{"id":"df0286ca-1733-45c2-8e17-60cba238d825","arxiv_id":"2507.07434","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"Epitaxial Fe3Ga4 films exhibit a large unconventional anomalous Hall effect from 100 to 380 K in low magnetic fields, attributed to fluctuation-driven spin chirality.","lead":"Epitaxial films of Fe3Ga4 show an unconventional anomalous Hall signal of about 0.36 microhm cm at 350 K in a field below 0.4 T. If the signal is real, the films could serve as a room-temperature spintronic platform.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The residual UAHE hump may be an artifact of assuming a field-independent anomalous Hall coefficient and a linear ordinary Hall term; the paper does not report the multi-band/field-dependent-coefficient test that would settle it.","rationale":"The reader's conditional verdict already captures the main weakness: the UAHE extraction assumes a linear ordinary Hall term and a field-independent anomalous Hall coefficient, and the paper lacks the explicit multi-band or field-dependent-coefficient checks needed to rule out a subtraction artifact. My stress-test pass agrees with this assessment, and I do not see a separate, more load-bearing concern. The sample quality evidence is strong: epitaxial growth is confirmed by XRD, RSM, and HAADF-STEM; the sharp TN, high RRR, and consistent metamagnetic critical fields from M(H) and rho_xx(H) support the phase diagram. The transport data also show the expected antisymmetrization for Hall measurements, which removes a common spurious longitudinal contribution. The MFM bubble-count scaling with rho_U^xy is suggestive but explicitly shown to be quantitatively inconsistent with a skyrmion topological Hall contribution by a factor of about 10^3, so the authors fall back on fluctuation-driven SSC; that attribution is plausible but indirectly inferred from bulk neutron results, and the note-added competing study by Baral et al. actually proposes nontrivial spin textures in the same phase. However, the materials claim of a robust large UAHE above room temperature would survive even if the precise fluctuation-driven SSC interpretation were replaced by a nontrivial spin-texture mechanism; what would not survive is a subtraction artifact. Therefore the decisive check is to test the subtraction assumptions directly. Since the reader already set the verdict to CONDITIONAL on essentially this basis, no verdict change is warranted.","tokens_in":17472,"tokens_out":2630,"duration_ms":35759,"concrete_test":"Re-analyze the 200 K, 300 K, and 350 K rho_xy(H) data in the forced-ferromagnetic high-field region (H > H_c4), where no UAHE is claimed, to extract R0 and S_H simultaneously. Then compute the residual rho_U^xy in the metamagnetic region using (i) the standard linear ordinary term, (ii) a two-band ordinary Hall term R0 H + beta H^3, and (iii) a field-dependent anomalous coefficient S_H(H) = S0(1 + gamma H) constrained by the high-field fit. If the 0.36 microhm cm hump near 0.35 T at 350 K disappears or is reduced by more than 50% under (ii) or (iii), the claimed UAHE is not robust; if the hump persists with essentially unchanged magnitude and field profile, the subtraction artifact concern is settled.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim rests on extracting rho_U^xy by subtracting rho_O^xy = R0 H and rho_A^xy = S_H rho_xx^2 M (or R_S M) from measured rho_xy, with R0 and S_H treated as field-independent constants (Methods, 'Analysis of Hall resistivity'; Fig. 5a,b). This subtraction is load-bearing: if the anomalous Hall coefficient is field dependent in the AFM state, or if multi-band transport makes the ordinary Hall term nonlinear, the residual hump between H_c1 and H_c3 could be an artifact rather than a genuine UAHE. The paper reports that using rho_xx^2 M versus M gives comparable rho_U^xy (Supplementary Fig. S13), but that check only varies the functional form of the M weighting; it does not test field dependence of the coefficient itself. In a multi-phase metamagnetic region, different magnetic phases can contribute with different R_S values, so scaling the AHE by the total M(H) may over- or under-subtract. No two-band analysis, no error bars, and no publicly released raw data are provided, so the robustness of the extraction cannot be independently assessed. The MFM-based estimate of the skyrmion topological contribution is about 10^3 times smaller than rho_U^xy, which weakens the skyrmion interpretation but does not by itself validate the fluctuation-driven SSC interpretation; the film's TCS phase is inferred from the bulk phase diagram rather than measured in the film. The reader's weakest assumption therefore identifies the correct point: the subtraction procedure is the most fragile link between the measured rho_xy and the claimed large UAHE above room temperature.