{"id":"6ef4a13e-2f82-4d60-a133-4a3726ed1ce3","arxiv_id":"2505.02936","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"Gallium doping gradually reorients magnetism in TmMn6Sn6 from easy-plane to easy-axis, with the reorientation temperature rising until it merges with the magnetic ordering temperature near x=2.","lead":"This paper maps how replacing tin with gallium in TmMn6Sn6, a kagome-lattice magnet, flips the preferred direction of the magnetic moments from in-plane to out-of-plane as gallium content grows. It offers a way to tune magnetic anisotropy in this materials family, which matters for designing skyrmions and other spin textures.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"DFT explanation rests on endpoint-only interpolation with a single Hubbard U; finite-x anisotropy and U sensitivity are untested, so the theoretical overlay is conditional even though the experimental SRT stands.","rationale":"The reader's weakest-assumption analysis correctly identifies the DFT interpolation and the Hubbard-U sensitivity as the fragile part of the paper. My stress-testing read agrees: the experimental evidence for the spin reorientation is consistent and direct, while the theoretical overlay is an endpoint calculation supplemented by an interpolation assumption. The manuscript itself provides the key internal warning in Sec. III.E when it acknowledges non-monotonic E(θ) and large higher-order anisotropy constants; this makes the two-point comparison insufficient to establish the concentration dependence of the anisotropy. A single Hubbard U with constrained 4f occupancy is also a known delicate treatment for rare-earth crystal-field anisotropy, so the sign change from x=0 to x=2 should be checked for robustness. None of these concerns undermines the central experimental claim, which is well supported by magnetization, transport, and heat-capacity measurements. Therefore the appropriate outcome remains CONDITIONAL, matching the reader's verdict, and no change to that verdict is needed.","tokens_in":15901,"tokens_out":4004,"duration_ms":45928,"concrete_test":"Compute E(θ) for TmMn6Sn5Ga1 (x=1) in a supercell with Ga at the Sn(2c) site, using the same Wien2k DFT+U protocol, and repeat for U = 6, 8, 10, and 12 eV. If the x=1 easy-axis energy difference is not between the x=0 and x=2 values, or if the sign of the x=2 easy-axis preference reverses within this U range, the Sec. III.E interpolation and the stated first-principles explanation of TSR(x) are unsupported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The experimental spin reorientation is well supported by M(T), M(H), transport, and heat-capacity data, so the measured easy-plane-to-easy-axis crossover is not in doubt. The load-bearing weak point is the theoretical explanation in Sec. III.E: the claim that TSR(x) is controlled by a monotonic crossover between endpoint anisotropies rests on DFT+U calculations for x=0 and x=2 only, using U=10 eV with a constrained Hund's-rule 4f occupancy. Two issues make this inference insecure. First, no intermediate-x calculation is reported; the sentence 'Interpolating between these two high-symmetry calculations, we conclude that the reorientation transition must appear at some small x...' assumes the anisotropy evolves monotonically and that no competing order (e.g., canted or multi-k state) intervenes. Second, the paper itself notes a non-monotonic E(θ) and large higher-order anisotropy constants in the R166 system, yet the endpoint energy difference E(θ=0)-E(θ=90°) cannot characterize the full angular anisotropy landscape at intermediate doping, especially if fourth- or sixth-order terms are significant. Finally, the Tm 4f crystal-field anisotropy is a delicate quantity; a single Hubbard U with controlled occupancy is not demonstrated to give a robust sign for the x=2 easy-axis state. If a moderate U change flips that sign, or if an explicit x=1 calculation does not lie between the endpoints, the 'first-principles explanation' fails even though the experimental SRT remains valid.