{"id":"1822a4a8-b99b-4cc3-99fd-648ab7d79390","arxiv_id":"2505.13001","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Epitaxial GdAlGe films from 1 to 90 monolayers show an altermagnetic state with an increasing ferromagnetic admixture toward the monolayer limit, intrinsic exchange bias, and a metal-insulator transition below 3 monolayers.","lead":"The paper grows thin films of the magnet GdAlGe, down to a single atomic layer, and reports that it behaves as an altermagnet with a growing ferromagnetic component in thinner films. This matters because tunable magnetic states in nanoscale spintronic materials could enable new control mechanisms for devices.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Thickness trend in FM admixture is confounded by the substrate switch: ultrathin films (≤4 ML) were grown on Ge/Si(001), thick films on Ge(001), with no same-substrate control.","rationale":"In good faith, the paper is a careful experimental study and the bulk-like observations (AHE, remnant moment, exchange bias in 17 ML) do indicate an FM admixture in thick GdAlGe films. However, the most load-bearing part of the central claim is not the existence of the admixture but its monotonic increase toward the 2D limit. That trend is inferred from films grown on two different substrates: thick films on Ge(001) wafers, and 1–4 ML films on Ge/Si(001). The manuscript itself acknowledges the substrate change, noting that the Ge wafer conductivity interferes with transport for ultrathin films; the same substrate difference can affect magnetic properties through strain, interface electronic reconstruction, or a small magnetic background from the Ge buffer/Si interface. Without a same-substrate control series, the data cannot distinguish an intrinsic thickness effect from a substrate-induced effect. The reader's weakest_assumption centered on the DFT sub-meV FiM energy and the unverified 4'm'm point group, which is a related but more secondary concern; the substrate confound is arguably more direct because it attacks the empirical trend rather than the interpretation. The reader's rationale did mention the substrate confound, so alignment is partial rather than complete. The verdict should remain CONDITIONAL: the observations are plausible and worth publishing, but the 2D-limit claim needs a control experiment before it can be regarded as established.","tokens_in":14749,"tokens_out":2976,"duration_ms":32058,"concrete_test":"Grow a 90 ML GdAlGe film on the same Ge/Si(001) substrate used for the ultrathin films, and measure M(H) at 2 K and remnant-moment temperature dependence. If the FM moment per Gd for 90 ML on Ge/Si matches the values for 1–3 ML films, the thickness trend in Fig. S9 is a substrate effect rather than a 2D-limit property. Conversely, growing a 4 ML film on the Ge(001) wafer and observing the same moment as on Ge/Si would validate the trend.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim, that the FM admixture increases as GdAlGe approaches the 2D limit, rests on comparing M(H) curves (Fig. S9) for 1, 2, 3 ML with 90 ML. However, the ultrathin films were synthesized on a different substrate stack (7–9 nm Ge on high-ohmic Si(001)) than the 90 ML film (Ge(001) wafer). The authors state in Section 2.1 that pristine Ge wafers were impractical for ≤4 ML films because the wafer's conductivity interferes with transport measurements, but magnetic measurements can also be affected by the substrate via strain, interface chemistry, or magnetic background. The increasing FM moment per Gd could reflect substrate-induced stabilization of the FiM/FM state rather than an intrinsic thickness effect in GdAlGe. Since the key trend lacks a same-substrate control, the headline claim is not yet established. The DFT support (Table S1: FiM only 0.9 meV/f.u. above AFM-1) is also within DFT+U accuracy, making the intrinsic-admixture interpretation underdetermined even for thick films.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a combined experimental and DFT study of epitaxial GdAlGe films spanning 1 to 90 monolayers (ML), grown by MBE on Ge(001) and Ge/Si(001) substrates. The paper claims that tetragonal GdAlGe is a candidate altermagnet with a small admixture of a ferromagnetic (or ferrimagnetic) state, that this FM contribution grows as the film thickness is reduced toward the monolayer limit, that coexistence of the magnetic states produces intrinsic exchange bias, and that ultrathin films undergo a metal-insulator transition with variable-range hopping transport. Supporting evidence includes SQUID magnetization, anomalous Hall effect (AHE), negative magnetoresistance, exchange bias in the 17 ML film, and DFT+U relative energies of six magnetic configurations, with the ferrimagnetic (FiM) state only 0.9 meV/f.u. above the altermagnetic AFM-1 ground state.","tokens_in":15006,"tokens_out":7964,"duration_ms":87396,"significance":"If the central claims hold, the paper would be a valuable contribution to