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the growth and characterization of epitaxial Fe3Ga4 films on SrTiO3 substrates, and claims the observation of a large unconventional anomalous Hall effect (UAHE) in the intermediate antiferromagnetic (AFM) phase, spanning roughly 100–380 K and peaking at about 0.36 μΩ cm at 350 K in 0.35 T. The Hall resistivity is decomposed into ordinary (linear in H), anomalous (proportional to M with a field-independent coefficient), and unconventional components. The unconventional term appears only in the AFM1 and AFM2 phases, which the authors identify with the transverse conical spiral (TCS) phase of bulk Fe3Ga4, and is attributed to fluctuation-driven scalar spin chirality rather than to skyrmions, based on an order-of-magnitude estimate from magnetic force microscopy bubble densities.","tokens_in":17795,"tokens_out":3202,"duration_ms":38199,"significance":"The reported UAHE, if genuine, would be notable: it appears well above room temperature and in a low magnetic field, which is rare for fluctuation-driven chiral transport. The paper is also strong in its systematic temperature- and field-dependent magnetization and transport measurements, the construction of a magnetic phase diagram, and the inclusion of a quantitative estimate that the skyrmion topological contribution is about 10^3 times smaller than the observed signal. The authors also provide some robustness checks on the choice of anomalous Hall scaling. However, the central claim rests on a subtraction procedure whose underlying assumptions are not fully tested, and the identification of the film's magnetic phases as TCS is inferred from bulk studies rather than measured directly in the film. These issues are load-bearing for the main conclusion, so the paper is of interest but requires additional validation.","major_comments":[{"comment":"The subtraction procedure assumes a field-independent anomalous Hall coefficient and a strictly linear ordinary Hall term (ρxy = R0H + SHρxx²M + ρU_xy). The robustness check in Supplementary S13 varies only the functional form of the M weighting (SHρxx² vs SHρxx vs SH), but does not test whether the coefficients themselves are field-dependent. In the metamagnetic region between Hc1 and Hc3, different magnetic phases coexist and may contribute with different anomalous Hall coefficients, so scaling a single coefficient by the total M(H) could over- or under-subtract and produce a spurious hump. The paper should report a more stringent test, for example a two-band or multi-coefficient analysis, an extraction of R0 and SH from the high-field forced-ferromagnetic regime where ρU_xy is assumed zero, or a direct measurement in the paramagnetic state if attainable. Without such checks, or without error bars on the extracted ρU_xy, the robustness of the central claim cannot be independently assessed.","section":"Methods, 'Analysis of Hall resistivity'; Fig. 5a–c; Supplementary S13"},{"comment":"The assignment of the film's AFM1 and AFM2 phases to the transverse conical spiral state is made by analogy with the bulk Fe3Ga4 phase diagram (Refs 38, 41, 42), and the same assignment is then used to explain the UAHE via fluctuation-driven scalar spin chirality. This is a potential circularity: if the phases are identified partly by the presence of the UAHE itself, then the mechanism argument is not independent. The authors should either provide direct structural or real-space evidence for the TCS state in the film (e.g., Lorentz transmission electron microscopy, resonant x-ray scattering, or a thickness-dependent study) or soften the claim to say that the observed UAHE is consistent with the TCS scenario for the film, as