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript reports a combined experimental and computational study of Ga-substituted TmMn6Sn6. Magnetization, magnetotransport, and heat-capacity measurements on single crystals show an easy-plane to easy-axis spin reorientation that appears above a critical Ga concentration and moves to higher temperature with increasing x. DFT+U calculations for the x=0 and x=2 endpoints show a change from easy-plane to easy-axis total anisotropy, and the authors interpolate between these endpoints to explain the doping dependence of the spin-reorientation temperature.","tokens_in":16192,"tokens_out":4991,"duration_ms":54103,"significance":"The experimental dataset is extensive and internally consistent: the spin-reorientation transition (SRT) is confirmed by three independent probes for x=1.2 and x=1.8, and the evolution with Ga content is systematic. If the theoretical mechanism is substantiated, the paper offers a practical route to tune anisotropy in RMn6Sn6 kagome magnets and to stabilize skyrmionic textures. The DFT parameter table (Table II) is a useful quantitative output. However, the theoretical explanation as presented is qualitative and rests on endpoint interpolation; the paper's central contribution is therefore better characterized as a well-documented experimental phase diagram with a plausible but not fully validated microscopic rationale.","major_comments":[{"comment":"The assertion that interpolating the x=0 and x=2 endpoint DFT calculations implies a reorientation at 'some small x' and that TSR 'should grow continuously' is not backed by any intermediate-x calculation or by any uncertainty estimate for the DFT+U anisotropy. In particular, the Tm 4f crystal-field anisotropy is a delicate quantity; with U=10 eV and constrained Hund's-rule occupancy, a small change in U or in the double-counting scheme could flip the sign of the easy-axis contribution, as the non-monotonic E(θ) in Fig. 11(a) already indicates high-order anisotropy terms are sizable. Please add an intermediate-x calculation (e.g., x=1) and a U-sensitivity test, or explicitly restrict the claim to 'a plausible scenario consistent with experiment.'","section":"Sec. III.E (paragraph beginning 'As discussed above, for a reorientation transition...')"},{"comment":"The phase diagram is derived from raw M(T) data at 0.1 T with no demagnetizing-field correction. For hexagonal platelet crystals, demagnetization makes M_c vs M_ab comparisons field-dependent, so the phase boundary (white region) and the reported TSR values may shift at other fields or with sample shape. Since the heat-capacity anomalies confirm the SRT for only x=1.2 and x=1.8, please provide demagnetization-corrected curves or a quantitative estimate of the correction's effect on the phase diagram.","section":"Sec. III.B and Fig. 6"},{"comment":"The claim to provide a 'first-principles explanation of the curious doping dependence of TSR' is not supported quantitatively: no TSR is computed from A, B1, B2, or from the temperature evolution of the sublattice anisotropies. The Heisenberg parameters in Table II are given only at x=0 and x=2, and the temperature dependence of Tm vs Mn anisotropy is invoked verbally. Please show how the model parameters would yield a transition temperature, or soften the claim to a qualitative mechanism.","section":"Sec. III.E and Table II"}],"minor_comments":[{"comment":"There is an errant comma in 'polycrystalline, samples' in the Abstract, and the symbol 'N`eel' is typeset incorrectly throughout the text (e.g., Sec. I and Sec. III.E).","section":"Abstract and Sec. I"},{"comment":"The word 'thullium' should be 'thulium', and 'FP-LAPW' should be 'FLAPW' for the standard acronym.","section":"Sec. II.B"},{"comment":"The word 'Intnesity' should be 'Intensity'.","section":"Fig. 2 caption"},{"comment":"The sign convention for J2 is not explicitly stated in the text; since the text refers to 'antiferromagnetic J2' and Table II lists positive values, please clarify that positive J corresponds to antiferromagnetic coupling in Eq. (1).","section":"Table II and Eq. (1)"},{"comment":"The heat capacity is labeled 'CV' in the caption, but the measurement is at constant pressure; please use 'Cp' or define the constant-volume usage explicitly.","section":"Fig. 10 caption"}],"recommendation":"major_revision","confidential_remarks":"The theoretical section relies heavily on Refs. [6,37,38] from the same group for the anisotropy decomposition and the crystal-field treatment; the editor may wish to ensure that the present manuscript is sufficiently self-contained and that the methodology is reproducible without those papers. This does not affect my recommendation, which is driven by the technical points above."