nanoscale altermagnet research: it identifies a new candidate AM (GdAlGe), demonstrates dimensionality-tuned competition between AM and FM states, and reports intrinsic exchange bias in a single material, with a natural comparison to GdAlSi. The study has notable strengths: a systematic thickness series down to a single monolayer with phase-pure epitaxial films characterized by RHEED and XRD; mutually consistent magnetization, magnetotransport, and exchange-bias observations across multiple thicknesses; DFT energy ordering of six magnetic configurations; and an honest use of the word 'candidate.' The cross-check with magnetization of polycrystalline bulk GdAlGe adds credibility to the bulk FM-admixture estimate. However, the headline thickness trend is currently underdetermined by a substrate switch, and the interpretation of the AHE and exchange bias rests on a sub-meV DFT energy and on an experimentally unverified magnetic point group, so the significance hinges on additional controls rather than on the data as presented.","major_comments":[{"comment":"The central claim that the FM admixture increases as the system approaches the 2D limit is confounded by a substrate switch at 4 ML. The M(H) comparison in Fig. S9 uses 1, 2, and 3 ML films grown on 7-9 nm Ge/Si(001) high-ohmic substrates and compares them with the 90 ML film grown on a pristine Ge(001) wafer, with no same-substrate control for either thickness regime. Strain, interface chemistry, or a magnetic background from the Ge/Si buffer could stabilize the FiM/FM state in the ultrathin films, and the paper does not characterize the strain state of the Ge buffer. A same-substrate control (e.g., a 90 ML or a 10/17 ML film grown on Ge/Si(001), or 3 ML films grown on a Ge(001) wafer measured magnetically) is required before the dimensionality trend in the FM admixture can be considered established; without it, the headline claim of the abstract is not supported by the presented data.","section":"Section 2.1 and Fig. S9"},{"comment":"The interpretation that the AHE, remnant moment, and exchange bias originate from an intrinsic FiM/FM admixture rests on a DFT+U energy difference of only 0.9 meV/f.u. between the FiM state and the AFM-1 ground state (Table S1), with a Hubbard U of 7 eV applied to Gd. This energy scale is well within the accuracy of DFT+U for Gd 4f electrons, and the paper itself contrasts this with the 2.3 meV FiM energy in GdAlSi as 'noticeably higher,' a distinction that the method cannot reliably support. The AHE attribution further assumes that the AM ground state has magnetic point group 4'm'm', which forbids AHE in the experimental geometry, but this point group is not verified experimentally (no spin-resolved ARPES, neutron diffraction, or symmetry-sensitive Hall measurements on the AM state alone). Since AHE is known to arise from AM states in other compensated materials (e.g., MnTe, cited as Ref. [45]), the paper should either provide a concrete, method-robust argument that the sub-meV FiM state is populated in the films at the measurement temperatures, or present independent evidence for the FiM phase, otherwise the 'intrinsic admixture' interpretation is underdetermined.","section":"Section 2.2 and Table S1"},{"comment":"The quantitative magnetic claims lack error bars and a documented extraction procedure. The paper quotes a total FM moment of about 0.13 mu_B/Gd (approximately 2% of the fully polarized value) for the 90 ML film and states that the FM moments in ultrathin films remain far from 7 mu_B/Gd, but no uncertainties, fits, or sample-to-sample reproducibility statistics are provided for the non-linear M(H) contribution. Given the acknowledged surface roughness of the films and the small size of the FM signal relative to the dominant AM/paramagnetic background, the subtraction procedure and its uncertainty should be described so the reader can judge whether the thickness trend in Fig. S9 is quantitative or qualitative.","section":"Section 2.2, Figs. 3b and S9"}],"minor_comments":[{"comment":"The substrate temperature is written as '190 C°'; it should read '190 °C'.","section":"Section 2.1"},{"comment":"The rationale for the substrate switch states that the Ge wafer conductivity is comparable to that of the ultrathin GdAlGe films; a quantitative statement of the wafer resistivity and the measured sheet resistances would help the reader assess whether the transport data on the two substrate types are directly comparable.","section":"Section 2.1"},{"comment":"The AHE scaling exponent of 1.61 for the 2 ML film is presented with the caveat that the spans of sigma_xx and sigma_xy are relatively small; the fit range, number of points, and an uncertainty estimate for the exponent should be given so the reader can judge how well the power law is constrained.","section":"Section 2.3 and Fig. 6d"},{"comment":"The statement that the magnetic point group 4'm'm' of the AM state does not allow observation of the AHE is central to