inferred from bulk measurements.","section":"Discussion, 'Now we discuss the possible mechanisms...'; Magnetic phase diagram, Fig. 5d"},{"comment":"The quantitative estimate that the skyrmion topological Hall resistivity is about 0.1–1×10⁻⁴ μΩ cm, three orders of magnitude smaller than the observed ρU_xy, is a strong argument against a skyrmion origin. However, the linear scaling between bubble density and ρU_xy(H) is noted as a feature also seen in skyrmion-hosting systems, so it does not by itself distinguish the fluctuation-driven mechanism. This is not a flaw, but the interpretation would be more convincing if the authors also showed that the extracted ρU_xy is insensitive to the details of the bubble-counting threshold and that the bubble density is truly representative of the film's bulk, given that the MFM probe is surface sensitive.","section":"Discussion, MFM estimate; Supplementary S17"}],"minor_comments":[{"comment":"There are a few typographical errors, e.g., 'refered' and 'unconvenional anomalous Hall resistivity' in the second paragraph; these should be corrected.","section":"Introduction, second paragraph"},{"comment":"The notation for crystal orientations (b axis, ac plane) is sometimes confusing because the relationship between the monoclinic b axis and the film growth direction is not explicitly shown in a coordinate diagram; a small schematic would help the reader follow the field-orientation dependence.","section":"Fig. 1 caption and main text"},{"comment":"The data availability statement says raw data are available 'upon request'. Given that the central claim depends on a subtraction procedure, depositing the raw Hall and magnetization data in a public repository would strengthen reproducibility and allow independent verification of the ρU_xy extraction.","section":"Data availability"}],"recommendation":"major_revision","confidential_remarks":"This paper reports a potentially important observation, and the authors have made good-faith efforts to check the robustness of their extraction. However, the subtraction procedure is the linchpin of the claim, and the current manuscript does not provide enough evidence that the hump is not an artifact of a field-dependent anomalous Hall coefficient or a nonlinear ordinary Hall term. The related work by Baral et al. (arXiv:2502.13343) should be discussed more explicitly, especially as it proposes a nontrivial spin texture in bulk Fe3Ga4 and may affect the novelty or interpretation of the present film results. I would advise major revision and encourage the authors to add a two-band analysis or a high-field consistency check, and to provide raw data or error bars for the extracted ρU_xy."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's the take: the paper delivers a genuine materials advance—epitaxial Fe3Ga4 films with a large UAHE-like hump between 100 and 380 K in low fields—but the central number depends on a Hall subtraction whose main assumption (field-independent ordinary and anomalous coefficients) is not directly tested in the text. The reader's stress-test concern lands; I'd want that fixed before trusting the magnitude, not necessarily before sending it out.\n\nWhat's new: not the mechanism—bulk Fe3Ga4 already shows UAHE and is attributed to fluctuation-driven scalar spin chirality—but the film form, the clean epitaxy (RRR 17.7, sharp TN), the systematic phase diagram, and the low-field (0.35 T) value at 350 K. The MFM bubble-density scaling and the order-of-magnitude estimate that skyrmion topological contribution is ~10^3 times smaller than the observed rho_U is a good, quantitative check. The supplementary robustness checks on different forms of the anomalous term (S13) and interface magnetization (S14/S15) are genuine efforts.\n\nSoft spots: the subtraction assumes R0 H is linear and RS (or SH rho_xx^2 M) has field-independent coefficients. In a metamagnetic region with multiple AFM phases, each phase can have different transverse conductivity; a multi-band or field-dependent coefficient could easily produce a spurious hump between H_c1 and H_c3. The paper does not report a two-band analysis, error bars, or raw data. The functional-form check in S13 only varies how M weights the anomalous term; it doesn't test whether the coefficient itself changes across the metamagnetic transitions. The identification of AFM1/AFM2 as TCS is inferred from bulk neutron work, not measured in the film, so the mechanism discussion carries some circularity. The note added about Baral et al. proposing nontrivial spin textures in bulk is honest but leaves the interpretation open.