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The thing to know: this paper gives the most complete single-crystal phase diagram yet for Ga doping in TmMn6Sn6, and the experimental spin reorientation from easy-plane to easy-axis is on solid footing. The DFT part is a plausible interpolation between x=0 and x=2 endpoints, not a fully verified microscopic theory, and the paper would be stronger if it said so more plainly.\n\nWhat is actually new: the consolidated phase diagram spanning x=0 to 1.9, the heat-capacity kinks at the reorientation temperature, the transport/MR/Hall data across the series, and the DFT+U/open-core decomposition separating Tm-4f and Mn contributions to the anisotropy. The earlier neutron study (ref 25) and single-crystal magnetization study (ref 33) already reported the easy-plane-to-easy-axis trend, so the title's claim of \"doping-induced spin reorientation\" is not new, but this paper extends and systematizes it. The authors are honest about that lineage.\n\nThe magnetization data (Mab vs Mc at 0.1 T, M(H) at selected temperatures) consistently support the reorientation, and the heat-capacity anomaly near TSR in x=1.2 and 1.8 is a nice thermodynamic confirmation. The formation-energy calculation correctly identifies the Sn3 (2c) site as the Ga substitution site, matching Rietveld and prior neutron work. I believe the experimental headline.\n\nThe soft spots are concentrated in the theory. The \"first-principles explanation\" of the TSR(x) trend rests entirely on DFT+U for x=0 and x=2, with a single Hubbard U (10 eV) and no intermediate-x calculation. The sentence \"Interpolating between these two high-symmetry calculations, we conclude that the reorientation transition must appear at some small x...\" assumes monotonic evolution of the anisotropy and no competing canted or multi-k state. Those are not guaranteed, and the paper itself notes non-monotonic E(θ) behavior and large higher-order anisotropy constants. A moderate change in U could shift the Tm-4f crystal-field sign, and an explicit x=1 calculation might not lie between the endpoints. So the DFT overlay is suggestive, not definitive. The phase diagram also lacks demagnetization corrections and error bars on composition and TSR, but those are minor given the consistency across multiple measurements.\n\nThis is a paper for the RMn6Sn6 community and anyone working on anisotropy engineering via chemical substitution. It deserves a serious referee. I would recommend acceptance after the authors either add an intermediate-x calculation or a U-sensitivity check, soften the interpolation claim, and include error bars in the phase diagram. The experimental contribution is solid enough on its own.","headline":"Solid single-crystal phase diagram for Ga-doped TmMn6Sn6; the endpoint-only DFT interpolation is a plausible but unproven mechanism.","tokens_in":16777,"tokens_out":1919,"would_cite":true,"duration_ms":21469,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["75.30.Gw","75.50.Gg"],"model":"deepseek-v4-flash","headline":"Gallium doping flips TmMn6Sn6 from in-plane to c-axis magnetism, with the switching temperature set by the doping level.","keywords":["kagome magnet","spin reorientation transition","magnetic anisotropy","TmMn6Sn6","gallium doping","magnetocrystalline anisotropy","rare-earth 4f electrons","kagome lattice"],"falsifier":"Find single crystals at x=0.6, 1.0, and 1.5 and map TSR from magnetization or heat capacity; if TSR does not rise smoothly toward the ordering temperature with increasing x, the interpolation between the x=0 and x=2 calculations is wrong. Independently, recompute the TmMn6Sn4Ga2 anisotropy with a different correlation correction for Tm 4f electrons or with unconstrained f occupancy, and check whether the easy-axis ground state survives.","tokens_in":1842,"feed_emoji":"🧲","tokens_out":2962,"duration_ms":132083,"temperature":0.7,"pith_summary":"This paper establishes that replacing a few percent of the tin atoms in the kagome magnet TmMn6Sn6 with gallium gradually rotates the preferred direction of the magnetic moments out of the kagome plane and onto the c-axis. The rotation is not instantaneous: at low Ga content the easy-plane state persists, around x≈0.5–0.6 a spin-reorientation transition appears, and the temperature of that transition rises with Ga content until near x≈2 the material is easy-axis at all temperatures. The authors support the picture with magnetization, heat-capacity, resistivity, and Hall measurements on single crystals, and with density-functional calculations tracing the effect to a Ga-induced change in the crystal field acting on thulium's 4f