{"id":"4d30800a-210c-4b47-880e-2f1144a0e265","arxiv_id":"2512.17715","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"Brillouin light scattering shows the magnon frequency in LnMn6Sn6 (Ln=Tb,Dy,Ho) decreases from Tb to Ho and tracks the lanthanide's magnetic anisotropy, with the field response tied to its angular momentum.","lead":"Using laser light scattering, the authors measured spin-wave (magnon) frequencies in three lanthanide kagome magnets and found the frequencies track the rare-earth ion's magnetic anisotropy and total angular momentum. The result suggests swapping the lanthanide can tune magnon behavior, but the effect is inferred from fits to a model that assumes an unverified laser-induced spin reorientation in the Tb compound.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central claim rests on the untested assumption that the BLS laser reorients TbMn6Sn6 into the planar state; if false, the Tb HA and γ fits collapse and the tuning rule is left with only two compounds.","rationale":"The reader's weakest-assumption analysis identifies the laser-induced spin reorientation in TbMn6Sn6 as the pivotal untested condition. My independent read agrees: the central claim has two legs—anisotropy controls the zero-field frequency and J controls γ. For Tb, both HA and γ are obtained by fitting field-dependent BLS data under the explicit assumption that the illumination has switched the magnetization from easy-axis to easy-plane. If that assumption is false, the Tb fit is invalid. The other two compounds are already in the planar state at room temperature, so the trend reduces to a two-point correlation. The manuscript's variable-temperature BLS experiments are suggestive but not conclusive, since the mode disappearance at low temperature has alternative explanations (e.g., changes in optical penetration depth). A direct laser-power ramp and/or an independent anisotropy measurement would settle the question. I also note an internal inconsistency between the arXiv abstract, which attributes zero-field frequency to the de Gennes factor (exchange coupling), and the main text/title, which attribute it to single-ion anisotropy; this deserves attention but is secondary to the experimental assumption that underpins the quantitative fits. No change to the reader's CONDITIONAL verdict is needed.","tokens_in":18960,"tokens_out":13873,"duration_ms":139794,"concrete_test":"Measure the zero-field BLS spectrum of TbMn6Sn6 at room temperature as a function of laser power (e.g., 0.5, 1, 2, 4, 8 mW). If the laser induces the spin reorientation, the magnon frequency should show a steep, nonlinear dependence on power as the local temperature crosses TSR=312 K; at the lowest power the mode should either vanish or display the decreasing-frequency behavior expected for an axially magnetized sample in an in-plane field. Corroborate with a focused-spot magneto-optical (e.g., MOKE) or Hall-probe measurement of the local magnetization orientation under identical illumination, or compare the extracted HA with an independent ferromagnetic-resonance measurement on an unilluminated crystal.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The quantitative foundation for the Tb member of the tuning rule is the hypothesis in §2 that the 8 mW, 3 µm BLS laser heats TbMn6Sn6 above TSR = 312 K, placing the magnetization in-plane so that eq. (1) with H|| parallel to M applies. This is explicitly introduced as 'we hypothesize,' not a measured state. If the sample remains axial under illumination, the observed monotonic increase of the magnon frequency with in-plane field (Fig. A-4a) is inconsistent with the standard perpendicular-field Kittel behavior, and the fitted HA = 3800 Oe and γ = 4.8×10^6 Hz/Oe for 1-Tb—the largest and most distinctive data point—are not meaningful. The variable-temperature BLS data (Fig. A-3) do not settle this: the disappearance of the mode at ≈253 K is consistent with local heating crossing TSR but also with temperature-dependent optical coupling, and no discontinuity at TSR is observed. Without independent evidence of the laser-induced reorientation, the claim that lanthanide anisotropy controls magnon frequency rests on only Dy and Ho, and the γ(J) trend loses its Tb anchor.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","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.","tokens_in":19269,"tokens_out":8372,"duration_ms":85922,"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":[{"comment":"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.","section":"§2 (Fig. A-2, A-3, A-4; Table 1)"},{"comment":"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.","section":"§2, Eq. (1)"},{"comment":"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.","section":"Table 1 and Fig. A-4b"},{"comment":"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.","section":"Abstract vs. §3 Conclusions"},{"comment":"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.","section":"§2, Table 1"}],"minor_comments":[{"comment":"The visual representation of 4f electron densities is qualitative; consider adding quantitative anisotropy parameters or Stevens factors to support the claim.","section":"Fig. A-4c"},{"comment":"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.","section":"Eq. (3)"},{"comment":"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.","section":"§2"},{"comment":"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.","section":"Figure S-1"},{"comment":"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.","section":"Fig. S-40"}],"recommendation":"major_revision","confidential_remarks":"The decisive issue is the TbMn6Sn6 laser-reorientation assumption. The authors should be asked to provide independent evidence—e.g., temperature-controlled BLS above TSR without laser heating, polarization-resolved BLS, or magneto-optical imaging under illumination—that the magnetization is truly planar under the BLS beam. Without this, the Tb fit is not credible and the central tuning claim loses its strongest anchor. Also, the internal inconsistency between the abstract (de Gennes factor/exchange coupling) and the main text (anisotropy) must be resolved before acceptance."