{"id":"fb5b00d9-77e1-4a29-aee6-5ed7a8ba1023","arxiv_id":"2509.07087","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"Polarized neutron scattering on a single-domain MnF2 crystal resolves two split magnon branches and shows a chiral scattering signal that reverses sign between branches, establishing MnF2 as an altermagnet.","lead":"Using polarized neutron beams, the paper shows that the textbook antiferromagnet MnF2 is actually an altermagnet, with spin waves of opposite handedness on its two magnetic sublattices. The result gives physicists a new, direct way to identify altermagnets by their magnetic excitations, which matters for future spintronics.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The chiral signal is attributed to an altermagnetic J7a–J7b imbalance, but the uniqueness argument explicitly defers 2n-spin couplings and assumes negligible spin–orbit terms; on the 4 μeV scale of the inferred imbalance, an omitted symmetry-allowed coupling could produce the same sign-reversing chi","rationale":"The paper provides a plausible and internally consistent interpretation: the domain imbalance (85:15), the resolved 178 μeV dipolar splitting, and the sign-reversing chiral difference are all measured, and the fitted J7a−J7b is constrained by both dispersion and chiral ratio. The chiral signal cannot be explained by dipolar coupling alone within the fitted model, and the supplementary's enumeration of two-spin couplings is a serious effort to rule out alternatives. I therefore do not regard the central claim as refuted.\n\nThe load-bearing soft spot is exactly the one the authors flag: the proof of uniqueness is not complete. The deferred 2n-spin couplings, or small spin-orbit terms, could in principle generate the same chiral observable at the micro-eV scale. Because the altermagnetic exchange imbalance inferred from the data is so small (≈4 μeV), the burden is not merely formal: an omitted coupling at that scale is physically plausible and would change the interpretation from 'altermagnetic exchange imbalance' to 'some symmetry-allowed coupling that produces the same signal.' This is why the verdict should remain conditional: the observation is exciting and likely correct in broad strokes, but the sharp claim that the chiral term 'clearly demonstrates altermagnetism' depends on closing the multi-spin/anisotropy loophole.\n\nI agree with the reader's weakest-assumption assessment. The concrete test—extending the symmetry enumeration and computing the chiral cross section for four-spin terms with J7a=J7b—would directly settle whether the loophole is real. Since the reader's CONDITIONAL verdict already encodes this, I do not propose a change.","tokens_in":22434,"tokens_out":17442,"duration_ms":222254,"concrete_test":"Extend the Supplementary symmetry enumeration to all spin operators up to fourth order (biquadratic and four-spin ring-exchange terms) on the seventh-neighbor shell and beyond, respecting P4_2/mnm. Implement these terms in the same flavor-wave code, set J7a=J7b, and scan the new couplings up to ~10 μeV while refitting the dispersion data. Compute the polarized INS chiral ratio at Q=(0.35,0.35,1). If any parameter set reproduces the observed sign-reversing 5–11% chiral ratio, the uniqueness premise fails and the altermagnetic interpretation must be re-attributed; if all such terms yield zero chiral term, the model-space restriction is safe.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central inference is that the measured sign-reversing chiral term in the polarized neutron cross section uniquely demonstrates altermagnetism in MnF2. This rests on the Supplementary proof that, within linear spin-wave theory, the only symmetry-allowed coupling that generates a non-vanishing chiral term is the seventh-neighbor Heisenberg imbalance J7a−J7b. That proof has two explicitly stated gaps: (i) it assumes spin-orbit/single-ion anisotropies are negligible; (ii) it considers only two-spin couplings, deferring '2n-spin (n≥2) couplings' to future work. These gaps are load-bearing because the extracted altermagnetic scale, |J7a−J7b|≈0.004 meV, is two orders of magnitude below the dipolar splitting (0.178 meV) and comparable to the size of tiny symmetry-allowed terms that are normally neglected. If, for example, a four-spin ring-exchange or biquadratic term allowed by P4_2/mnm produces a chiral term of the observed 5–11% with J7a=J7b, then the measured chiral signal is not a unique fingerprint of altermagnetic exchange; the conclusion 'MnF2 is altermagnetic' would survive only in the weaker sense that the full crystal symmetry already permits altermagnetism, not in the demonstrated sense claimed. A statistical-significance statement for the difference map is also absent, but the model-completeness gap is the more specific threat to the interpretive claim.