\n\nWho this is for: experimentalists working on Hall-effect signatures of spin textures and on thin-film spintronics. It deserves a serious referee round—the central claim is plausible and important if true, but the extraction needs hardening. I'd ask for a two-band fit or an explicit field-dependent-coefficient test, error propagation, and deposition of raw Hall and magnetization curves.\n\nRecommendation: send to peer review with expectations of major revision. Don't desk reject.","headline":"A real materials advance with a load-bearing Hall-subtraction assumption; worth refereeing, but the magnitude of the UAHE signal needs independent validation.","tokens_in":18423,"tokens_out":1938,"would_cite":true,"duration_ms":20864,"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":"Epitaxial Fe3Ga4 films show an unconventional anomalous Hall resistivity of about 0.36 µΩ cm at 350 K in a field of only 0.35 T, spanning roughly 100 to 380 K.","keywords":["unconventional anomalous Hall effect","scalar spin chirality","transverse conical spiral","Fe3Ga4 epitaxial films","metamagnetic transitions","room-temperature spintronics","helimagnetism"],"falsifier":"Measure the Hall resistivity of the same film in the forced-ferromagnetic phase above Hc3 at 350 K and apply the identical subtraction procedure, since the paper reports the residual vanishes there: a surviving hump would indicate the subtraction is missing a field-dependent background. Alternatively, fit $\\rho_{xy}(H)$ with a two-band ordinary Hall model and repeat the subtraction; if $\\rho^U_{xy}$ at 0.35 T disappears or changes shape, the claimed chirality contribution is not separately established.","tokens_in":2020,"feed_emoji":"🧲","tokens_out":2400,"duration_ms":94232,"temperature":0.7,"pith_summary":"Epitaxial Fe3Ga4 films show a large unconventional anomalous Hall effect far above room temperature. In the intermediate antiferromagnetic phases, between roughly 100 K and 380 K, an extra Hall contribution builds up and peaks at about 0.36 µΩ cm at 350 K in a field of only 0.35 T. The paper argues that this hump comes from fluctuation-driven scalar spin chirality in the field-induced transverse-conical-spiral magnetic state, not from skyrmion-like topological textures. The result matters because most materials with such chirality-driven Hall signals require cryogenic temperatures or large magnetic fields, whereas this film operates near room temperature in a small field, making it a candidate for spintronic devices.","feed_headline":"Hall-effect hump appears at 350 K in Fe3Ga4 films","feed_subtitle":"A 0.35 T field yields a ~0.36 µΩ cm chirality-driven Hall signal across 100–380 K.","key_machinery":"The central object is the scalar spin chirality $\\chi_{ijk} = \\mathbf{S}_i \\cdot (\\mathbf{S}_j \\times \\mathbf{S}_k)$, the solid angle subtended by three neighboring spins, which acts as a virtual magnetic field on conduction electrons. The field-induced transverse-conical-spiral (TCS) magnetic phase carries the noncoplanar spin arrangement whose thermal or dynamic fluctuations produce a finite $\\chi_{ijk}$. The analysis machinery is the additive decomposition of the measured Hall resistivity into $\\rho_{xy} = R_0 H + S_H \\rho_{xx}^2 M + \\rho^U_{xy}$, where the first two terms are the ordinary and conventional anomalous Hall contributions and the residual hump $\\rho^U_{xy}$ is the claimed unconventional contribution. A comparison of $\\rho^U_{xy}$ with the topological Hall signal estimated from magnetic-bubble density is what rules out skyrmion textures as the dominant source.","core_discovery":"The central discovery is that a modest magnetic field applied perpendicular to the film plane transforms the helical-spiral order of epitaxial Fe3Ga4 into a transverse-conical-spiral state whose fluctuating spin chirality generates an unconventional anomalous Hall resistivity as large as 0.36 µΩ cm at 350 K in a field of 0.35 T. The signal