electrons. The practical payoff is a single family of crystals whose magnetic anisotropy can be dialed continuously, which is the regime where skyrmionic spin textures have been observed.","feed_headline":"Gallium doping flips TmMn6Sn6 from in-plane to c-axis magnetism","feed_subtitle":"The spin switch happens at a temperature that rises with Ga content until the c-axis becomes the easy axis everywhere.","key_machinery":"The load-bearing object is an effective one-dimensional Heisenberg chain Hamiltonian for the Mn layers, $H=E_0+\\sum_{i<j}J_{ij}\\mathbf{m}_i\\cdot\\mathbf{m}_j+A\\sum_i(m_i^z)^2+\\sum_{i<j}B_{ij}m_i^z m_j^z$, whose parameters are obtained by fitting density-functional energies for five ordered spin configurations. In this Hamiltonian, the single-site term $A$ controls the easy-plane versus easy-axis balance, and the anisotropic-exchange terms $B_{ij}$ are what keep the Mn sublattice easy-plane in the parent compound. The doping dependence enters through the Tm 4f contribution to the total anisotropy, which the authors isolate by comparing DFT+U and open-core calculations; Ga substitution at the Sn(2c) site modifies the Tm crystal field enough to reverse that contribution, while the Mn anisotropic exchange changes comparatively little.","core_discovery":"The paper's central discovery is a doping-controlled spin-reorientation transition in TmMn6Sn6−xGax. For x≲0.5 the magnetization stays in the ab-plane as in the parent compound; starting near x≈0.5–0.6 the moments reorient from the ab-plane to the c-axis below a doping-dependent temperature TSR, which increases from about 196 K at x=1.2 to about 241 K at x=1.8 and merges with the Néel temperature near x≈2, leaving easy-axis order at all temperatures. First-principles calculations show the total magnetic anisotropy flipping from easy-plane in TmMn6Sn6 to easy-axis in TmMn6Sn4Ga2, with the Ga atoms occupying the Sn(2c) site. The authors interpret the flip as a reversal of the single-ion Tm 4f anisotropy via a changed crystal field; in the intermediate regime the reorientation reflects competition between the low-temperature Tm anisotropy and the high-temperature Mn-sublattice anisotropy.","pith_inferences":["The crystal-field mechanism implies Ga is probably not unique: any substitution that occupies the 2c Sn site and shrinks the lattice—Ge, In, or Si variants, for instance—should shift the Tm 4f anisotropy in a similar direction, though the magnitude would differ.","Between x=1.8 and x=2.0 the reorientation temperature should approach the magnetic ordering temperature, so a finely spaced doping series in that window would reveal whether the reorientation merges smoothly or is cut off by the collinear ordering.","A natural microscopic question the paper leaves open is how the reorientation proceeds: uniform rotation of all moments versus nucleation and motion of domain walls between easy-plane and easy-axis regions could be distinguished by small-angle neutron scattering through TSR.","The fitted exchange and anisotropy parameters could be fed directly into atomistic spin simulations to predict skyrmion size, lifetime, and current-driven motion across the series, without requiring new measurements for each doping."],"forward_implications":["The spin-reorientation temperature is a continuous design parameter: choosing x between about 1.2 and 1.8 places the easy-axis switch anywhere between roughly 196 K and 241 K.","Because skyrmion bubbles are observed just below the reorientation transition in the x=1.8 crystal, fine Ga adjustments provide a route to tune the stability, size, and temperature window of such spin textures.","The fitted exchange parameters indicate that Ga reverses J2 between Mn layers; in the Tm system this predicts a crossover from the parent long-pitch spiral to a collinear ferrimagnetic state as x grows.","The same anisotropy-compensation mechanism that drives the spontaneous reorientation in TbMn6Sn6 can be induced intentionally by substitution in other magnetic RMn6Sn6 compounds, broadening the materials space for reorientation-tuned spintronic behavior."],"supporting_citations":[{"why":"Neutron study of TmMn6Sn6−xGax that established the easy-plane-to-easy-axis anisotropy change and the parent spiral; the present single-crystal data are the high-quality confirmation and extension.","marker":"[25]"},{"why":"Earlier magnetization study on TmMn6Sn6−xGax single crystals (0.15≤x≤1.90) that the current phase diagram compares with and