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REVIEW 5 major objections 5 minor 7 references

Lanthanide Ion Electronic Structure Controls Magnetic Excitations in Topological Quantum Ferrimagnets $LnMn_{6}Sn_{6}$ (Ln = Tb, Dy, Ho)

T0 review · 5 major / 5 minor · reviewed 2026-08-03 · deepseek-v4-flash

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Pith's one-line read Brillouin light scattering measurements show that in LnMn6Sn6, the choice of lanthanide ion sets the magnon frequency through single-ion anisotropy and the field response through total angular momentum.

desk verdict First comparative BLS magnon data on LnMn6Sn6 (Tb, Dy, Ho) are valuable, but the tuning rule rests on an unverified laser-reorientation assumption and model-dependent fits. read the letter →

arxiv 2512.17715 v2 pith:7BREBWTD submitted 2025-12-19 cond-mat.mtrl-sci

Kelsey A. Collins , Jacob Pfund , Michael R. Page , Menka Jain , Michael A. Susner , Michael J. Newburger This is my paper · ORCID
classification cond-mat.mtrl-sci
keywords LnMn6Sn6Brillouinlightscatteringmagnonskagomemagnetslanthanidesingle-ionanisotropygyromagneticratioferrimagnetspinreorientation
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 the first comparative Brillouin light scattering study of three LnMn6Sn6 topological ferrimagnets (Ln = Tb, Dy, Ho) and claims that two properties of the lanthanide ion independently control the material's magnetic excitations: the single-ion anisotropy sets the zero-field magnon frequency, and the total angular momentum sets the gyromagnetic ratio that governs how the frequency changes with applied field. If correct, swapping one lanthanide for another is a predictable dial for magnon frequency, which matters for magnonic and spintronic devices operating in the GHz range. The quantitative evidence comes from fitting field-dependent magnon frequencies to the Kittel equation, yielding anisotropy fields of 3800, 1900, and 670 Oe for Tb, Dy, and Ho respectively, while the gyromagnetic ratio is highest for Ho (6.9 x 10^6 Hz/Oe), intermediate for Tb (4.8 x 10^6), and lowest for Dy (4.3 x 10^6). The central zero-field magnon frequencies follow the anisotropy trend: about 17.6 GHz for TbMn6Sn6, 9 GHz for DyMn6Sn6, and 5 GHz for HoMn6Sn6.

What carries the argument

The analysis is carried by the Kittel equation for a ferrimagnet with the field applied parallel to the magnetization: f = (gamma/2pi) * sqrt[(H_parallel + H_A + H_ex)(H_parallel + H_A + H_ex + 4pi M_S)]. Fitting field-dependent BLS magnon frequencies with this equation yields the anisotropy field H_A and gyromagnetic ratio gamma. A crucial supporting assumption is that the BLS laser induces a spin-reorientation transition in TbMn6Sn6, so the magnetization is planar (in the ab plane) under measurement and the applied in-plane field is therefore parallel to it; the authors state this as a hypothesis, motivated by the proximity of the spin-reorientation temperature (312 K) and the similarity o

What would settle it

Measure the TbMn6Sn6 magnon field dependence at a laser power below the expected heating or photomagnetic threshold, or with the magnetic field applied along the c-axis. If the zero-field magnon frequency and its field slope change qualitatively (for example, the frequency initially decreasing in low field, as expected when the field is perpendicular to the easy axis), the laser-induced spin reorientation is not occurring and the fitted H_A and gamma for TbMn6Sn6 are artefacts. A more direct test is to image the magnetization direction under 532 nm laser illumination, for instance with magneto

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Extended reading notes

Core claim

The paper claims that in LnMn6Sn6, the zero-field magnon frequency and the field response are each controlled by independent lanthanide electronic-structure parameters. The zero-field frequency follows the single-ion anisotropy of the Ln3+ ion (Tb3+ most anisotropic, Ho3+ least), yielding anisotropy fields HA of 3800, 1900, and 670 Oe and corresponding magnon frequencies near 17.6, 9, and 5 GHz. The field-dependent slope is governed by the gyromagnetic ratio, which the paper ties to the lanthanide's total angular momentum and the crystal-field-split mJ ground state: Ho3+ (J = 8) gives the highest g|| = 4.9 and the steepest slope, while Dy3+ (a Kramers ion) gives the lowest g|| = 3.1. The aut

Load-bearing premise

The fitting for TbMn6Sn6 depends on the untested hypothesis that the focused laser beam flips the magnetization from the easy c-axis into the ab plane, so that the applied in-plane field is parallel to the magnetization; if that reorientation does not occur, the extracted anisotropy field and gyromagnetic ratio for TbMn6Sn6 lose their quantitative foundation.

