{"id":"7a790326-7bee-4cbb-909d-5a713450b910","arxiv_id":"2501.01355","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"EuMn2P2 shows weak manganese-based magnetic ordering around 48 to 51 K, on top of europium antiferromagnetic order at 18 K, according to ESR, heat capacity, and NMR data.","lead":"A multi-technique study of EuMn2P2 finds small but consistent anomalies near 48 to 51 K that the authors interpret as weak magnetic ordering of manganese, sitting below the europium antiferromagnetic transition at 18 K. The result offers a candidate explanation for why earlier neutron work saw no manganese order in this frustrated triangular-lattice material.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The weak Mn-order claim rests on indirect bulk probes; all element-specific Mn measurements are null, so a trace impurity or dynamic-fluctuation origin is not excluded.","rationale":"The reader's weakest_assumption already identifies the indirect nature of the evidence and the null XMCD/XMLD results; my independent pass reaches the same conclusion. The paper is honest about the null element-specific probes, which is why outright rejection is not warranted, but the central claim is precisely as fragile as the reader states. My reading adds that the magnetization data cited as supporting the claim show no clear feature, and that the entropy release is small enough that trace inclusions (Sn content is explicitly 1–5%) cannot be excluded by the bulk measurements alone. The ESR and NMR anomalies are also compatible with dynamic spin fluctuations rather than static order, so the conclusion requires a direct probe of the Mn sublattice. The reader's CONDITIONAL verdict is appropriate; I would not change it, but the requested neutron diffraction check should be a condition for full acceptance.","tokens_in":20495,"tokens_out":7912,"duration_ms":90651,"concrete_test":"Perform a zero-field, high-statistics neutron diffraction experiment (single crystal or large powder) on the same Sn-flux-grown EuMn2P2 at 60, 50, 45, 40, and 5 K, and refine possible magnetic Bragg reflections on the Mn sublattice with a detection limit below the claimed 0.06 μB ordered moment. If no Mn magnetic reflection appears below 51 K, the static weak-Mn-order interpretation is falsified, and the ESR/HC/NMR anomalies must instead be attributed to dynamic Mn fluctuations or a trace impurity phase.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing assumption is that the ESR kink at TM ≈ 47 K, the two heat-capacity anomalies at 48.5 and 51 K, and the 31P NMR wipe-out below ≈ 50 K are intrinsic static magnetic transitions of the Mn sublattice. This assumption is underdetermined because none of these probes directly measures Mn order: ESR observes an Eu2+ resonance whose anomalies could reflect changes in the Eu–Mn coupling or a separate small phase; NMR observes 31P, whose line broadening and intensity loss are also produced by enhanced spin fluctuations without static order; heat capacity is non-element-specific. The only Mn-specific probes, XMCD and XMLD (Sec. III E), detect no Mn signal at 23 K/9 T and at 5–51 K, respectively; the authors attribute this to a c-axis or frustrated arrangement (Sec. IV), but this is an unverified escape. The heat-capacity entropy is only 0.035 J/mol K (0.6% of R ln2), so a trace Sn-rich or Mn-defect impurity phase—samples contain 1–5% Sn inclusions—would give signals of comparable magnitude. The 0.06 μB 'upper limit' is a model-dependent estimate from 31P linewidth, not a measured ordered moment. Magnetization itself shows no clear anomaly at T1/T2 (Sec. III C), so citing it as a confirmatory technique is an overstatement. Since prior neutron diffraction [19] saw no Mn order, positive identification is required before the statement that the results 'convincingly show' weak Mn order can be accepted.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a multi-technique study of EuMn2P2 single crystals, combining ESR, heat capacity, magnetization, 31P NMR, XMCD/XMLD, and resistivity. The authors confirm the known Eu2+ antiferromagnetic transition at TN = 18 K and present evidence for an additional temperature scale near 47-51 K: a kink in the ESR linewidth/resonance field/intensity at TM = 47 K, two small heat-capacity anomalies at T1 = 51 K and T2 = 48.5 K, and a strong suppression of the 31P NMR echo intensity below about 50 K. These observations are attributed to weak magnetic ordering of the Mn sublattice, with an estimated Mn moment below 0.06 μB. The XMCD and XMLD measurements, however, detect no Mn signal, and the authors speculate that the ordering is either out-of-plane or frustrated. The central claim is that ESR, heat capacity, magnetization, and