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports polarized inelastic neutron scattering experiments on MnF2, a collinear antiferromagnet in space group P4_2/mnm, using a crystal with an approximately 85±5% single-domain population. Unpolarized data resolve a small (~0.178 meV) splitting of the two magnon branches, attributed mainly to long-range magnetostatic dipolar coupling. Half-polarized measurements with the incident polarization aligned along Q yield a difference spectrum between +P_in and -P_in that reverses sign between the two magnon branches. The authors fit a Heisenberg exchange model through seventh-neighbor couplings plus dipolar interactions to the dispersion, obtaining J7a-J7b ≈ 0.004 meV, and argue from a symmetry enumeration of two-spin couplings that the only such coupling generating a nonzero chiral term in linear spin-wave theory is this Heisenberg imbalance. They conclude that the sign-reversing chirality 'clearly demonstrates altermagnetism in MnF2.'","tokens_in":22726,"tokens_out":9540,"duration_ms":99316,"significance":"If the inference holds, this is an important advance: a direct, polarized-neutron observation of magnon chirality in a compensated collinear antiferromagnet, and a demonstration that magnetic fine structure below the dipolar scale can be probed through the chiral cross section. The experiment is carefully designed: the domain population is characterized by flipping-ratio measurements, the chiral signal is a direct difference of two polarization channels, the sign reversal is not a fit parameter, and the spin-wave calculations include long-range dipolar interactions via Ewald summation and fit more than 1600 dispersion points. These are genuine strengths. However, the central claim that the chiral signal uniquely demonstrates altermagnetism is tied to a model-completeness assumption that the manuscript itself leaves open, the statistical significance of the difference signal is not quantified, and the relation to a very recent null result for altermagnetic splitting in MnF2 is not addressed. The result is plausible and potentially important, but the claims as written outrun the evidence.","major_comments":[{"comment":"The uniqueness proof that the observed chiral term is the fingerprint of the J7a-J7b imbalance is explicitly conditional: it considers only two-spin couplings out to seventh neighbors, only linear spin-wave theory, and assumes single-ion/spin-orbit terms are negligible. The paragraph immediately before the conclusion defers '2n-spin (n≥2) couplings' to future studies. The inferred altermagnetic scale, |J7a-J7b| ≈ 0.004 meV, is two orders of magnitude below the 0.178 meV dipolar splitting and comparable to the single-ion anisotropy scale D_c ≈ 0.027 meV used in a recent fit (Ref. 42). A symmetry-allowed four-spin ring-exchange or biquadratic term compatible with P4_2/mnm could, in principle, produce a sign-reversing chiral signal of the observed 5-11% with J7a=J7b. To support the abstract's 'clearly demonstrates altermagnetism,' the authors should either extend the enumeration to multi-sp","section":"Supplementary Text, 'A priori constraints on the anisotropic couplings' (final paragraph and Table S3)"},{"comment":"The central evidence for a nonzero chiral term is the difference between the +P_in and -P_in intensities, yet the difference spectra in Fig. 3(e-f) and Fig. S5 are plotted without error bars. The integrated chiral ratio is quoted as 'between 5% and 11%' and '~11±1%' (Fig. S6), but no statistical significance test is reported. The statement that the signal 'goes to zero, within errors' at H=1/2 presupposes an error estimate that is not shown. Please propagate counting statistics (including the 85±5% domain dilution) and provide confidence intervals on the fitted Gaussian areas in the difference channel, or otherwise quantify the significance of the sign-reversing difference. Without this, the reader cannot assess whether the chiral signal is statistically established.","section":"Main text, 'Chirality of magnon bands' and Figs. 3(e-f), S5, S6"},{"comment":"The paper cites a very recent study, Morano et al., 'Absence of Altermagnetic Magnon Band Splitting in MnF2' (Phys. Rev. Lett. 134, 226702, 2025), but only as a comparison of exchange parameters. This prior work appears to have searched specifically for altermagnetic magnon band splitting in MnF2 and reports its absence. The present manuscript reports a well-resolved splitting and a chiral signal in the same material. The discrepancy must be addressed explicitly: is it due to different resolution, a different sample/domain state, different