Editorial extensions

If this is right

  • For Ln = Tb, Dy, Ho, the zero-field magnon frequency is predicted to scale with lanthanide single-ion anisotropy, offering a route to selecting GHz-range magnon frequencies by rare-earth substitution.
  • The gyromagnetic ratio, and hence the magnon field sensitivity, is set by the lanthanide's total angular momentum and crystal-field ground state; high-J lanthanides such as Ho should produce steeper frequency-versus-field slopes.
  • The measured magnon frequencies lie in the 5-18 GHz range at fields up to 200 mT, a range relevant for high-frequency magnonic device operation.
  • Because the saturation magnetization is nearly identical across the three compounds, differences in magnon behavior can be attributed cleanly to anisotropy and gyromagnetic ratio rather than to magnetization changes.
  • Alloying different lanthanides in the same crystal structure could allow fine-tuning of the magnon spectrum between the Tb, Dy, and Ho extremes; the paper explicitly proposes this as future work.

Reading between the lines

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

  • If the laser-induced spin reorientation hypothesis is correct, the same BLS measurement could serve as a local probe of the spin-reorientation transition, with the appearance or disappearance of the magnon signal marking the transition; the observed disappearance of the magnon on cooling already hints at this.
  • The abstract's mention of the de Gennes factor suggests that exchange coupling, not just anisotropy and total J, might contribute to the zero-field frequency; a direct test would be to measure an isotropic-lanthanide compound such as GdMn6Sn6 and compare its magnon frequency with this series.
  • The small Stokes/anti-Stokes asymmetry in TbMn6Sn6 (about 0.5 GHz) could indicate a Dzyaloshinskii-Moriya interaction; if confirmed, BLS asymmetry could become a route to extracting DMI strength in these topological magnets.
  • The broad magnon linewidths (~11-13 GHz) imply very short magnon lifetimes; comparing linewidths across the series could reveal whether lifetime, not just frequency, is also tunable by lanthanide choice.
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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

5 major / 5 minor

Summary. The paper reports a comparative Brillouin light scattering study of magnetic excitations in the ferrimagnetic kagome metals TbMn6Sn6, DyMn6Sn6, and HoMn6Sn6. A single magnon mode is observed in each compound; its zero-field frequency decreases from Tb (≈17.6 GHz) to Dy (≈9 GHz) to Ho (≈5 GHz), and increases monotonically with in-plane magnetic field up to 200 mT. Fitting the field dependence with the Kittel equation (Eq. 1) yields anisotropy fields HA = 3800 Oe (Tb), 1900 Oe (Dy), 670 Oe (Ho) and gyromagnetic ratios γ = 4.8, 4.3, 6.9 × 10^6 Hz/Oe, respectively. The authors conclude that the single-ion anisotropy of the lanthanide controls the magnon frequency and that the total angular momentum governs the gyromagnetic ratio, proposing lanthanide substitution as a tuning knob for magnon properties. The central quantitative claim relies on two modeling assumptions: that the BLS laser reorients the Tb magnetization into the ab plane, and that a simple collinear Kittel equation applies to easy-cone ferrimagnets.

Significance. If the central claims hold, this would be the first comparative study of magnetic dynamics across the LnMn6Sn6 family and would provide a practical design rule for magnon frequencies in topological kagome magnets. The paper has notable strengths: the raw BLS spectra, per-field Lorentzian fits, variable-temperature data, and magnetometry are provided in the Supporting Information; the qualitative anti-correlation between known lanthanide anisotropy and zero-field magnon frequency is suggestive; and the linewidth and temperature behavior are reported in detail. However, the quantitative conclusions rest on unverified modeling choices, especially the hypothesized laser-induced spin reorientation in TbMn6Sn6, and the inference that anisotropy controls the magnon frequency is partly a restatement of the fitting model. The significance is therefore conditional on additional validation.

