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Altermagnetizing the FeSe-like two-dimensional materials and approaching to giant tunneling magnetoresistance with Janus Cr4BN(B2) MBene electrode

T0 review · 4 major / 5 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read Nitrogen substitution plus bilayer stacking is predicted to turn a FeSe-like CrB bilayer into a stable 2D altermagnet, Cr4B3N, whose Cr-B edged vacuum-barrier junction reaches a 91,001% tunneling magnetoresistance.

desk verdict A plausible new 2D altermagnet with an unsupported headline TMR: the 91,001% value rides on an untested near-zero denominator, and the printed TMR formula contradicts the reported numbers. read the letter →

arxiv 2502.03165 v2 pith:M5K3YWTA submitted 2025-02-05 cond-mat.mtrl-sci cond-mat.mes-hall

Weiwei Sun , Mingzhuang Wang , Baisheng Sa , Shaui Dong , Carmine Autieri This is my paper · ORCID
classification cond-mat.mtrl-scicond-mat.mes-hall
keywords altermagnetismtwo-dimensionalmagnetsMBenetunnelingmagnetoresistancemagnetictunneljunctiondensityfunctionaltheorynon-equilibriumGreen'sfunctionJanusstructure
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using density functional theory, the paper predicts a new two-dimensional metal-boride magnet, Cr4B3N, formed by substituting nitrogen into one side of a FeSe-like CrB bilayer and stacking the bilayers. The substitution breaks the symmetry that kept the parent monolayer a conventional antiferromagnet, producing an altermagnet: a metal with zero net magnetization but spin-split bands whose up- and down-spin channels are swapped under a fourfold rotation in momentum space. Because the material is metallic, the authors use it as the electrode of an in-plane magnetic tunnel junction with a 7 Å vacuum barrier. They find that when both electrodes terminate in Cr-B edges, the parallel-spin configuration conducts while the antiparallel configuration is essentially blocked, giving a tunneling magnetoresistance of 91,001%. If this holds up, it would be one of the highest TMR ratios predicted for any altermagnetic junction and a concrete route to using antiferromagnet-like electrodes in spintronic devices.

What carries the argument

The load-bearing object is the spin-group operation $[C_2 \parallel C_4^z]$ — a spin flip combined with a fourfold real-space rotation — which replaces the translation or inversion connection of a conventional antiferromagnet and makes spin-up and spin-down bands split oppositely along $X\!-\!\Gamma$ and $\Gamma\!-\!Y$ while staying degenerate along $\Gamma\!-\!M$. This symmetry is what makes the material a d-wave altermagnet. The transport claim is carried by the Cr4B3N/vacuum/Cr4B3N in-plane MTJ with matched Cr-B vertical edges, whose $k_\parallel$-resolved transmissions are computed with the non-equilibrium Green's function method; the near-total suppression of $T_{AP}$ relative to $T_P$ is the mechanism behind the 91,001% TMR.

What would settle it

The decisive check is a convergence study of the conf-3 antiparallel transmission: recomputing $T_{AP}$ with $k$-point grids from 101×101 up to 401×401, with larger basis sets and vacuum gaps from 5 to 15 Å. If $T_{AP}$ converges to a nonzero value above roughly $10^{-4}$, the TMR drops below 10,000%; if it stays below $10^{-6}$, the giant ratio is confirmed as a genuine symmetry effect.

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Extended reading notes

Core claim

The central claim is that the FeSe-like CrB monolayer, whose ground state is a conventional antiferromagnet, can be converted into an altermagnet by nitrogen substitution plus bilayer stacking: the resulting Janus MBene Cr4B3N belongs to magnetic space group $Pm'm2'$ (#25.59), has an AFM1 ground state with antiferromagnetic exchange couplings to the first four Cr neighbors, and shows a clear d-wave spin splitting of its metallic bands. The Fermi surface is spin-split in a momentum-dependent way, and the same material, used as both electrodes of a Cr4B3N/vacuum/Cr4B3N in-plane tunnel junction, gives an edge-dependent TMR: 149% for asymmetric Cr-B/Cr-B-N edges, 2,151% for matched Cr-B-N edges, and 91,001.6% for matched Cr-B edges. In the best configuration the antiparallel transmission is effectively zero in the sampled $k$-space while parallel transmission survives as strips, and the giant ratio persists across an energy window of $E_f \pm 0.5$ eV.

Load-bearing premise

The 91,001% tunneling magnetoresistance hinges on the assumption that the antiparallel-state current is truly zero to numerical resolution; any small unresolved nonzero transmission would lower the ratio by orders of magnitude.

