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Nonreciprocal transport in a room-temperature chiral magnet

T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read This paper claims that the nonreciprocal resistivity of the chiral magnet Co8Zn9Mn3 decomposes into two distinct components, one from chiral spin scattering and one from conical-state band asymmetry.

desk verdict Room-temperature nonreciprocal transport with a plausible but imposed two-component separation; worth a serious referee, not a desk reject. read the letter →

arxiv 2412.02272 v1 pith:ZRXPSGN6 submitted 2024-12-03 cond-mat.mtrl-sci

Daisuke Nakamura , Mu-Kun Lee , Kosuke Karube , Masahito Mochizuki , Naoto Nagaosa , Yoshinori Tokura , Yasujiro Taguchi This is my paper · ORCID
classification cond-mat.mtrl-sci
keywords nonreciprocaltransportelectricalmagnetochiraleffectchiralmagnetconicalspinstatechiralitybandasymmetryCo-Zn-Mnsecondharmonicresistivity
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports that the nonreciprocal (direction-dependent) electrical resistivity of the chiral magnet Co8Zn9Mn3 contains two physically distinct contributions that can be separated by their magnetic-field and temperature dependence. One contribution, largest just above the Curie temperature and vanishing toward zero temperature, comes from asymmetric scattering of electrons off chiral spin fluctuations (magnons below Tc, critical spin clusters above Tc). The other, which appears in the conical spin state and survives at the lowest temperatures, comes from an asymmetry in the electronic band dispersion induced by the exchange coupling between conduction electrons and the conical spin texture. Using a Kondo-lattice model and Boltzmann transport theory, the paper derives a closed-form expression for the band term and shows it scales as $\cos\alpha \sin^2\alpha$, where $\alpha$ is the tilt angle of the conical magnetization. Because both mechanisms appear in one material, the experiment provides a clean platform for studying nonreciprocal transport at and above room temperature.

What carries the argument

The central construct is the additive decomposition $r_{xx,2f}(T,B) = r_{2f}^{\mathrm{scat}}(T,B) + r_{2f}^{\mathrm{band}}(T,B)$, with the scattering part fixed as $r_{2f}^{\mathrm{scat}}(T,B) = A_{\mathrm{scat}}(T) M(T,B)$ below 290 K and the band part defined as the remainder, normalized so that it vanishes in the high-field forced-ferromagnetic limit. The theory is built on a Kondo-lattice Hamiltonian for conduction electrons coupled to a conical spin texture, and the Boltzmann equation with a single relaxation time is expanded to second order in the electric field. For the band term this yields the closed identity $r_{2f}^{\mathrm{band}} \propto \cos\alpha(B) \sin^2\alpha(B)$, where $\alpha$ is the tilt angle of the conical magnetization defined by $\cos\alpha = M(B)/M_{\mathrm{sat}}$; the same framework, with the Dzyaloshinskii-Moriya interaction treated perturbatively, produces the magnon-induced vector-chirality scattering with a $T^{3/2}$ low-temperature law. The decomposition rule is what lets the experiment separate the two mechanisms.

What would settle it

Measure $r_{xx,2f}$ at fixed temperature below 100 K from the conical phase through $B_c$ into the forced ferromagnetic state. The paper's identification predicts that the band term follows $\cos\alpha(B) \sin^2\alpha(B)$ inside the conical phase and drops to zero above $B_c$, while the scattering term contributes a $T^{3/2}$ background; a clear field-dependent nonreciprocal signal above $B_c$ at low temperature, or a deviation of $r_{2f}^{\mathrm{scat}}$ from strict proportionality to $M(T,B)$ at intermediate fields, would break the decomposition and falsify the two-mechanism assignment.