major comments (5)
  1. [§2 (Fig. A-2, A-3, A-4; Table 1)] The central fit for 1-Tb rests on the assumption that the 8 mW BLS laser reorients the magnetization into the ab plane, so that H || M and Eq. (1) applies. This is introduced explicitly as a hypothesis ("we hypothesize that the incident laser is inducing the spin reorientation transition"), and the variable-temperature data in Fig. A-3 do not confirm it: no discontinuity is observed at TSR, and the disappearance of the mode near 253 K is interpreted as either reorientation or an optical effect. If the sample is not planar, the monotonic increase in frequency with in-plane field contradicts the perpendicular-field Kittel behavior, and the quoted HA = 3800 Oe and γ = 4.8×10^6 Hz/Oe for 1-Tb are not meaningful. Because 1-Tb is the largest zero-field frequency and anchors both the HA and γ trends, the central claim would be left with only two compounds.
  2. [§2, Eq. (1)] The Kittel equation used is for a collinear ferromagnet with H parallel to M. For 2-Dy and 3-Ho the ground state is an easy cone (φ = 45° and 49°, Fig. A-1c), and all three materials are two-sublattice ferrimagnets. The manuscript does not justify that Eq. (1) describes the uniform (or finite-q) mode in these systems; no ferrimagnetic resonance formalism is given. The fitted HA and γ are therefore effective parameters whose connection to the single-ion anisotropy of the Ln3+ ion is model-dependent. The authors should either derive the appropriate dispersion for a two-sublattice easy-cone ferrimagnet or present evidence that the single-mode Kittel form is a good approximation in the field/geometry range used.
  3. [Table 1 and Fig. A-4b] The conclusion that lanthanide anisotropy 'controls' the zero-field magnon frequency is largely a restatement of the fitting model: HA is a free parameter in Eq. (1), and f(0) is computed from the fitted HA and γ. No independent measurement of HA (e.g., from magnetization, torque, or FMR) is provided. To avoid circularity, the authors should compare the fitted HA values with known anisotropy fields for these materials or present the zero-field frequency as a prediction from independently determined anisotropy.
  4. [Abstract vs. §3 Conclusions] The abstract states that the zero-field magnon frequency is "primarily dictated by the strength of the lanthanide exchange coupling, as modeled by its relationship with the de Gennes factor," but the main text and Table 1 attribute the zero-field frequency to the anisotropy field HA (3800/1900/670 Oe), and no de Gennes factor analysis appears anywhere in the paper. These two claims are mutually incompatible, and the central message needs to be reconciled.
  5. [§2, Table 1] The claim that the total angular momentum governs the gyromagnetic ratio is not supported by a monotonic J dependence: J(Dy3+) = 15/2 > J(Tb3+) = 6, yet γ_Dy = 4.3×10^6 Hz/Oe < γ_Tb = 4.8×10^6 Hz/Oe. The observed ordering γ(Ho) > γ(Tb) > γ(Dy) requires crystal-field and Kramers/non-Kramers considerations beyond the value of J, as the text itself acknowledges. The concluding statement that total angular momentum 'governs' γ overstates the regularity of the trend; please qualify or identify a more precise design parameter.
minor comments (5)
  1. [Fig. A-4c] The visual representation of 4f electron densities is qualitative; consider adding quantitative anisotropy parameters or Stevens factors to support the claim.
  2. [Eq. (3)] g|| = 2 g_J M_J is derived for a specific axial crystal-field state; the text should clarify that the g|| extracted from γ via Eq. (2) is an effective value, not necessarily the isolated-ion value.
  3. [§2] The statement "saturation magnetization values of ~210 Oe" is dimensionally unusual; 4πMs in Oe is acceptable, but the text should specify how the value was obtained from VSM and clarify units.
  4. [Figure S-1] TC for 1-Tb is not observed in the data; the values quoted in the text (423 K, 393 K, 370 K) should be explicitly referenced to the source, as they are used to argue that the exchange stiffness varies only weakly.
  5. [Fig. S-40] The linewidth data are presented but not analyzed quantitatively; since the linewidths are comparable to the frequency spacings (FWHM ≈ 11–13 GHz), a brief discussion of how the fits are affected by this broadening would strengthen the presentation.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity; conclusions are standard Kittel-model fits corroborated by external lanthanide-anisotropy and g-factor data.

full rationale

Walked the derivation chain from BLS spectra through Eq. (1) to Table 1. The BLS frequencies are measured independently; HA and gamma are fitted parameters, explicitly described as such ('when we analytically fit the magnon frequencies of the three materials using the Kittel equation... we extract a HA of 3800 Oe...'). The conclusions that lanthanide anisotropy controls the zero-field frequency and that J governs gamma are model interpretations of the fitted values, not predictions derived from the same quantities. Crucially, the fitted ordering HA(Tb) > HA(Dy) > HA(Ho) and gamma(Ho) > gamma(Tb) ~ gamma(Dy) is compared to external, independently established lanthanide single-ion anisotropy and g-tensor trends (refs 48, 52, 53, 56), so the central claim does not reduce to the fitting model. The self-citations (refs 41, 57) support routine methodological statements and are not load-bearing. The main caveat—the hypothesis that the BLS laser reorients TbMn6Sn6 in-plane—is an unverified physical assumption and a correctness risk, but it is not a circularity. No equation is defined in terms of the quantity it is used to predict, and no fitted parameter is renamed as an independent prediction.