NMR 'convincingly show' weak Mn order below 51 K.","tokens_in":20865,"tokens_out":3861,"duration_ms":37942,"significance":"If the interpretation is correct, EuMn2P2 would be a rare example of weak magnetic ordering of Mn on a frustrated triangular lattice in the 122 family, with an ordered moment below 0.06 μB. The paper is valuable for its rich experimental dataset, the internal consistency between the ESR, heat-capacity, and NMR anomalies, and the honest reporting of the null XMCD/XMLD results. The explicit upper limit on the Mn moment and the comparison with non-magnetic isostructural compounds are useful constraints. However, the central claim currently rests on indirect bulk probes, and the element-specific direct probes are negative, so the significance of the result depends on whether the indirect anomalies can be tied to intrinsic Mn order rather than to impurities or dynamic fluctuations.","major_comments":[{"comment":"The statement that ESR, heat capacity, magnetization, and NMR 'convincingly show the presence of weak Mn order below 51 K' is not supported by the element-specific measurements in the same paper. The XMCD data give an upper limit of μMn < 0.03 μB at 23 K and 9 T, and the XMLD measurements find no Mn signal at 5-51 K, including at T = 51 K which is the claimed onset of Mn order. Because these direct probes are negative, the indirect bulk anomalies could also arise from impurity phases (the samples contain 1-5% Sn inclusions) or from enhanced spin fluctuations without static order. The suggestion of a c-axis-oriented or frustrated Mn order is an unverified escape. The authors should either soften the conclusion to 'suggestive of' weak Mn ordering or provide additional evidence, such as neutron diffraction with polarization analysis or a quantitative model of how a static 0.03-0.06 μB Mn moment produces the observed ESR and NMR effects.","section":"Section IV Discussion, Section III E"},{"comment":"The thermal-resolution description of the heat capacity measurements appears inconsistent with the claim of two distinct anomalies. The paper states that to resolve weak anomalies, large heating pulses with typical temperature rises of 6 K were used. A 6 K thermal pulse averages the specific heat over a window that is larger than the 2.5 K separation between T1 = 51 K and T2 = 48.5 K, making the resolution of two separate peaks implausible. Please clarify whether '6 K' is a typo, how the single-slope analysis handles such large pulses, and what the effective temperature resolution actually is. This is load-bearing because the two anomalies T1 and T2 are the only thermodynamic evidence for two distinct transitions.","section":"Section II C, Figure 4"},{"comment":"The abstract and summary list 'magnetization' among the techniques that show weak Mn order, but the magnetization data in the manuscript show no clear feature at T1 and T2: Sec. III C states that 'no clear feature was seen around the T1 and T2 transition temperatures in χ(T)' and Fig. 6(b) shows only a slight change in slope. Citing magnetization as a confirmatory technique overstates the experimental evidence. The manuscript should be corrected to reflect what the magnetization data actually show.","section":"Section III C, Section V Summary"},{"comment":"The combined entropy of the T1 and T2 anomalies is given as S = 0.035 J mol−1 K−1, only 0.6% of R ln 2 expected for ordering of a local Mn moment. This extremely small entropy is not discussed quantitatively. Such a small value could indicate that the anomalies are not bulk thermodynamic transitions of a local-moment system, or that only a tiny fraction of the sample orders. A comparison with the expected impurity contribution from Sn inclusions (1-5%) or from other minority phases would help the reader judge whether the anomalies are intrinsic. Please provide an estimate of the impurity-phase contribution to the heat capacity in this temperature range.","section":"Section IV Discussion, heat capacity entropy"}],"minor_comments":[{"comment":"The heading 'X-ray magnetic circular and linear dicroism' contains a typo; it should read 'dichroism'.","section":"Section III E heading"},{"comment":"The ESR data in Fig. 3 appear to be plotted without error bars, and the fits use the same TM = 47 K value that is read from the data being fitted. The authors should state the uncertainty in TM and show whether the (T − 47)−1 form is uniquely favored over other divergences.","section":"Figure 3"},{"comment":"The sentence 'the lineshape keeps to be Lorentzian across the 47 K scale' is awkward; it should read 'remains Lorentzian across the 47 K anomaly.'","section":"Section IV Discussion"},{"comment":"The resistivity shoulder near 142 