Watch

Extended reading notes

Core claim

The central claim is that the measured nonreciprocal resistivity $r_{xx,2f}$ in Co8Zn9Mn3 is the sum of two components with different physical origins. For $T < 290$ K, the paper decomposes $r_{xx,2f} = r_{2f}^{\mathrm{scat}} + r_{2f}^{\mathrm{band}}$, where $r_{2f}^{\mathrm{scat}}$ is assumed proportional to the magnetization $M(T,B)$ and $r_{2f}^{\mathrm{band}}$ is the remainder; the normalization is fixed by requiring the band term to vanish in the forced ferromagnetic state at high field. For $T > 290$ K, the entire signal is assigned to $r_{2f}^{\mathrm{scat}}$. The identified scattering term peaks near $T_c = 301$ K, follows the magnetization's field dependence, and is explained by magnon-induced vector spin chirality (with a $T^{3/2}$ low-temperature law) and by critical chiral spin-cluster fluctuations above $T_c$. The band term is largest in the conical phase, persists to zero temperature at roughly half its maximum, and is reproduced by the zero-temperature Boltzmann formula $r_{2f}^{\mathrm{band}} \propto \cos\alpha \sin^2\alpha$, reflecting the tilt angle of the conical magnetization. The paper concludes that these are the same two mechanisms long discussed separately in chiral magnets, here separated in one room-temperature material.

Load-bearing premise

The decomposition rests on assuming that below 290 K the scattering component is strictly proportional to magnetization with a coefficient fixed by demanding the band component vanish at high field, and that above 290 K the entire signal is scattering; if either assumption fails, the reconstructed band component is an artifact.

Editorial extensions

If this is right

  • The two mechanisms can be separated in a single material because they occupy different field and temperature regimes: the scattering term peaks near $T_c$ and vanishes at zero temperature, while the band term lives in the conical state and survives to zero temperature.
  • The band term's field dependence is tied directly to the tilt angle of the conical magnetization through $\cos\alpha \sin^2\alpha$, so nonreciprocal transport can be used as a probe of the conical state's structure.
  • Nonreciprocal resistivity is not always a fluctuation or scattering phenomenon: the conical-state band asymmetry produces a finite signal even in the zero-temperature limit.
  • Below $T_c$, the scattering component is consistent with magnon-induced vector spin chirality, reproducing the experimentally observed $T^{3/2}$ growth at low temperatures.
  • Above $T_c$, the same scattering channel is driven by critical spin fluctuations with finite vector chirality, analogous to the behavior reported in MnSi.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the decomposition is correct, then $r_{2f}^{\mathrm{scat}}(T,B)/M(T,B)$ should be a function of temperature only below 290 K; checking this directly on a denser field grid would test the proportionality assumption without relying on the high-field limit.
  • The paper's result $r_{2f}^{\mathrm{band}} \propto \cos\alpha \sin^2\alpha \propto M\langle\chi_{\mathrm{vec}}\rangle$ suggests that both the band-asymmetry and scattering mechanisms may be governed by the same vector-chirality order parameter, so materials with larger helical pitch or stronger DMI could be screened by measuring this single scaling.
  • The observed $\mathbf{j}\times\mathbf{B}$ component, which the authors assign to the electrode interface, implies that nonreciprocal measurements on microdevices can mix intrinsic bulk signals with surface or Rashba-type contributions; angle-dependent checks are necessary for quantitative separation in other materials.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The manuscript reports measurements of nonreciprocal (second-harmonic) resistivity in the chiral magnet Co8Zn9Mn3 over a wide temperature range, including above room temperature. The authors observe a field- and temperature-dependent signal and decompose it into two components: a scattering contribution r2f_scat that is assumed proportional to the magnetization for T<290 K and dominant near Tc, and a band-asymmetry contribution r2f_band that appears in the conical state and persists to low temperature. A Kondo-lattice model with a conical spin structure yields a theoretical r2f_band proportional to cos(alpha) sin^2(alpha), and a magnon calculation yields r2f_scat ~ T^(3/2) at low temperatures. The central claim is that these two microscopic mechanisms are separated and identified in a single room-temperature chiral magnet.