Editorial extensions

If this is right

  • Cr4B3N is a metallic 2D altermagnet whose AFM1 ground state is stabilized by antiferromagnetic exchange up to the fourth Cr neighbor.
  • A Cr4B3N/vacuum/Cr4B3N junction with matched Cr-B edges gives a predicted TMR of 91,001.6%, while Cr-B-N edges give 2,151% and asymmetric edges give 149%.
  • The near-total blocking of antiparallel transmission persists across $E_f \pm 0.5$ eV, so the junction should tolerate Fermi-level shifts from doping or gating.
  • The combined N-substitution and bilayer-stacking route can in principle be applied to other FeSe-like lattices to create a family of 2D altermagnets.
  • The predicted TMR exceeds previously reported altermagnetic and antiferromagnetic tunnel junctions, making the material a candidate electrode for MRAM and magnetic-sensor applications.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Beyond the paper, the same nitrogen-substitution-plus-stacking recipe could be screened across other FeSe-like metal borides to find additional 2D altermagnets; the paper sketches this mapping but does not enumerate candidates.
  • The >9 eV built-in work-function difference between the Janus surfaces, which the paper reports but does not exploit, may make Cr4B3N interesting for field emission or electrocatalysis applications beyond spintronics.
  • The extreme TMR sensitivity to the antiparallel denominator suggests that real devices will need very clean Cr-B edges; atomic disorder or edge reconstruction could destroy the channel mismatch and collapse the ratio.
  • Because the spin channels are momentum-split rather than net-magnetized, the junction's conductance might be switchable by rotating the Néel vector instead of applying a magnetic field, but the paper does not compute spin-transfer torques.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The manuscript uses DFT (VASP) and NEGF (QuantumATK) to predict a two-dimensional altermagnetic MBene, Cr4B3N, obtained by substituting one B atom by N in a FeSe-like CrB bilayer and then stacking bilayers. It reports phonon stability, magnetic exchange couplings via the LKAG formula, and three in-plane vacuum-barrier MTJs with different edge terminations. The central quantitative claim is a TMR ratio of 91,001.6% for the symmetric Cr-B edge junction (conf-3), which the authors attribute to near-blocked antiparallel transmission. The paper also claims a built-in work-function difference larger than 9 eV between the two sides of the Janus layer.

Significance. If the material and the transport values were fully established, the work would be significant: it proposes a design route (N substitution plus bilayer stacking) toward 2D altermagnetic MBenes and an MTJ with an ultrahigh TMR, exceeding many previously reported AFM-based junctions. The manuscript includes standard DFT+NEGF calculations, phonon spectra, and a real-space LKAG exchange analysis, and it makes falsifiable predictions about a new material and its device behavior. However, the headline TMR value is not currently supported because the printed TMR formula cannot produce the reported ratios and the near-zero antiparallel transmission is not demonstrated to be converged. These issues affect the main claim rather than presentation.

major comments (4)
  1. [Section 2.3, TMR definition] The manuscript defines TMR=(T_P-T_AP)/T_P in Section 2.3. With T_AP greater than or equal to zero, this ratio cannot exceed 100%. The reported values of 149%, 2151%, and 91,001% for conf-2, conf-1, and conf-3 are therefore incompatible with the printed definition. If the intended definition is the conventional ratio (T_P-T_AP)/T_AP, this must be stated explicitly and all reported values should be recomputed and interpreted accordingly. As written, the central quantitative claim is internally inconsistent.
  2. [Section 2.3 and Figure 4] The 91,001% TMR for conf-3 depends on an integrated antiparallel transmission T_AP that is described as 'almost blocked' (Section 2.3). No convergence or sensitivity tests are reported for this quantity: the Methods state a 101 x 101 transmission k-grid, a single-zeta-polarized basis, a fixed 7.0 A vacuum barrier, and Ueff=4.0 eV, but no variations of these parameters are given. Since T_AP appears in the denominator of the standard TMR formula, an unresolved numerical floor in T_AP can change the TMR by orders of magnitude (e.g., a floor of 10^-6 of T_P caps TMR near 10^5%, and a floor of 10^-4 caps it near 10^4%). The authors should provide convergence tests for T_AP and T_P with respect to k-grid, basis-set size, and vacuum thickness, and report the numerical noise floor.
  3. [Section 2.1, Abstract, Conclusion] The text repeatedly claims a built-in work-function difference of 'over 9 eV' (Abstract, Conclusion), but Section 2.1 reports work functions of 4.85 and 5.31 eV for the prototype and 2.73 and 4.86 eV for the optimized structure, giving differences of 0.46 eV and 2.13 eV. This is a quantitative contradiction in a stated material property; the claim should be corrected or the calculations should be reconciled.
  4. [Section 2.2] The exchange-coupling paragraph states that 'the exchanges between 1st to 4th neighbors are all in AFM coupling with negative values for the four Jij, with an exception of J1 for the upper layer.' This is self-contradictory and leaves unclear which couplings are negative. Because the robustness of the altermagnetic ground state is supported by these J values, the authors need to present the Jij values explicitly (including which J1 is meant) and correct the wording.
minor comments (5)
  1. [Throughout] The chemical formula is inconsistently written as Cr4B3N, Cr4BN(B2), and Cr4B3NB2; the title, abstract, and main text should use one notation consistently.
  2. [Section 2.3] conf-2 is described as having an 'almost zero TMR ratio of about 149%'; 149% is not almost zero, and the sentence should be reworded to describe the transmission difference rather than the TMR ratio.
  3. [Figure 5] The figure caption lists panels (a) and (b), but the text refers to 'Figure 5c'; the figure and text should be aligned.
  4. [Section 2.3] Since Cr4B3N has zero net magnetization, the meaning of 'parallel' and 'antiparallel' states of the two electrodes should be defined explicitly (e.g., relative orientation of the Neel vector or of the staggered magnetization).
  5. [Throughout] The manuscript contains numerous typographical errors (e.g., 'vaccum', 'electode', 'fucntion', 'disctribition', 'strcuture', 'P4'/n'm'm') that should be corrected in a thorough proofreading pass.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the altermagnetic state and 91,001% TMR are DFT/NEGF outputs, not restatements of fitted inputs; the TMR-formula inconsistency and missing T_AP convergence tests are correctness risks, not circularity.