model assumptions (single-ion vs. dipolar anisotropy), or a different interpretation? Without this reconciliation, the experimental claim is incomplete in an important respect.","section":"References and Notes, Ref. [42]; Supplementary 'Parameterization of the dispersion relations'"}],"minor_comments":[{"comment":"The references to Fig. 3 panels are inconsistent: the sum and difference maps are panels (a) and (c), not (c) and (e); 'Focussing on panel (e)' should refer to the difference map (c), while the constant-Q scans are in panels (e-f).","section":"Main text, 'Chirality of magnon bands'"},{"comment":"The sentence 'J7a = -0.006 meV and J7a = -0.002 meV' should read J7b for the second coupling.","section":"Main text, fit parameters"},{"comment":"The text 'The figure constrains |J7a-J7b| ≈ 0.04' should be 0.004 meV, consistent with Fig. S10 axes and the main text δJ7 < 5 μeV.","section":"Supplementary, 'Parameterization of the dispersion relations'"},{"comment":"The phrase 'a fold-fold rotation symmetry' should read 'a four-fold rotation symmetry.'","section":"Main text, microscopic origins paragraph"},{"comment":"The caption contains a duplicated 'of of'; please correct to 'Q dependence of the ratio.'","section":"Fig. S6 caption"}],"recommendation":"major_revision","confidential_remarks":"The paper is timely and the experimental approach is interesting, but the model-completeness gap and the absence of quantitative significance testing are load-bearing for the central claim. I would also want the authors to explicitly reconcile with the 2025 null result for MnF2. These issues are fixable within the manuscript's scope, so I do not recommend rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a genuine experimental result—a sign-reversing chiral term in the polarized neutron cross section of MnF2, with the dipolar splitting resolved for the first time. The authors interpret it as altermagnetism. The data look credible, the analysis is mostly careful, but the claim that the signal uniquely demonstrates an altermagnetic exchange imbalance has a gap you should know about.\n\nWhat's actually new: the polarized INS difference map, the sign reversal between the two magnon branches, and the fact that the chiral signal is a direct observable, not a fit output. They also visualize the long-predicted dipolar splitting. The domain population is measured, the geometry is thoughtfully set up, and the paper makes a solid case that polarized neutrons can see a chiral term in a compensated magnet. That is a real step forward.\n\nThe main soft spot is the uniqueness argument. The supplementary argues that within linear spin-wave theory, the only two-spin coupling that generates a net chiral term is the J7a–J7b imbalance. But the argument explicitly defers 2n-spin couplings and assumes spin-orbit and single-ion terms are negligible. The extracted imbalance is about 4 micro-eV, two orders of magnitude below the dipolar splitting. At that scale, a symmetry-allowed four-spin term could plausibly produce the same sign-reversing signal. So the strong claim—that the data uniquely demonstrate altermagnetic exchange—is not fully closed. The weaker claim, that MnF2 is altermagnetic in the sense that its symmetry permits it, was already known. This paper moves the evidence from 'symmetry allows' to 'we see a chiral signal consistent with a small exchange imbalance,' which is real progress, but it is not a proof.\n\nTwo smaller issues: there are no error bars on the chiral difference intensities, and the 5–11% integrated chiral ratio has no stated statistical significance. That is a missing check, not a fatal flaw. And the recent null result from Morano et al. is not fully reconciled; the authors should address it head-on.\n\nWho is this for? Experimentalists working on altermagnets and neutron scattering, and theorists interested in magnon chirality. It deserves a serious referee. My recommendation: send it out, but the authors should be pushed to give error-propagated significance, release the reduced data, and explicitly discuss the limits of the uniqueness argument. The central observation is likely robust; the interpretation needs careful wording.","headline":"Real signal, credible experiment, but the leap to 'altermagnetism demonstrated' rests on a model-completeness assumption the paper itself leaves open.","tokens_in":23331,"tokens_out":2224,"would_cite":true,"duration_ms":23531,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"MnF2, long treated as the textbook antiferromagnet, is shown to be an altermagnet: the chiral term in polarized neutron scattering flips sign between its two magnon branches, and the paper ties this uniquely to a micro-eV J7a-J7b exchange