Significance. If the decomposition were independently validated, this paper would be a significant contribution: it would demonstrate the coexistence and separation of scattering-based and band-based nonreciprocal transport in a single material with a quantitative theoretical framework. The experimental dataset is extensive, the exclusion of the spurious I x B component via polar-angle measurements is careful, and the Boltzmann-theory derivation of the band-asymmetry term, including the exact diagonalization of the Kondo-lattice Hamiltonian, is a useful theoretical contribution. However, the central decomposition rests on an assumption that is not directly measured, and the theoretical comparisons involve fitted or assumed normalizations, so the mechanism identification is less secure than the text suggests.

major comments (4)
  1. [Main text, Eq. (1) and following paragraph] The decomposition rxx,2f = r2f_scat + r2f_band is imposed rather than measured. The text states that the sign and normalization of r2f_scat is determined with the assumption that r2f_band vanishes in the large-field limit; with that anchor, r2f_band is defined as the residual. Consequently, the statement that r2f_band appears only in the conical state and is absent in the forced-ferromagnetic state is built into the analysis, not tested. An independent determination of r2f_scat (for example, from a measurement in a clearly saturated forced-ferromagnetic state at fields well above Bc, or from a microscopic calculation of the scattering term without the M-proportionality ansatz) is needed to support the central claim.
  2. [Fig. S12 and Eq. (7)] The theoretical comparison uses alpha(B) extracted from cos alpha(B) = M(B)/M(0.1 T), with the assumption that M is saturated at 0.1 T. Figs. 1(b) and S3 show that Bc and BH-C rise steeply below 100 K; at 10 K, 0.1 T is very likely inside the conical or helical state, so alpha(0.1 T) is not zero. The theoretical r2f_band proportional to cos(alpha) sin^2(alpha) is then not required to vanish for B > 0.1 T. The residual signal above 0.1 T, which the authors attribute to the empirical form of r2f_scat, could instead be a band term that persists because of an incorrect normalization. Conversely, subtracting an M-proportional term using an unsaturated M(B) can generate an artificial band-like contribution in the conical region. This normalization is load-bearing for the mechanism identification.
  3. [Fig. 4(d) and SI D-4.4] The magnon comparison is not parameter-free: the blue dotted curve in Fig. 4(d) is obtained by a least-squares fit of rho_scat = c T^(3/2) to the experimental data, with c chosen to match the data. The statement that the T^(3/2) curve agrees with experiment therefore does not constitute an independent quantitative test of the scattering mechanism; it shows only that the data are consistent with a T^(3/2) law after choosing the overall scale. A prediction of the absolute magnitude, or an independent determination of c from known material parameters, would strengthen the identification.
  4. [Main text, Eq. (2)] For T > 290 K (that is, below Tc = 301 K), the entire measured rxx,2f is assigned to r2f_scat based on the sign of the signal, with no quantitative test for a possible surviving band contribution. Since the band-asymmetry term is argued to exist in the conical state for all T < Tc, a coexistence of the two components in the 295-300 K range is plausible and should be ruled out by a fit or a bound rather than by inspection of sign alone.
minor comments (5)
  1. [References] Reference [4] contains a typo: 'Annual Review and of Condensed and Matter Physics' should be 'Annual Review of Condensed Matter Physics'.
  2. [Notation] The notation for the nonreciprocal resistivity switches between rxx,2f in the main text and rho_2f in the Supplementary Information; please unify the notation across the manuscript.
  3. [Fig. 4(c) caption] The caption states that the theoretical curve reproduces the trend, but the curve is normalized to the experimental data; please state the normalization explicitly in the caption.
  4. [Main text, after Eq. (1)] The sentence describing the determination of r2f_scat should specify the exact large-field limit used (which field value), because the resulting decomposition depends on that choice.
  5. [SI D-4.4, Eq. (D23)] The numerical value 3.47 in Eq. (D23) is quoted without derivation; a brief explanation of the integration method or a reference would improve reproducibility.

Circularity Check

2 steps flagged · score 6.0 of 10

The separation of r2f_band is enforced by anchoring its high-field value to zero, and the theoretical cosα sin²α comparison is forced to zero above 0.1 T by defining α(0.1 T)=0 from the same magnetization curve.

  1. self definitional [Eq. (1) and description in Fig. 2(a)]
    "Note that the sign and normalization of r2fscat is determined with an assumption that r2fband, which is depicted as the pink hatched region in panel (a), vanishes in the large magnetic field limit. The residual component in rxx,2f for T < 290 K is ascribed to r2fband."