is confined to the intermediate antiferromagnetic phases, AFM1 and AFM2 below the third metamagnetic field Hc3, and spans about 100 to 380 K. It nearly vanishes in the low-temperature ferromagnetic state and above Hc3. The paper shows that the magnitude expected from the emergent magnetic field of topological magnetic bubbles is roughly a thousand times smaller than the measured residual, so skyrmion textures cannot be the main origin; instead the study credits fluctuation-driven scalar spin chirality of the noncoplanar conical state, a mechanism previously invoked for kagome metals and supported by a concurrent neutron-scattering study of bulk Fe3Ga4.","pith_inferences":["The paper's finding that magnetic-bubble density scales linearly with the residual Hall signal, while the calculated skyrmion topological contribution is a thousand times too small, suggests the bubble-like domains visible in magnetic force microscopy may be the conical state itself rather than skyrmions; direct Lorentz transmission electron microscopy or neutron scattering on the same films could","A natural extension is a thickness series below about 20 nm with varied capping layers, which would isolate any interfacial Dzyaloshinskii-Moriya contribution and show whether the transverse-conical-spiral phase and its Hall signal survive or shift.","The subtraction model's robustness could be tested by adding a two-band ordinary Hall term or by allowing the anomalous Hall coefficient to depend on field; if the residual hump at 0.35 T disappears or changes shape under such fits, the chirality interpretation would need revision.","The concurrent neutron-scattering study noted in the paper proposes a nontrivial spin texture in the same phase window; if that texture is a fluctuation-stabilized conical state rather than a skyrmion lattice, the fluctuation-chirality picture would be confirmed."],"forward_implications":["Epitaxial Fe3Ga4 films provide a platform where a large chirality-driven Hall response is available near room temperature in fields below about 0.4 T, useful for magnetic sensing or electrical readout in spintronic devices.","The unconventional anomalous Hall signal tracks the metamagnetic phase boundaries, so field, temperature, and strain can tune the Hall response; piezoelectric or strain engineering should modulate it.","Because the signal arises from the transverse-conical-spiral phase rather than from skyrmions, noncoplanar conical spin order alone can produce large topological-like Hall signals, widening the search to centrosymmetric helimagnets.","Applying the same decomposition procedure to other helimagnetic intermetallics with helical-spiral-to-conical transitions may reveal comparable room-temperature unconventional Hall effects.","The large signal in films about 80 nm thick indicates that interfacial Dzyaloshinskii-Moriya interaction is not required, and thinner films with different capping layers should allow the mechanism to be tested and tuned."],"supporting_citations":[{"why":"Supplies the bulk Fe3Ga4 magnetic phase sequence and the earlier observation of an unconventional Hall effect below about 400 K, giving the baseline this film study extends.","marker":"[38]"},{"why":"Provides the theoretical spin-spiral and topological-Hall mechanism for Fe3Ga4 that the paper applies to its films.","marker":"[41]"},{"why":"Establishes the helical-spiral order and the field-induced transverse and longitudinal conical-spiral states in bulk Fe3Ga4, the magnetic structures invoked to explain the film's Hall signal.","marker":"[42]"},{"why":"Documents fluctuation-driven scalar spin chirality in the kagome metal YMn6Sn6 within its transverse-conical-spiral phase, the closest mechanistic analog used by the paper.","marker":"[29]"},{"why":"Reports the topological Hall effect induced by chiral fluctuations in ErMn6Sn6, reinforcing the fluctuation-chirality mechanism in the same material family.","marker":"[30]"},{"why":"Shows field-induced nontrivial spin chirality and a large topological Hall effect in ScMn6Sn6, another transverse-conical-spiral kagome example the paper compares with.","marker":"[31]"},{"why":"A concurrent neutron-scattering study of bulk Fe3Ga4 proposing a nontrivial spin texture in the same phase where the Hall signal