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The headline is that this is the first comparative BLS magnon study of Tb/Dy/Ho Mn6Sn6, and the raw data look real. The zero-field frequencies (17.6, 9, 5 GHz) and the field slopes are a solid experimental contribution. But the central claim—that lanthanide single-ion anisotropy controls the magnon frequency and total angular momentum controls gamma—rests on fitting the Kittel equation with parameters that are not independently pinned down, and for Tb it requires an unverified laser-induced spin reorientation.\n\nWhat's genuinely good: the synthesis, VSM, and BLS are careful. The spectra and per-field fits are in the SI. The authors are upfront that the Tb planar state is a hypothesis, and the qualitative ordering of HA (Tb > Dy > Ho) matches the known anisotropy series, which is reassuring.\n\nSoft spots:\n\n1. The Tb analysis assumes the 8 mW laser heats the crystal above TSR so M is in-plane. This is not measured. The variable-temperature BLS shows no discontinuity at TSR and the mode disappears at 253 K, which does not confirm the reorientation. If the sample stays axial, the Kittel fit for Tb is not meaningful, and the tuning rule loses its most distinctive point.\n\n2. The Kittel equation for a collinear ferromagnet with H||M is applied to easy-cone ferrimagnets. Ferrimagnets have two-sublattice modes; using a single uniform-mode formula is a strong simplification. It may be okay for the acoustic branch, but it is not justified here.\n\n3. HA and gamma are fit parameters. The statement that anisotropy 'controls' the magnon frequency is partly a restatement of the model. The ordering is externally plausible, but the numerical HA values are not independent predictions.\n\n4. There is an abstract mismatch: the arXiv abstract attributes the zero-field frequency to the de Gennes factor (exchange coupling), while the paper's own abstract and text attribute it to single-ion anisotropy. That is a real inconsistency.\n\n5. Three data points make a trend, not a law. The gamma(J) story is hand-wavy and relies on crystal-field mJ arguments not developed here.\n\nWho is this for: people working on kagome magnets, magnonics, or BLS of anisotropic magnets. The experimental data set deserves a serious referee—there is enough new material to justify the effort. But the paper needs either stronger evidence for the laser reorientation (angle-dependent BLS, direct heating measurement) or a reframing that presents the Tb result as tentative. As is, the tuning rule is an interesting hypothesis, not an established result.\n\nI would send it to peer review with the expectation of revision.","headline":"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.","tokens_in":19820,"tokens_out":3663,"would_cite":true,"duration_ms":35243,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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.","keywords":["LnMn6Sn6","Brillouin light scattering","magnons","kagome magnets","lanthanide single-ion anisotropy","gyromagnetic ratio","ferrimagnet","spin reorientation"],"falsifier":"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","tokens_in":18829,"feed_emoji":"🧲","tokens_out":3638,"duration_ms":35543,"temperature":0.7,"pith_summary":"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.","feed_headline":"Rare-earth ion sets spin-wave frequency in kagome magnets","feed_subtitle":"Tb, Dy, and Ho versions of LnMn6Sn6 show magnons spanning 5-18 GHz; anisotropy and total angular momentum are the dials.","key_machinery":"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","core_discovery":"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","pith_inferences":["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."],"forward_implications":["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."],"fun_headline_variants":["Lanthanide anisotropy sets zero-field magnon frequency","Tb, Dy, Ho: choosing rare-earth tunes magnon frequency","Magnon field response hinges on lanthanide's g-factor","Spin-wave frequency and slope dialed by Ln3+ ion choice","Rare-earth's magnetic character directs kagome magnons"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Lanthanide anisotropy sets zero-field magnon frequency","Tb, Dy, Ho: choosing rare-earth tunes magnon frequency","Magnon field response hinges on lanthanide's g-factor","Spin-wave frequency and slope dialed by Ln3+ ion choice","Rare-earth's magnetic character directs kagome magnons"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000671,"raw_usage":{"total_tokens":2938,"prompt_tokens":834,"completion_tokens":2104,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":578,"completion_tokens_details":{"reasoning_tokens":2024}},"tokens_in":578,"tokens_out":2104,"duration_ms":15174,"temperature":1.0,"reasoning_tokens":2024,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-03T15:10:10.335423+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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","supporting_citations":[],"review_version":1}