    The decomposition defines r2f_band as the residual after subtracting Ascat(T)M(T,B), but Ascat is not measured independently: it is chosen so that the residual vanishes at large field. The later claim that the band term appears only in the conical state and is absent in the forced-ferromagnetic state is therefore an input to the decomposition, not a tested consequence. The distinct field dependence of r2f_band is generated by subtracting an M-proportional curve, so part of the 'separated' component is manufactured by the chosen normalization.

  2. self definitional [Fig. S12 caption and Fig. 4(c) note]
    "cosα(B)=M(B)/M(0.1 T) by assuming the saturation of M as at B=0.1 T. ... Note that for B>0.1 T, theoretical r2fband becomes zero because we assumed a(B = 0.1 T) = 0."

    Since α(0.1 T) is defined to be zero, the prefactor cosα sin²α in Eq. (7) vanishes identically for B ≥ 0.1 T. The theoretical curve is thus forced to mimic the experimental decomposition's high-field zero, which itself was imposed by the normalization of Ascat. The agreement that r2f_band is confined to the conical region and disappears in the forced-ferromagnetic state is a shared input of the two analyses, not an independent prediction. The saturation assumption is especially questionable at 10 K because the paper's own phase diagram shows Bc rising steeply below 100 K, so 0.1 T may not be in the forced-ferromagnetic state.

full rationale

The Hamiltonian diagonalization and Boltzmann-transport derivation leading to Eq. (7) are genuine first-principles calculations and are not circular. The magnon vector-chirality calculation in SI D-4.4 is also self-contained, although its final comparison reduces to a one-parameter T^(3/2) fit. The circularity is concentrated in the experimental separation and in the theory-experiment comparison. Eq. (1) defines r2f_band as the residual after subtracting Ascat(T)M(T,B), with Ascat anchored by requiring that residual to vanish at high field; Fig. S12 then defines α(B) from the same M(B) by assuming saturation at 0.1 T, which forces Eq. (7)'s cosα sin²α to zero for B ≥ 0.1 T. Therefore, the central claims that the band term exists only in the conical phase, that its field dependence follows cosα sin²α, and that it vanishes in the forced-ferromagnetic state are partly built into the shared inputs rather than independently established. The residual signal inside the conical region is real data, and the temperature profiles (broad peak near 150 K and persistence to zero temperature) are not directly imposed by the decomposition, so the paper retains substantial independent content. Score 6 reflects partial, not total, circularity.

Assumptions & free parameters 3 free parameters · 8 assumptions · 0 invented entities

The central claim rests on a two-component decomposition of the measured second harmonic resistivity. The decomposition introduces a per-temperature proportionality constant A_scat, a saturation normalization for alpha(B), and a least-squares coefficient c for the magnon scaling, while the microscopic Hamiltonians, Boltzmann approximation, and bulk-to-device magnetization mapping are borrowed or assumed.

free parameters (3)
  • A_scat(T) = per temperature, not tabulated
    Scale of r2f_scat = A_scat(T) M(T,B); chosen so r2f_band vanishes at high field. This choice determines the separated band term, so it is a fitted normalization.
  • c in rho_scat = c T^(3/2) = not reported
    Least-squares fit to experimental r2f_scat below 280 K; makes the magnon comparison a one-parameter fit rather than an absolute prediction.
  • M(0.1 T) saturation normalization for alpha(B) = M_sat at 0.1 T
    cos alpha = M(B)/M(0.1 T) assumes magnetization is saturated at 0.1 T; this defines the theoretical curve in Fig. 4(c) and couples the residual and the theory through the same measured M(B).
assumptions (8)
  • domain assumption Kondo-lattice Hamiltonian with classical conical local moments (Eq. 3 and Eq. 4) describes the relevant nonreciprocal transport in Co8Zn9Mn3.
    Used for both band and magnon calculations; no microscopic justification beyond the known helical order.
  • standard math Boltzmann transport with a single relaxation time applies to the second harmonic resistivity.
    SI D-4.2 follows Refs [8,27]; ignores scattering anisotropy and vertex corrections.
  • ad hoc to paper For T<290 K, r2f_scat is exactly proportional to M(T,B), and r2f_band vanishes in the large field limit.
    Main text after Eq. (1); the paper says the sign and normalization of r2f_scat are determined by this assumption. This is the central separation premise.
  • ad hoc to paper For T>290 K, the measured rxx,2f is entirely r2f_scat.
    Main text Eq. (2); assumes no band contribution above Tc.
  • domain assumption Vector spin chirality fluctuations generated by spin clusters or magnons are the cause of r2f_scat.
    Uses the mechanism of Ref [13]; no direct measurement of the spin chirality is provided.
  • domain assumption Bulk magnetization measured on a separately polished crystal represents the microdevice magnetization when computing A_scat and alpha.
    Methods and SI Sec. B; only the aspect ratio is matched, not the exact device geometry.
  • domain assumption The observed I x B component is extrinsic and does not contaminate the j || B measurements.
    SI D-3; based on polar angle 360-degree periodicity rather than fourfold cubic symmetry.
  • domain assumption DMI is small (D/JF ~ 1e-2) so first-order expansion in D and the continuous low-temperature limit are valid for magnon chirality.
    SI D-4.4 uses D/JF ~ 1e-2 from literature and drops the Zeeman field in the continuous limit.