full rationale

The paper's derivation chain is self-contained rather than circular. The altermagnetic ground state of Cr4B3N is selected by total-energy comparison of four magnetic configurations (AFM1, AFM2, AFM3, FM) computed with DFT, and the AFM ordering is then independently characterized through LKAG exchange parameters (Eq. 2); the J_ij values are outputs of the calculation, not inputs fitted to reproduce the target magnetism. The only externally set electronic-structure parameter, U_eff = 4.0 eV on Cr 3d, is taken from prior Cr2B2 XBene literature (Ref. [62]) and is not tuned to any reported TMR value. The transport claim is obtained from DFT+NEGF transmission calculations for the Cr4B3N/vacuum/Cr4B3N junctions: T_P and T_AP are computed quantities, and the TMR ratio is derived from them rather than imposed. Minor self-citations occur (e.g., Ref. [41] for the parent CrB monolayer and related MBenes), but they are background or structural starting points, not load-bearing uniqueness arguments, and the SI independently recomputes the monolayer Cr2B2 and Cr2BN magnetic orderings. The more serious concerns are non-circular: Section 2.3 defines TMR=(T_P-T_AP)/T_P, which cannot exceed 100%, yet values of 149%, 2151%, and 91001% are reported, implying either a typographical error or the use of the alternative denominator T_AP; additionally, no k-grid, basis, or vacuum-gap convergence tests are provided for the near-zero AP transmission that dominates the denominator. These are numerical-consistency and robustness issues that should be addressed, but they do not make the central result equivalent to its inputs by construction.

Assumptions & free parameters 2 free parameters · 4 assumptions · 1 invented entities

The central claims rest on a standard DFT+U and DFT-NEGF pipeline. The only externally set numeric inputs are Ueff=4.0 eV transferred from Cr2B2 literature and the 7.0 Å vacuum gap; neither is varied or validated. The dominant assumptions are methodological and convergence-related, not circular.

free parameters (2)
  • Effective Hubbard U on Cr 3d (Ueff) = 4.0 eV
    Transferred from Cr2B2 XBene literature (Ref. 62), not fitted here. It can control the magnetic ground state, band spin splitting, and TMR; no Ueff variation is reported.
  • Vacuum barrier thickness = 7.0 Å
    Chosen as the tunneling barrier in all three edge configurations without a thickness sweep or convergence test; TMR is exponentially sensitive to barrier width.
assumptions (4)
  • domain assumption PBE+U with Ueff=4.0 eV on Cr 3d is an adequate description of Cr4B3N electronic structure and magnetism.
    Methods section; the value is adopted from Cr2B2 XBene literature and not tested for this compound, so the AM state and all derived transport are conditional on it.
  • domain assumption The near-zero AP transmission in NEGF is a physical result rather than a numerical artifact.
    Section 2.3 and Fig. 4d-f; the 91,001% TMR comes from a denominator close to the numerical noise floor, with no convergence or noise-floor analysis.
  • domain assumption A 7 Å vacuum gap is a valid and stable tunneling barrier in the in-plane MTJ geometry.
    Section 2.3; no dependence on gap width, lateral size, or electrode termination beyond the three shown configurations is tested.
  • domain assumption Phonon stability of the relaxed AFM1 structure implies the material is experimentally accessible.
    Section 2.1; only phonon dispersions on a 3x3x1 supercell are given, with no formation energy, AIMD, or synthesis evidence.
invented entities (1)
  • Janus altermagnetic MBene Cr4B3N (also written Cr4BN(B2)) independent evidence
    purpose: The predicted 2D electrode material that is the central object of the study and the basis for the 91,001% TMR junction.
    The paper predicts its structure, phonon stability, magnetic ordering, and transport; these are falsifiable by synthesis and measurement, but no external verification exists. The formula is used inconsistently across title, abstract, and conclusion.