extends.","marker":"[33]"},{"why":"Establishes the anisotropy-compensation mechanism for spin reorientation in kagome RMn6Sn6 and supplies the effective one-dimensional chain model used here.","marker":"[6]"},{"why":"Defines the kagome RMn6Sn6 magnetic structure and exchange parameters J1, J2, J3, including the anisotropic-exchange picture of Mn easy-plane anisotropy.","marker":"[1]"},{"why":"Provides the DFT+U and open-core methodology for separating Tm 4f and non-4f anisotropy contributions, plus the crystal-field perturbation treatment.","marker":"[37]"},{"why":"Mean-field theory of double-flat-spiral magnetic structures used to interpret the helimagnetic phase and to predict the collinear state upon Ga doping.","marker":"[39]"},{"why":"Reports biskyrmion textures near the spin-reorientation transition in TbMn6Sn6, motivating the search for tunable spin textures in TmMn6Sn6−xGax.","marker":"[27]"},{"why":"Reports skyrmion bubbles just below TSR in TmMn6Sn4.2Ga1.8, the observation that the tunability discussion builds on.","marker":"[28]"}],"fun_headline_variants":["Gallium doping flips Kagome magnet's spin axis","Doping tunes spin reorientation in TmMn6Sn6","Easy-axis flip in Kagome magnet via Ga substitution","Ga doping switches magnetic anisotropy in TmMn6Sn6","Spin reorientation driven by Ga in Kagome magnet"],"cache_read_input_tokens":18816,"weakest_assumption_plain":"The explanation assumes that the anisotropy of intermediate Ga concentrations changes smoothly between the two calculated endpoints, and that the way the calculation treats thulium's inner f electrons—including the chosen correlation correction and the imposed Hund's-rule configuration—gets the sign of the switch right.","fun_headline_variants_meta":{"raw":{"variants":["Gallium doping flips Kagome magnet's spin axis","Doping tunes spin reorientation in TmMn6Sn6","Easy-axis flip in Kagome magnet via Ga substitution","Ga doping switches magnetic anisotropy in TmMn6Sn6","Spin reorientation driven by Ga in Kagome magnet"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000266,"raw_usage":{"total_tokens":1610,"prompt_tokens":943,"completion_tokens":667,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":559,"completion_tokens_details":{"reasoning_tokens":587}},"tokens_in":559,"tokens_out":667,"duration_ms":7183,"temperature":1.0,"reasoning_tokens":587,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T00:39:30.417109+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Find single crystals at x=0.6, 1.0, and 1.5 and map TSR from magnetization or heat capacity; if TSR does not rise smoothly toward the ordering temperature with increasing x, the interpolation between the x=0 and x=2 calculations is wrong. Independently, recompute the TmMn6Sn4Ga2 anisotropy with a different correlation correction for Tm 4f electrons or with unconstrained f occupancy, and check whether the easy-axis ground state survives.","supporting_citations":[{"cited_title":"Lefevre, G","cited_arxiv_id":null,"evidence_quote":"Neutron study of TmMn6Sn6−xGax that established the easy-plane-to-easy-axis anisotropy change and the parent spiral; the present single-crystal data are the high-quality confirmation and extension."},{"cited_title":"Canepa, M","cited_arxiv_id":null,"evidence_quote":"Earlier magnetization study on TmMn6Sn6−xGax single crystals (0.15≤x≤1.90) that the current phase diagram compares with and extends."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the anisotropy-compensation mechanism for spin reorientation in kagome RMn6Sn6 and supplies the effective one-dimensional chain model used here."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Defines the kagome RMn6Sn6 magnetic structure and exchange parameters J1, J2, J3, including the anisotropic-exchange picture of Mn easy-plane anisotropy."},{"cited_title":"Rosenfeld and N","cited_arxiv_id":null,"evidence_quote":"Mean-field theory of double-flat-spiral magnetic structures used to interpret the helimagnetic phase and to predict the collinear state upon Ga doping."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports biskyrmion textures near the spin-reorientation transition in TbMn6Sn6, motivating the search for tunable spin textures in TmMn6Sn6−xGax."},{"cited_title":"Skyrmion Bubbles by Design in a Centrosymmetric Kagome Magnet","cited_arxiv_id":"2504.19045","evidence_quote":"Reports skyrmion bubbles just below TSR in TmMn6Sn4.2Ga1.8, the observation that the tunability discussion builds on."}],"review_version":1}