the AHE attribution, yet it is given without derivation or a supporting reference; a brief symmetry argument would make the claim checkable.","section":"Section 2.2"},{"comment":"The terms 'AM state' and 'AFM state' are used somewhat interchangeably; since the AM state of GdAlGe is a compensated collinear state that is also an antiferromagnet, one sentence defining the relationship at first use would remove ambiguity.","section":"Abstract and Section 3"}],"recommendation":"major_revision","confidential_remarks":"The paper is part of a productive series from this group on Gd-based 2D magnets and altermagnets, and the incremental novelty (Ge analogue, exchange bias, MIT, EB) is appropriate for the journal. The decisive risk is the substrate switch confound: if the authors cannot supply a same-substrate control, the headline thickness claim should be softened to a qualitative observation restricted to the Ge/Si(001) platform. The reviewer should also weigh whether one set of films per thickness is sufficient for the quantitative claims, given the acknowledged roughness and nominal thickness uncertainties."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is a well-executed experimental paper from a group that demonstrably knows how to make these Gd-based films. The headline—FM admixture increasing as GdAlGe approaches the monolayer—is plausible but not yet fully nailed down, because the ultrathin films were grown on a different substrate stack than the thick ones.\n\nWhat is genuinely new: first epitaxial GdAlGe films, from 90 ML down to 1 ML, first altermagnet framing for this compound, and a thickness series that reveals an MIT below 3 ML. The characterization is thorough: RHEED and XRD establish the tetragonal phase, SQUID and transport measurements are mutually consistent, and the observations hang together. AHE, negative MR, remnant moments, and the exchange bias in the 17 ML film all point to coexisting magnetic states. The side-by-side comparison with GdAlSi is useful and suggests a real chemical difference.\n\nThe main soft spot is the substrate switch. The authors say that pristine Ge wafers were impractical for ≤4 ML films because of substrate conductivity in transport measurements, so ultrathin films went on Ge(001)/Si(001) with 7–9 nm of Ge, while thick films went on bulk Ge(001). That is a confound for the magnetic measurements too: strain, interface chemistry, and substrate background can all change the measured moment per Gd. Without a same-substrate control, the increasing FM contribution toward the 2D limit is not uniquely a thickness effect. I read the paper as somewhat aware of this but not treating it as a serious limitation.\n\nSecond, the DFT support for the ferrimagnetic state is a 0.9 meV/f.u. energy difference above the AFM-1 state—inside typical DFT+U accuracy. So the interpretation of the FM admixture as arising from a specific FiM state is underdetermined. Third, the assignment of the AHE to the FM admixture assumes the AM point group 4'm'm, which is not experimentally verified here.\n\nNone of this kills the paper. The experimental observations are likely real, and the core claim of coexisting magnetic states with intrinsic exchange bias is credible. But the strongest assertion—that the FM admixture grows toward the 2D limit—is not yet established to the standard the authors imply.\n\nWho is this for? People working on altermagnet thin films and rare-earth intermetallics will find value in the growth recipes, the transport data, and the comparison to GdAlSi. It deserves a serious referee, but the referee should push for same-substrate controls and quantified FM moments with error bars. I would send it to review with that expectation.","headline":"First epitaxial GdAlGe films show credible AM/FM coexistence and an interesting exchange bias, but the central thickness-dependent FM-admixture claim is confounded by a substrate switch and needs same-substrate controls before it is established.","tokens_in":15554,"tokens_out":2063,"would_cite":false,"duration_ms":21196,"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":"Thin films of the candidate altermagnet GdAlGe carry a ferromagnetic admixture that grows as thickness drops, and the two coexisting magnetic states produce intrinsic exchange bias.","keywords":["altermagnetism","GdAlGe","anomalous Hall effect","exchange bias","metal-insulator transition","two-dimensional limit","magnetic state competition","rare-earth intermetallics"],"falsifier":"Spin-resolved photoemission on a bulk-like GdAlGe film that fails to show the predicted non-relativistic spin splitting would falsify the altermagnet designation; conversely, demonstrating that the anomalous Hall effect persists in a purely compensated single-domain sample, or neutron diffraction that rules out any ferrimagnetic volume fraction, would falsify the FM-admixture explanation. A simpler check is whether the exchange-bias shift and the ferromagnetic moment track each other