K is described as sample-dependent; the connection to the onset of Mn fluctuations in Ref. [20] should therefore be phrased as tentative, since the sample dependence weakens the association.","section":"Section III F and Fig. S9"}],"recommendation":"major_revision","confidential_remarks":"The manuscript fits the scope of cond-mat.str-el and presents a substantial experimental effort. The main obstacle to acceptance is the mismatch between the strength of the central claim ('convincingly show') and the absence of direct element-specific confirmation; the null XMCD/XMLD results and the very small heat-capacity entropy leave room for impurity or dynamic-fluctuation interpretations. If the authors can provide a clarifying modification of the claim, a careful resolution analysis for the heat capacity, and a quantitative impurity assessment, the paper could become publishable. I would also encourage the editor to consider whether the authors should be asked to obtain neutron diffraction data, though that may be beyond the manuscript's current scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is a careful experimental paper, and the main thing you should know is that the central claim--weak Mn ordering near 50 K in EuMn2P2--is plausible but not proven. The authors do a lot right, and they are honest about what they cannot see.\n\nThe new results: first ESR on this compound, with clean anomalies in linewidth, resonance field, and intensity around 47 K, while the line stays Lorentzian. Two tiny heat-capacity anomalies are resolved at 48.5 K and 51 K, and the 31P NMR intensity drops sharply below about 50 K, consistent with enhanced Mn fluctuations. The field dependence of the two heat-capacity peaks is argued carefully: T1 broadens and shifts isotropically, T2 sharpens and shifts anisotropically. That is a real experimental contribution.\n\nThe soft spot is the attribution. The entropy under the heat-capacity anomalies is 0.6% of Rln2, which is about what a trace impurity phase would give, and the crystals contain 1-5% Sn inclusions. There are no error bars on those peaks. XMCD gives an upper limit of 0.03 Bohr magneton for the Mn moment at 23 K and 9 T, and XMLD sees nothing at 51 K--the exact temperature of the claimed transition. The authors' response is that the moment could be out of plane or frustrated, which is an unverified escape. The NMR-based 0.06 Bohr magneton estimate is also model-dependent. Magnetization shows essentially nothing at T1/T2. So the statement that these results \"convincingly show\" weak Mn order overshoots the evidence; they show that something happens around 50 K, and that Mn is the plausible source, but the element-specific probes do not see it.\n\nThe circularity concern is minor. The ESR TM is read from the same data used for the (T-47)^-1 fits, but the heat-capacity and NMR anomalies provide independent anchors.\n\nBottom line: this is a solid, honest experimental study with a genuine puzzle in a specific material. It deserves peer review, with the expectation that the authors soften the \"convincingly show\" language and address the impurity/error-bar issue. I would cite it as a data point on EuMn2P2, not as proof of Mn order. For a reading group, it is a good example of how to present null element-specific results without burying them.\n\nSend it to a referee.","headline":"A careful multi-technique study that plausibly identifies weak Mn order near 50 K in EuMn2P2, but the Mn-specific probes are null and the evidence is circumstantial; deserves peer review.","tokens_in":21429,"tokens_out":2267,"would_cite":false,"duration_ms":21966,"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":"EuMn2P2 hosts a weak manganese magnetic ordering near 50 K, coexisting with the europium antiferromagnetic transition at 18 K.","keywords":["EuMn2P2","weak magnetic ordering","magnetic frustration","triangular lattice","electron spin resonance","nuclear magnetic resonance","heat capacity","Eu-based 122 pnictide"],"falsifier":"A search for a magnetic Bragg peak by neutron diffraction, or an internal-field signature by muon spin rotation, on a tin-free EuMn2P2 crystal between 45 and 55 K would settle the claim: if no magnetic signal appears at a sensitivity below $0.05\\,\\mu_B$ while the heat-capacity and ESR anomalies persist, the weak-Mn-ordering interpretation would be falsified.","tokens_in":20321,"feed_emoji":"🧲","tokens_out":13761,"duration_ms":115431,"temperature":0.7,"pith_summary":"EuMn2P2, a layered compound with europium on triangular planes separated by manganese-phosphorus blocks, has been known to order antiferromagnetically at 18 K through its europium moments. This paper argues that the manganese sublattice itself