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Cite this review

Pith. "Pith review of Nonreciprocal transport in a room-temperature chiral magnet." pith.science (2026). https://pith.science/paper/ZRXPSGN6

@misc{pith2026241202272,
  author       = {Pith},
  title        = {Pith review of: Nonreciprocal transport in a room-temperature chiral magnet},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZRXPSGN6}},
  note         = {Machine review of arXiv:2412.02272}
}
read the original abstract

Chiral magnets under broken time-reversal symmetry can give rise to rectification of moving electrons, called nonreciprocal transport. Several mechanisms, such as the spin-fluctuation-induced chiral scattering and asymmetry in the electronic band dispersion with and without the relativistic spin-orbit interaction, have been proposed, but clear identification as well as theoretical description of these different contributions are desired for full understanding of nonreciprocal transport phenomena. Here, we investigate a chiral magnet Co8Zn9Mn3 and find the nonreciprocal transport phenomena consisting of different contributions with distinct field- and temperature-dependence across the magnetic phase diagram over a wide temperature range including above room-temperature. We successfully separate the nonreciprocal resistivity into different components and identify their mechanisms as spin-fluctuation-induced chiral scattering and band asymmetry in a single material with the help of theoretical calculations.

Figures

Figures reproduced from arXiv: 2412.02272 by the authors.

Figure 1
Figure 1. Characteristics of Co8Zn9Mn3. (a) b-Mn type crystal structure of Co-Zn-Mn chiral magnet. The atoms at 8c and 12d sites are colored with blue and red, respectively. (b) Magnetic phase diagram of Co8Zn9Mn3, obtained from the magnetization curves for a bulk crystal with the same aspect ratio as the microfabricated sample. (c) Schematic spin arrangement in helical and conical phases. (d) Scanning electron microscopy (SE… view at source ↗
Figure 2
Figure 2. Nonreciprocal transport observed in Co8Zn9Mn3. (a), (b) Magnetic field dependence of nonreciprocal resistivity (rxx,2f) at selected temperatures below 275 K [(a), blue curves] and above 295 K [(b), green curves]. The black dashed curves in (a) represent the component (magnon-induced r2f scat) proportional to M, which is estimated from the M-B curve of bulk crystal with the same aspect ratio with the microfabricated … view at source ↗
Figure 3
Figure 3. Temperature profiles of decomposed contributions to the nonreciprocal resistivity. (a), (b) The color contour maps of the nonreciprocal resistivity contributions r2fband and r2f scat in the magnetic phase diagram. In (a), H and C stand for the helical and conical spin states, respectively. In (b), the color scale is reversed to that of (a) due to the opposite sign between r2fband and r2f scat. The open circles indic… view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Magnon-induced scalar spin chirality in Kagome and honeycomb ferromagnets

    cond-mat.mes-hall 2025-01 accept novelty 7.0 of 10

    In collinear Kagome and honeycomb ferromagnets, Dzyaloshinskii-Moriya interactions give thermally excited magnons a spin texture that produces a finite scalar spin chirality at finite temperature.

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Works this paper leans on

2 extracted references · 1 canonical work pages · cited by 1 Pith paper

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    Nonreciprocal transport in a room-temperature chiral magnet

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