as thickness, strain, or field-cooling conditions change.","tokens_in":14575,"feed_emoji":"🧲","tokens_out":7617,"duration_ms":69804,"temperature":0.7,"pith_summary":"The paper sets out to show that tetragonal GdAlGe is a candidate altermagnet whose films, from bulk-like down to one monolayer, do not stay in a purely compensated magnetic state: a ferromagnetic or ferrimagnetic contribution is present at all thicknesses and grows as the film approaches the two-dimensional limit. This matters because altermagnets promise spintronic functionality without net magnetization, and a controllable ferromagnetic admixture would add a handle for switching and reading such devices while largely preserving altermagnetic spin splitting. The coexistence of the two magnetic states is used to explain an intrinsic exchange bias observed in intermediate-thickness films, alongside an anomalous Hall effect and negative magnetoresistance. The authors interpret the thickness evolution as evidence that small chemical substitutions, here Si to Ge, and dimensional confinement can move the same magnetic family into different regimes of altermagnet/ferromagnet competition.","feed_headline":"Thin altermagnet GdAlGe gains a ferromagnetic admixture","feed_subtitle":"Ferromagnetic content rises toward one monolayer, yielding intrinsic exchange bias and a metal-insulator transition.","key_machinery":"The load-bearing object is the energy landscape of magnetic configurations of tetragonal GdAlGe computed by DFT+U: an altermagnetic AFM-1 ground state with magnetic point group 4'm'm, a ferrimagnetic state at 0.9 meV per formula unit that the films can populate, and a fully ferromagnetic state at 45.7 meV per formula unit. The near-degeneracy of the AFM-1 and ferrimagnetic states is the mechanism that lets a small ferromagnetic-like moment coexist with altermagnetic order and grow toward the 2D limit; that coexistence in turn generates the intrinsic exchange bias. On the transport side, the anomalous Hall effect is attributed to the ferromagnetic admixture because the 4'm'm point group of the altermagnetic state forbids AHE, and Mott variable-range hopping analysis is used to identify 3D localization in 2 ML films and 2D localization in 1 ML films.","core_discovery":"The central claim is that epitaxial GdAlGe films host two competing magnetic orders simultaneously: the altermagnetic state, which has zero net moment and non-relativistic spin splitting, and a weak ferromagnetic or ferrimagnetic state whose weight is roughly two percent of full Gd polarization in bulk-like films and rises as thickness drops to a single monolayer. Evidence comes from magnetization loops with small hysteretic moments, remnant moments, an anomalous Hall contribution that appears below the magnetic transition, and a shifted hysteresis loop in 17 ML films interpreted as intrinsic exchange bias. DFT+U places a ferrimagnetic configuration only 0.9 meV per formula unit above the altermagnetic AFM-1 ground state, close enough to be stabilized in the films, whereas the fully ferromagnetic state lies 45.7 meV above. The paper contrasts GdAlGe with GdAlSi, whose films show no measurable ferromagnetic admixture, and finds that GdAlGe undergoes a metal-insulator transition below roughly three monolayers, with the 1 ML film showing 2D variable-range hopping and unusually large negative magnetoresistance.","pith_inferences":["A testable extension is chemical alloying: if the 0.9 meV separation between the AFM-1 and ferrimagnetic states controls the admixture, GdAlSi1-xGex films should show a continuous progression from GdAlSi-like compensation to GdAlGe-like ferromagnetic admixture as x rises.","The near-degeneracy also opens the possibility of switching the admixture externally: strain, electric field, or substrate termination perturb energies at the meV scale, so field- or gate-controlled changes in the ferromagnetic fraction could turn the exchange bias on and off, though the paper does not demonstrate such control.","The sub-meV energy difference suggests the ferromagnetic signal may come from spatially localized ferrimagnetic domains rather than a uniform weak canting; magnetic imaging or thickness-dependent exchange-bias measurements could distinguish a homogeneous admixture from phase separation."],"forward_implications":["GdAlGe films offer the same altermagnetic band splitting as GdAlSi while adding a thickness-tunable ferromagnetic response, so the Si-to-Ge substitution provides a materials lever for adjusting the altermagnet/ferromagnet balance.","Intrinsic exchange bias in single-material films removes the need for a separate antiferromagnetic interface in proposed altermagnet-based memory elements.","The ferromagnetic contribution strengthens toward the monolayer, so scaling to the 2D limit can enhance the spintronic responses that a compensated altermagnet suppresses.","Films of 4 ML and thicker reproduce bulk-like transport and AHE hysteresis, while 1-2 ML