undergoes a weak, likely frustrated magnetic ordering near 51 K and 48.5 K, with an electron-spin-resonance temperature scale of about 47 K. The argument rests on four independent measurements: anomalies in the Eu$^{2+}$ ESR linewidth, resonance field and intensity; two small heat-capacity anomalies; and a sharp loss of ${}^{31}$P NMR intensity below about 50 K. The ordered manganese moment is estimated to be below about 0.06 Bohr magnetons, which is why the transition is nearly invisible in magnetization, resistivity, and element-specific X-ray dichroism. If the claim is correct, EuMn2P2 becomes a test bed for weak 3d magnetism on a frustrated triangular lattice coexisting with dominant 4f moments.","feed_headline":"Weak manganese order surfaces near 50 K in EuMn2P2","feed_subtitle":"Heat capacity, ESR and NMR independently see the tiny transition that magnetization and X-ray dichroism miss.","key_machinery":"The central object is the Eu$^{2+}$ electron spin resonance line, which acts as a local probe of the Mn spin background: the line stays a single exchange-narrowed Lorentzian across the 47 K scale, while its linewidth and resonance field follow a Curie-Weiss form $\\propto (T-47)^{-1}$ approaching $T_M = 47$ K from above and then kink below it. The supporting machinery is the combination of heat capacity with large heating pulses to resolve the two tiny anomalies at 48.5 and 51 K, and ${}^{31}$P NMR with its hyperfine coupling constant $A_{\\rm hf} = -5.11$ kOe/$\\mu_B$, which converts the observed wipe-out and line broadening into an upper bound of $0.06\\,\\mu_B$ for the Mn moment change. The interpretation framework is magnetic frustration on the triangular Mn sublattice, which the paper invokes to explain why such a weak order coexists with the large paramagnetic Eu background and why it escapes X-ray magnetic dichroism.","core_discovery":"The paper's central claim is that EuMn2P2 exhibits intrinsic weak magnetic ordering of the Mn spins below about 51 K, in addition to the A-type antiferromagnetic order of the Eu$^{2+}$ ions at $T_{\\rm N} = 18$ K. The evidence is the set of coincident anomalies at $T_1 = 51$ K and $T_2 = 48.5$ K: the ESR linewidth and resonance field of the Eu$^{2+}$ resonance show kinks and a $(T-47)^{-1}$ Curie-Weiss divergence above the 47 K scale; the heat capacity shows two small peaks whose field dependence (isotropic broadening and shift upward for $T_1$, anisotropic sharpening and upward shift for $T_2$) is inconsistent with a purely structural transition; and the ${}^{31}$P NMR spin-echo intensity loses about two-thirds of its value between 72.5 K and 40 K, with a wipe-out onset near 50 K. From the NMR hyperfine coupling the paper derives an upper bound of about $0.06\\,\\mu_B$ for the Mn moment variation, while XMCD puts the field-aligned Mn spin moment below $0.03\\,\\mu_B$. The authors interpret the small entropy of the transitions (about 0.6% of $R\\ln 2$) as the signature of a strongly frustrated triangular Mn sublattice that orders only weakly, with a possible ferromagnetic component at $T_1$ and a frustrated component at $T_2$.","pith_inferences":["If the weak order is intrinsic, the same local-probe strategy could be applied to other Eu$T_2$Pn$_2$ compounds to search for hidden 3d magnetism that bulk measurements miss.","Measuring the ${}^{31}$P spin-lattice relaxation rate across the 50 K wipe-out would test whether the signal loss is caused by critical slowing down of Mn fluctuations rather than by static broadening.","Hydrostatic pressure, which has been shown to tune Eu magnetism in related 122 compounds, could strengthen or suppress the Mn order and thereby discriminate between frustration-limited and impurity-stabilized ordering.","The near-equality of the Curie-Weiss scale (47 K), the heat-capacity transitions (48.5 and 51 K), and the NMR wipe-out onset (50 K) suggests a single underlying temperature scale, so a unified microscopic model should reproduce all three simultaneously."],"forward_implications":["Below 51 K EuMn2P2 hosts two coexisting magnetic subsystems: a dominant Eu$^{2+}$ antiferromagnet ordering at 18 K and a much weaker Mn order, so local probes (ESR, NMR, heat capacity) are needed to see the Mn transition at all.","The field dependence of the Mn transitions—$T_1$ shifting isotropically to higher temperatures and broadening, $T_2$ sharpening and shifting anisotropically—rules out a simple structural origin and points to a ferromagnetic component at $T_1$ and frustration at $T_2$.","The estimated ordered Mn moment of $<0.06\\,\\mu_B$ and the XMCD upper bound of $0.03\\,\\mu_B$ imply that any Mn moment must be either very small, mostly along the $c$-axis where the dichroism geometry is