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Cite this review

Pith. "Pith review of Altermagnetizing the FeSe-like two-dimensional materials and approaching to giant tunneling magnetoresistance with Janus Cr4BN(B2) MBene electrode." pith.science (2026). https://pith.science/paper/M5K3YWTA

@misc{pith2026250203165,
  author       = {Pith},
  title        = {Pith review of: Altermagnetizing the FeSe-like two-dimensional materials and approaching to giant tunneling magnetoresistance with Janus Cr4BN(B2) MBene electrode},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/M5K3YWTA}},
  note         = {Machine review of arXiv:2502.03165}
}
read the original abstract

Altermagnetism is an emerging series of unconventional magnetic materials characterized by time-reversal symmetry breaking and spin-split bands in the momentum space with zero net magnetization. Metallic altermagnets offer unique advantages for exploring applications in spintronics as conductive metals allows for serving as electrode in magnetic tunneling junction (MTJ) and/or manipulation of spincurrent through external field. Through density functional theory calculations, the 2D altermagnet Cr4B3N was predicted to be stable, resulting from a N atom substitution in the FeSe-like CrB bilayer. Both intra- and inter-layered magnetic exchanges between Cr atoms are in antiferromagnetic with the first three neighbours. Leveraging the anisotropic spin-splittings with momentum dependency revealed in band structure, we designed three edge dependent Cr4B3N/square/Cr4B3N in-plane MTJs with the 7 \AA vaccume the barrie. We found that the Cr-B vertical edge-assembled electrodes based MTJs exhibited giant tunneling magnetoresistance (TMR) ratios of 91001% , by aligning the conduction channels of the electrodes in parallel and anti-parallel states in the momentum space. Our work deepens and generalizes understanding toward altermagnetic 2D metallic electrode with a newly established metal boride (MBene), and broadens applications of the nanoscale spintronics.

Figures

Figures reproduced from arXiv: 2502.03165 by the authors.

Figure 1
Figure 1. (a) Origin of translation symmetry breaking in ML-Cr2B2 and ML-Cr2BN. (b) Optimized altermagnetic BL-Cr4B3N obtained from the prototype constructed by buckled MXene layers, employing a combination of N substitution and bilayer stacking strategy. The green, black, and silver spheres represent B, Cr, and N atoms, respectively. (c-e) Band structure of (c) ML-Cr2B2 with conventional AFM ground state, (d) ML-Cr2BN with F… view at source ↗
Figure 2
Figure 2. (a) Four different magnetic ground states considered for Cr4B3N. The red and blue vectors indicate spin-up and spin-down directions, respectively. Both side view (upper) and top view (lower) are diaplayed. (b) Relative energies of the four magnetic configurations of Cr4B3N. (c) The phonon dispersion curves (PDCs) along the path of G-X-M-G and the asssociated phonon density of states (PHDOS). (d) The rotation symmetr… view at source ↗
Figure 3
Figure 3. (a) The electron localization functions (ELFs) projected into (001) planes. Two diagrams are corresponding to the uppder and lower Cr layers, respectively. (b) ELFs projected into (110) and (100) planes. The distances from the origin of these two planes are is 1 𝑎⃗ and 0.5 [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: (a) Schematic representation of a MTJ device based on bilayer Cr4B3N (b) Side view of atomic models of Cr4B3N/vacuum/Cr4B3N MTJ for three different edges. The edges of Cr￾B and Cr-B-N are marked in blue and orange, respectively. The conf-1 has Cr-B-N edges on both elec…
Figure 5
Figure 5. Figure 5: (a) The atomic model of conf-3 with the Cr-B edge on both electrodes and the corresponding bilayer-resolved density of states with the spin polarization. Each panel represents the DOS of a given bilayer, and panels from left to right correspond to the MTJ model. (b) To…

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