Assumptions & free parameters 6 free parameters · 4 assumptions · 0 invented entities

The analysis introduces no new physical entities. It relies on standard Kittel-equation fitting and on domain assumptions about the magnetic geometry under BLS illumination. The main free parameters are HA and γ for each compound, which are fit to the same data used to support the central correlation.

free parameters (6)
  • HA (TbMn6Sn6) = 3800 ± 100 Oe
    Fitted from the field-dependent BLS magnon frequency using the Kittel equation (Table 1).
  • HA (DyMn6Sn6) = 1900 ± 200 Oe
    Fitted from the field-dependent BLS magnon frequency using the Kittel equation (Table 1).
  • HA (HoMn6Sn6) = 670 ± 90 Oe
    Fitted from the field-dependent BLS magnon frequency using the Kittel equation (Table 1).
  • γ (TbMn6Sn6) = (4.8 ± 0.1) × 10^6 Hz/Oe
    Fitted gyromagnetic ratio from the field-dependent BLS frequency (Table 1).
  • γ (DyMn6Sn6) = (4.3 ± 0.3) × 10^6 Hz/Oe
    Fitted gyromagnetic ratio from the field-dependent BLS frequency (Table 1).
  • γ (HoMn6Sn6) = (6.9 ± 0.3) × 10^6 Hz/Oe
    Fitted gyromagnetic ratio from the field-dependent BLS frequency (Table 1).
assumptions (4)
  • domain assumption The Kittel equation (eq. 1) describes the BLS magnon mode of these ferrimagnets.
    Applied for easy-cone ferrimagnets and a hypothesized planar reorientation; no two-sublattice derivation is provided. Entered in Results and discussion around eq. (1).
  • domain assumption The BLS laser reorients TbMn6Sn6 magnetization in-plane, making H parallel to M.
    Stated as a hypothesis in Results and discussion; not directly verified. Without it, the Kittel fit for 1-Tb is not valid.
  • domain assumption The exchange stiffness term Hex can be neglected because Dq² ~ 10 Oe.
    Estimated in the text; excluded from fitting without direct measurement, affecting the extracted HA and γ.
  • standard math g|| = 2 gJ MJ (eq. 3) applies to these axially symmetric Ln sites.
    Standard crystal-field/EPR relation used to connect γ to the lanthanide electronic structure; its quantitative application here is not independently benchmarked.

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Cite this review

Pith. "Pith review of Lanthanide Ion Electronic Structure Controls Magnetic Excitations in Topological Quantum Ferrimagnets $LnMn_{6}Sn_{6}$ (Ln = Tb, Dy, Ho)." pith.science (2026). https://pith.science/paper/7BREBWTD

@misc{pith2026251217715,
  author       = {Pith},
  title        = {Pith review of: Lanthanide Ion Electronic Structure Controls Magnetic Excitations in Topological Quantum Ferrimagnets $LnMn_6Sn_6$ (Ln = Tb, Dy, Ho)},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7BREBWTD}},
  note         = {Machine review of arXiv:2512.17715}
}
abstract

The $LnMn_{6}Sn_{6}$ family of topological magnets is a promising platform for next-generation spintronic and magnonic technologies. However, the influence of the lanthanide ion ($Ln^{3+}$) on the excited-state spin dynamics, or magnons, remains a critical knowledge gap. Here, we present the first comparative study of the magnetic dynamics in $LnMn_{6}Sn_{6}$ materials (Ln = Tb, Dy, Ho) using Brillouin light scattering. Our findings reveal a direct correlation between the lanthanide ion's intrinsic properties and the magnon behavior. We demonstrate that the magnon frequency in the absence of an applied magnetic field is primarily dictated by the strength of the lanthanide exchange coupling, as modeled by its relationship with the de Gennes factor. The response of the magnon to an applied field is influenced by material's gyromagnetic ratio and the overall anisotropy of the material, which are dictated by total angular momentum and the anisotropy of the lanthanide sublattice, respectively. These results establish that simple lanthanide substitution provides a powerful and predictable method for tuning magnon properties, enabling the rational design of materials for advanced technological applications.

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Works this paper leans on

7 extracted references · 1 linked inside Pith

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