blind, or strongly frustrated.","The ${}^{31}$P NMR wipe-out constitutes a sensitive indicator of the hidden Mn order, since neither magnetization nor resistivity shows any feature at $T_1$ and $T_2$."],"supporting_citations":[{"why":"Neutron diffraction established the A-type Eu antiferromagnetic order at 16.5 K and found no Mn order, providing the baseline this paper overturns.","marker":"[19]"},{"why":"A prior heat-capacity and susceptibility study concluded that Mn long-range order is suppressed by chemical pressure and suggested Mn short-range correlations around 250–100 K, providing the dataset and interpretation this paper refines.","marker":"[20]"},{"why":"It shows the related compound EuMn2As2 with an Mn transition at 135 K, giving a family comparison for Mn ordering.","marker":"[21]"},{"why":"It shows EuMn2Sb2 with an Mn transition at 128 K, another family comparison.","marker":"[22]"},{"why":"An ESR study of isostructural EuZn2P2 with nonmagnetic Zn, whose absence of the 47 K anomalies supports assigning them to Mn.","marker":"[27]"},{"why":"An ESR study of EuFe$_{2-x}$Co$_x$As$_2$ that connects linewidth features to Eu ordering and the spin-density-wave transition, serving as the interpretive template for an Eu$^{2+}$ resonance probing a second spin system.","marker":"[31]"},{"why":"A magnetic study of (Sr,Ca)Mn2P2 showing how weak Mn magnetism contributes to susceptibility, used to argue that the Mn contribution is about four orders of magnitude smaller than the Eu paramagnetic signal.","marker":"[43]"}],"fun_headline_variants":["Tiny Mn order appears at 50 K in EuMn2P2","Frustrated EuMn2P2 shows weak Mn order near 50 K","Weak Mn ordering at 51 K emerges despite frustration in EuMn2P2","Hidden weak Mn order at 50 K in frustrated EuMn2P2","Mn spins order weakly at 51 K in EuMn2P2, NMR confirms"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the very small heat-capacity peaks at 48.5 and 51 K, the ESR kinks near 47 K, and the NMR wipe-out below 50 K are intrinsic magnetic transitions of the Mn sublattice and not artifacts of tin inclusions, sample variation, or the strong europium paramagnetic response.","fun_headline_variants_meta":{"raw":{"variants":["Tiny Mn order appears at 50 K in EuMn2P2","Frustrated EuMn2P2 shows weak Mn order near 50 K","Weak Mn ordering at 51 K emerges despite frustration in EuMn2P2","Hidden weak Mn order at 50 K in frustrated EuMn2P2","Mn spins order weakly at 51 K in EuMn2P2, NMR confirms"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000844,"raw_usage":{"total_tokens":3792,"prompt_tokens":1182,"completion_tokens":2610,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":798,"completion_tokens_details":{"reasoning_tokens":2505}},"tokens_in":798,"tokens_out":2610,"duration_ms":18517,"temperature":1.0,"reasoning_tokens":2505,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T22:29:31.323204+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A search for a magnetic Bragg peak by neutron diffraction, or an internal-field signature by muon spin rotation, on a tin-free EuMn2P2 crystal between 45 and 55 K would settle the claim: if no magnetic signal appears at a sensitivity below $0.05\\,\\mu_B$ while the heat-capacity and ESR anomalies persist, the weak-Mn-ordering interpretation would be falsified.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Neutron diffraction established the A-type Eu antiferromagnetic order at 16.5 K and found no Mn order, providing the baseline this paper overturns."},{"cited_title":"Berry, N","cited_arxiv_id":null,"evidence_quote":"A prior heat-capacity and susceptibility study concluded that Mn long-range order is suppressed by chemical pressure and suggested Mn short-range correlations around 250–100 K, providing the dataset and interpretation this paper refines."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It shows the related compound EuMn2As2 with an Mn transition at 135 K, giving a family comparison for Mn ordering."},{"cited_title":"Schellenberg, M","cited_arxiv_id":null,"evidence_quote":"It shows EuMn2Sb2 with an Mn transition at 128 K, another family comparison."},{"cited_title":"Krug von Nidda, S","cited_arxiv_id":null,"evidence_quote":"An ESR study of EuFe$_{2-x}$Co$_x$As$_2$ that connects linewidth features to Eu ordering and the spin-density-wave transition, serving as the interpretive template for an Eu$^{2+}$ resonance probing a second spin system."},{"cited_title":"Benner and J","cited_arxiv_id":null,"evidence_quote":"A magnetic study of (Sr,Ca)Mn2P2 showing how weak Mn magnetism contributes to susceptibility, used to argue that the Mn contribution is about four orders of magnitude smaller than the Eu paramagnetic signal."}],"review_version":1}