peaks, providing independent support for the interpretation.","marker":"[48]"},{"why":"Gives the Pt/Cr2O3 bilayer result with an unconventional Hall signal above the Néel temperature but smaller than 1 nΩ cm, the key comparison for showing the Fe3Ga4 film signal is large.","marker":"[37]"}],"fun_headline_variants":["Fe3Ga4 films show large Hall effect at 350 K","Chirality-driven Hall signal in Fe3Ga4 films at 350 K","Room-temperature Hall anomaly from spin chirality in Fe3Ga4","Fe3Ga4 films: large unconventional Hall effect at 350 K","Unconventional Hall effect in Fe3Ga4 films at 350 K"],"cache_read_input_tokens":20352,"weakest_assumption_plain":"The load-bearing premise is that the measured Hall resistivity is exactly a field-linear ordinary term plus a magnetization-proportional anomalous term with field-independent coefficients, so the leftover hump is a real magnetic-chirality contribution; if either coefficient changes with magnetic field, or the ordinary part curves because there are two types of carriers, the leftover hump could be a subtraction artifact.","fun_headline_variants_meta":{"raw":{"variants":["Fe3Ga4 films show large Hall effect at 350 K","Chirality-driven Hall signal in Fe3Ga4 films at 350 K","Room-temperature Hall anomaly from spin chirality in Fe3Ga4","Fe3Ga4 films: large unconventional Hall effect at 350 K","Unconventional Hall effect in Fe3Ga4 films at 350 K"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001027,"raw_usage":{"total_tokens":4327,"prompt_tokens":944,"completion_tokens":3383,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":560,"completion_tokens_details":{"reasoning_tokens":3286}},"tokens_in":560,"tokens_out":3383,"duration_ms":26161,"temperature":1.0,"reasoning_tokens":3286,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T18:41:31.341611+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the Hall resistivity of the same film in the forced-ferromagnetic phase above Hc3 at 350 K and apply the identical subtraction procedure, since the paper reports the residual vanishes there: a surviving hump would indicate the subtraction is missing a field-dependent background. Alternatively, fit $\\rho_{xy}(H)$ with a two-band ordinary Hall model and repeat the subtraction; if $\\rho^U_{xy}$ at 0.35 T disappears or changes shape, the claimed chirality contribution is not separately established.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the bulk Fe3Ga4 magnetic phase sequence and the earlier observation of an unconventional Hall effect below about 400 K, giving the baseline this film study extends."},{"cited_title":"& Mazin, I","cited_arxiv_id":null,"evidence_quote":"Provides the theoretical spin-spiral and topological-Hall mechanism for Fe3Ga4 that the paper applies to its films."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the helical-spiral order and the field-induced transverse and longitudinal conical-spiral states in bulk Fe3Ga4, the magnetic structures invoked to explain the film's Hall signal."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Documents fluctuation-driven scalar spin chirality in the kagome metal YMn6Sn6 within its transverse-conical-spiral phase, the closest mechanistic analog used by the paper."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the topological Hall effect induced by chiral fluctuations in ErMn6Sn6, reinforcing the fluctuation-chirality mechanism in the same material family."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows field-induced nontrivial spin chirality and a large topological Hall effect in ScMn6Sn6, another transverse-conical-spiral kagome example the paper compares with."},{"cited_title":"Fluctuation-driven topological Hall effect in room-temperature itinerant helimagnet Fe3Ga4","cited_arxiv_id":"2502.13343","evidence_quote":"A concurrent neutron-scattering study of bulk Fe3Ga4 proposing a nontrivial spin texture in the same phase where the Hall signal peaks, providing independent support for the interpretation."},{"cited_title":"& Yang, F","cited_arxiv_id":null,"evidence_quote":"Gives the Pt/Cr2O3 bilayer result with an unconventional Hall signal above the Néel temperature but smaller than 1 nΩ cm, the key comparison for showing the Fe3Ga4 film signal is large."}],"review_version":1}