i","keywords":["altermagnetism","MnF2","polarized inelastic neutron scattering","magnon chirality","spin waves","antiferromagnet","dipolar coupling","chiral neutron cross section"],"falsifier":"Measure the chiral difference map on a crystal whose majority domain is reversed (for example, field-cooled into the opposite 4:1 imbalance): if MnF2's altermagnetism is the cause, the chiral signal flips sign with the Néel vector while the unpolarized sum stays unchanged. Alternatively, a linear spin-wave calculation adding a symmetry-allowed two-spin or 2n-spin coupling that produces a comparable sign-reversing chiral term would falsify the paper's uniqueness claim.","tokens_in":22263,"feed_emoji":"🧲","tokens_out":11082,"duration_ms":109619,"temperature":0.7,"pith_summary":"This paper sets out to show that MnF2—a material that has served for decades as the canonical example of a two-sublattice antiferromagnet—is actually an altermagnet, a collinear magnet whose crystal symmetry allows spin-split excitations without a net magnetization. Using polarized inelastic neutron scattering on a crystal biased toward one antiferromagnetic domain, the authors resolve a roughly 0.2 meV splitting of the two magnon (spin-wave) branches and, more importantly, isolate a chiral term in the cross section that changes sign between the lower and upper modes. Within linear spin-wave theory, a symmetry-restricted enumeration of two-spin couplings identifies the microscopic source as the tiny inequality J7a≠J7b between two distinct seventh-neighbor Heisenberg exchange bonds—an energy scale of order 10 μeV, far below the instrumental resolution. If correct, the result reclassifies a textbook antiferromagnet as a d-wave altermagnet and shows that polarized neutrons can detect altermagnetism directly from magnetic excitations, even when the altermagnetic splitting is dwarfed by non-altermagnetic ones like the dipolar splitting.","feed_headline":"Polarized neutrons expose altermagnetism in a textbook antiferromagnet","feed_subtitle":"A sign-reversing chiral signal between the two magnon modes turns a textbook antiferromagnet into an altermagnet.","key_machinery":"The load-bearing object is the chiral term of the polarized neutron cross section, isolated by subtracting intensities for +Pin and -Pin: I_+ - I_- ∝ (-1)^n (Pin·k)(k·N) C_k δ(ω-ε_k^n). Because this term changes sign with the magnon index n, it is the direct fingerprint of magnon chirality. The supporting machinery is a symmetry-lowered spin-wave model: the fluoride ions reduce the lattice symmetry from body-centered tetragonal to a four-fold rotation plus half-translation and time reversal, and only the two distinct seventh-neighbor Heisenberg bonds J7a and J7b carry that symmetry. An altermagnetic splitting appears when J7a≠J7b; the long-range dipolar interaction also splits the modes but,","core_discovery":"The central discovery is a nonzero chiral term in MnF2's polarized neutron cross section, isolated by subtracting intensities for opposite incident polarizations. The term is proportional to (-1)^n (Pin·k)(k·N), reversing sign between the two magnon branches; it is visible only because the crystal is biased 85:15 toward one antiferromagnetic domain. Along (H,H,1) the chiral signal peaks near H=0.35 and dies at the zone boundary, matching the d-wave chirality pattern dictated by symmetry. Spin-wave fits give J7a-J7b on the micro-eV scale, and an enumeration of symmetry-allowed two-spin couplings singles out this seventh-neighbor imbalance as the only one that yields a net chiral term. The pap","pith_inferences":["A clean control experiment would be to cool a fresh crystal through TN in a field that selects the minority domain: the chiral signal should reverse sign while the unpolarized spectrum remains unchanged.","The same half-polarized protocol could be applied to other collinear antiferromagnets in the rutile family; a small survey would show whether micro-eV seventh-neighbor exchange imbalances are generic or peculiar to MnF2.","If the chiral intensity is as sensitive as reported, polarized inelastic neutron scattering may become a way to constrain anisotropic and multi-spin exchange terms that are invisible in fits to the unpolarized dispersion, using chirality as a fine-structure magnifier."],"forward_implications":["If the result holds, MnF2 becomes a confirmed altermagnet whose fingerprint is read from the magnetic excitations themselves, not from electronic bands, placing it beside MnTe and CrSb in the list of established altermagnets.","The measured chiral ratio (5–11% of the integrated intensity) pins J7a-J7b to the micro-eV range, showing that a symmetry-breaking energy two orders of magnitude below the magnon bandwidth can still