivation for scaling behaviors, expressions for the orbital and spin magnetic moments, comparison between the constraints from SO-tensor and magnetic groups, the relationship between the direction and mag- nitudes of magnetic moments and the direction of the N´ eel order, material selection...
-
[36]
M. Milivojevi´ c, M. Orozovi´ c, S. Picozzi, M. Gmitra, and S. Stavri´ c, Interplay of altermagnetism and weak ferro- magnetism in two-dimensional RuF 4, 2D Materials11, 035025 (2024)
work page 2024
-
[37]
T. Berlijn, P. C. Snijders, O. Delaire, H.-D. Zhou, T. A. Maier, H.-B. Cao, S.-X. Chi, M. Matsuda, Y. Wang, M. R. Koehler, P. R. C. Kent, and H. H. Weitering, Itiner- ant Antiferromagnetism in RuO 2, Phys. Rev. Lett.118, 077201 (2017)
work page 2017
-
[38]
M. Hiraishi, H. Okabe, A. Koda, R. Kadono, T. Muroi, D. Hirai, and Z. Hiroi, Nonmagnetic Ground State in RuO2 Revealed by Muon Spin Rotation, Phys. Rev. Lett. 132, 166702 (2024)
work page 2024
- [39]
-
[40]
S. V. Gallego, J. M. Perez-Mato, L. Elcoro, E. S. Tasci, R. M. Hanson, K. Momma, M. I. Aroyo, and G. Madariaga, MAGNDATA: towards a database of mag- netic structures. I. The commensurate case, J. Appl. Crystallogr.49, 1750 (2016)
work page 2016
-
[41]
X. Chen, Y. Liu, P. Liu, Y. Yu, J. Ren, J. Li, A. Zhang, and Q. Liu, Unconventional magnons in collinear mag- nets dictated by spin space groups, Nature640, 349 (2025)
work page 2025
- [42]
-
[43]
V. I. Anisimov, J. Zaanen, and O. K. Andersen, Band theory and mott insulators: Hubbard u instead of stoner i, Phys. Rev. B44, 943 (1991)
work page 1991
-
[44]
G. Kresse and J. Furthm¨ uller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci.6, 15 (1996)
work page 1996
-
[45]
G. Kresse and J. Furthm¨ uller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B54, 11169 (1996)
work page 1996
-
[46]
J. P. Perdew, K. Burke, and M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865 (1996)
work page 1996
-
[47]
Q. Wu, S. Zhang, H.-F. Song, M. Troyer, and A. A. Soluyanov, WannierTools: An open-source software package for novel topological materials, Comput. Phys. Commun.224, 405 (2017)
work page 2017
-
[48]
G. Pizzi, V. Vitale, R. Arita, S. Bl¨ ugel, F. Freimuth, G. G´ eranton, M. Gibertini, D. Gresch, C. Johnson, T. Koretsune, J. Iba˜ nez-Azpiroz, H. Lee, J.-M. Lihm, D. Marchand, A. Marrazzo, Y. Mokrousov, J. I. Mustafa, Y. Nohara, Y. Nomura, L. Paulatto, S. Ponc´ e, T. Pon- weiser, J. Qiao, F. Th¨ ole, S. S. Tsirkin, M. Wierzbowska, N. Marzari, D. Vanderbi...
work page 2019
-
[49]
V. Wang, N. Xu, J.-C. Liu, G. Tang, and W.-T. Geng, VASPKIT: A user-friendly interface facilitating high- throughput computing and analysis using VASP code, Comput. Phys. Commun.267, 108033 (2021)
work page 2021
-
[50]
G.-X. Zhi, C. Xu, S.-Q. Wu, F. Ning, and C. Cao, Wannsymm: A symmetry analysis code for wannier or- bitals, Comput. Phys. Commun.271, 108196 (2022)
work page 2022
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