films enter a strongly localized regime with markedly larger negative magnetoresistance.","The observed AHE, remnant moments, and shifted hysteresis are all consistent with coexistence of altermagnetic and ferromagnetic orders, making GdAlGe a model platform for studying that coexistence across dimensions."],"supporting_citations":[{"why":"Sister study on epitaxial GdAlSi films down to one monolayer; provides the altermagnet-to-2D baseline and the contrast case with no measurable FM admixture.","marker":"[30]"},{"why":"Establishes tetragonal GdAlSi as a candidate altermagnet with nonrelativistic spin splitting, the structural and electronic template for GdAlGe.","marker":"[49]"},{"why":"Determines the tetragonal crystal structure and lattice parameters of GdAlGe used to identify the epitaxial polymorph.","marker":"[52]"},{"why":"Reports bulk magnetization of tetragonal GdAlGe consistent with a small ferromagnetic moment, supporting the FM-admixture interpretation.","marker":"[56]"},{"why":"Shows thickness-dependent competing magnetic states in related rare-earth Si/Ge 2D magnets, the pattern the paper extends to GdAlGe.","marker":"[35]"},{"why":"Documents polymorphism in GdAlSi and stabilization of a layered polymorph, informing the substrate choice for tetragonal GdAlGe.","marker":"[55]"},{"why":"Supplies the anomalous Hall scaling relation used to interpret the AHE in 2 ML GdAlGe.","marker":"[67]"}],"fun_headline_variants":["Ferromagnetic admixture grows in thin altermagnet GdAlGe","GdAlGe films gain ferromagnetic character as thickness drops","Thin GdAlGe altermagnet reveals ferromagnetic admixture rise","Ferromagnetic state strengthens in GdAlGe films toward monolayer","GdAlGe altermagnet picks up ferromagnetism as it thins"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The central conclusion assumes that the films realize the DFT-predicted ferrimagnetic state, only 0.9 meV per formula unit above the altermagnetic ground state, and that the altermagnetic state's 4'm'm point group forbids the anomalous Hall effect that is then attributed entirely to the ferromagnetic admixture.","fun_headline_variants_meta":{"raw":{"variants":["Ferromagnetic admixture grows in thin altermagnet GdAlGe","GdAlGe films gain ferromagnetic character as thickness drops","Thin GdAlGe altermagnet reveals ferromagnetic admixture rise","Ferromagnetic state strengthens in GdAlGe films toward monolayer","GdAlGe altermagnet picks up ferromagnetism as it thins"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001012,"raw_usage":{"total_tokens":4278,"prompt_tokens":952,"completion_tokens":3326,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":568,"completion_tokens_details":{"reasoning_tokens":3232}},"tokens_in":568,"tokens_out":3326,"duration_ms":21551,"temperature":1.0,"reasoning_tokens":3232,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T20:21:23.185288+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Spin-resolved photoemission on a bulk-like GdAlGe film that fails to show the predicted non-relativistic spin splitting would falsify the altermagnet designation; conversely, demonstrating that the anomalous Hall effect persists in a purely compensated single-domain sample, or neutron diffraction that rules out any ferrimagnetic volume fraction, would falsify the FM-admixture explanation. A simpler check is whether the exchange-bias shift and the ferromagnetic moment track each other as thickness, strain, or field-cooling conditions change.","supporting_citations":[{"cited_title":"E., Averyanov, D","cited_arxiv_id":null,"evidence_quote":"Sister study on epitaxial GdAlSi films down to one monolayer; provides the altermagnet-to-2D baseline and the contrast case with no measurable FM admixture."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes tetragonal GdAlSi as a candidate altermagnet with nonrelativistic spin splitting, the structural and electronic template for GdAlGe."},{"cited_title":"T., Parthé, E., Structure of YAlGe and Isotypic Rare-Earth-Aluminium Germanides, Acta Cryst","cited_arxiv_id":null,"evidence_quote":"Determines the tetragonal crystal structure and lattice parameters of GdAlGe used to identify the epitaxial polymorph."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports bulk magnetization of tetragonal GdAlGe consistent with a small ferromagnetic moment, supporting the FM-admixture interpretation."},{"cited_title":"V., Sokolov, I","cited_arxiv_id":null,"evidence_quote":"Shows thickness-dependent competing magnetic states in related rare-earth Si/Ge 2D magnets, the pattern the paper extends to GdAlGe."},{"cited_title":"V., Sokolov, I","cited_arxiv_id":null,"evidence_quote":"Documents polymorphism in GdAlSi and stabilization of a layered polymorph, informing the substrate choice for tetragonal GdAlGe."},{"cited_title":"H., Ong, N","cited_arxiv_id":null,"evidence_quote":"Supplies the anomalous Hall scaling relation used to interpret the AHE in 2 ML GdAlGe."}],"review_version":1}