produce a detectable polarization signal.","Symmetry guarantees the chiral term reverses between (H,H,1) and (H,-H,1); a future measurement at the symmetry-related wavevector that fails to show the opposite sign would refute the altermagnetic assignment.","The protocol—domain-biased single crystal, half-polarized neutrons, and a +Pin/−Pin difference map—provides a general way to identify altermagnets even when the altermagnetic splitting is buried under larger non-altermagnetic splittings.","The fitted exchange parameters, including J7a and J7b, give a benchmark for modeling d-wave altermagnetism in rutile-structure insulators with pure-spin moments."],"supporting_citations":[{"why":"supplies the polarized-neutron cross-section formula whose chiral term is the measured observable and predicts the sign reversal between magnon modes","marker":"[34]"},{"why":"defines the spin-symmetry classification of altermagnets that MnF2 is claimed to realize","marker":"[3]"},{"why":"establishes the altermagnetism classification and the spin-splitting criterion that motivates probing magnon chirality","marker":"[4]"},{"why":"reports the previous inelastic-neutron observation of a chiral split magnon in MnTe, the comparison case that this experiment goes beyond by resolving chirality","marker":"[9]"},{"why":"measured altermagnetic magnon chirality by RIXS circular dichroism in CrSb, showing the energy-resolution limitation that polarized neutrons overcome","marker":"[21]"},{"why":"a recent claim of no altermagnetic magnon band splitting in MnF2 that the present dipolar-plus-exchange fit must be distinguished from","marker":"[42]"},{"why":"supplementary material containing the domain-population measurements, spin-wave fits, and the enumeration of two-spin couplings behind the uniqueness claim","marker":"[45]"},{"why":"supplies the flipping-ratio method used to quantify the 85:15 domain imbalance in the sample","marker":"[47]"}],"fun_headline_variants":["Neutron chirality flips textbook antiferromagnet to altermagnet","Polarized neutrons unmask MnF2 as altermagnet","Chiral neutron signal proves MnF2 is altermagnetic","MnF2's hidden altermagnetism seen via polarized neutrons","Sign-reversing chirality turns MnF2 into altermagnet"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The identification of altermagnetism rests on the unproven premise that, among all couplings allowed by symmetry in MnF2, only the small J7a-J7b imbalance can create the observed sign-reversing chiral signal; the paper itself leaves 2n-spin (n≥2) couplings to future work and assumes spin-orbit and single-ion anisotropies are negligible, so an omitted coupling with the same fingerprint would weaken the conclusion.","fun_headline_variants_meta":{"raw":{"variants":["Neutron chirality flips textbook antiferromagnet to altermagnet","Polarized neutrons unmask MnF2 as altermagnet","Chiral neutron signal proves MnF2 is altermagnetic","MnF2's hidden altermagnetism seen via polarized neutrons","Sign-reversing chirality turns MnF2 into altermagnet"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000441,"raw_usage":{"total_tokens":2139,"prompt_tokens":876,"completion_tokens":1263,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":620,"completion_tokens_details":{"reasoning_tokens":1170}},"tokens_in":620,"tokens_out":1263,"duration_ms":10526,"temperature":1.0,"reasoning_tokens":1170,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-04T22:51:07.717655+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the chiral difference map on a crystal whose majority domain is reversed (for example, field-cooled into the opposite 4:1 imbalance): if MnF2's altermagnetism is the cause, the chiral signal flips sign with the Néel vector while the unpolarized sum stays unchanged. Alternatively, a linear spin-wave calculation adding a symmetry-allowed two-spin or 2n-spin coupling that produces a comparable sign-reversing chiral term would falsify the paper's uniqueness claim.","supporting_citations":[{"cited_title":"Cichutek, P","cited_arxiv_id":null,"evidence_quote":"supplies the polarized-neutron cross-section formula whose chiral term is the measured observable and predicts the sign reversal between magnon modes"},{"cited_title":"(a) inelastic neutron spectra along high- symmetry directions within the Brillouin zone (inset) obtained with an incident energy of𝐸 𝑖 = 9 meV","cited_arxiv_id":null,"evidence_quote":"defines the spin-symmetry classification of altermagnets that MnF2 is claimed to realize"},{"cited_title":"Yamani, Z","cited_arxiv_id":null,"evidence_quote":"a recent claim of no altermagnetic magnon band splitting in MnF2 that the present dipolar-plus-exchange fit must be distinguished from"}],"review_version":1}