Turning an Antiferromagnet into an Altermagnet: Surface Functionalization Opens the Way to Giant Magnetoresistance ✛ ✦
Context
The discovery of altermagnetism—a third class of collinear magnets combining antiferromagnetic spin compensation with momentum-dependent splitting—has ushered in a new era in magnetism. Unlike ferromagnets, altermagnets produce no stray fields, which is crucial for ultrafast and ultra-dense spintronics. However, the search for two-dimensional (2D) altermagnets, needed for integration into van der Waals heterostructures, is limited. The approach proposed in the article allows obtaining them from known antiferromagnets through controlled chemical functionalization, paving the way for scalable and non-volatile solutions.
Methods
The study was performed using first-principles calculations within density functional theory (VASP code) and non-equilibrium Green's functions for transport (QuantumATK package). To account for correlations in the iron d-shell, a Hubbard correction (U = 0.8 eV) was applied. Full atomic relaxation of the FeSe monolayer with one-sided attachment of hydrogen, oxygen, or fluorine was modeled. Symmetry analysis of magnetic space groups was carried out to identify the conditions for altermagnetism, after which band structures, spin textures, and transmission coefficients were calculated.
Results
Attachment of adatoms to one surface of FeSe lowers the symmetry from P4/nmm to P4mm, breaking inversion and mirror symmetry while preserving the rotational symmetry that connects opposite spin sublattices. This transforms the spin-degenerate antiferromagnet into a d-wave altermagnet with well-defined momentum-dependent splitting. In the band structure of Fe₂Se₂H, pairs of Weyl points are found near −0.5 eV with opposite Chern numbers, and with spin-orbit coupling a topological gap and quantized spin Hall response emerge. In Au/Fe₂Se₂H/GeF₄/Fe₂Se₂H/Au tunnel junctions, parallel orientation of the Néel vectors provides an efficient tunneling channel due to matching spin filters, while the antiparallel configuration blocks transport. This leads to giant tunneling magnetoresistance (TMR) up to 1.87×10³ %, far surpassing known analogs. Numerical experiments with different mutual arrangements of the functionalized layers showed that TMR strongly depends on interface geometry, confirming the key role of chemical design.
Implications
The work demonstrates for the first time that surface functionalization is an effective and accessible method for engineering altermagnetism in 2D materials. This shifts the paradigm from passive search to active design of altermagnets, where the choice of functional group, adsorption site, and orientation relative to the interface acts as a powerful 'tuning knob'. The obtained TMR values outperform those of many ferromagnetic tunnel junctions, making altermagnet devices competitive for applications.
Future development
The proposed approach can be extended to other antiferromagnetic monolayers (MnPSe₃, CrI₃, etc.) and various chemical treatments (chalcogenization, metallization). The discovered coexistence of altermagnetism and topological states opens prospects for dissipationless spin transport and quantum computing. Of particular interest is integration with superconducting materials: FeSe is known as a high-temperature superconductor, and hybrid structures may reveal nontrivial proximity effects.
Impact
The results will impact the development of spintronics (MRAM, spin transistors, filters), quantum technologies (topological qubits), and materials science of 2D functionalized systems.
Next steps
The priority task is experimental verification of the predicted effects—synthesis of functionalized FeSe monolayers and measurement of their band structure and transport properties. A detailed study of the role of defects, temperature, and interface states on altermagnetic splitting is also required.
Key open problems
The research is directly linked to the fundamental problem of the co-existence of altermagnetism and superconductivity in layered materials. The influence of spin fluctuations and possible charge ordering on the stability of the altermagnetic phase remains unclear. The results also highlight the need to develop a theory of spin-dependent transport in systems without net magnetic moment, which could help solve the dissipation problem in spintronics.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The work demonstrates that one-sided functionalization of an FeSe monolayer with hydrogen, oxygen, or fluorine lowers the symmetry and turns an antiferromagnet into an altermagnet. Using first-principles calculations and transport simulations, a giant tunneling magnetoresistance of up to 1.87×10³ % is shown in contacts based on functionalized FeSe. Weyl points and a topological gap are found when spin–orbit coupling is taken into account.
Where its strength is
A practical and scalable method for engineering altermagnetism via chemical surface functionalization is proposed, opening a route to the active design of 2D altermagnets. The obtained giant TMR surpasses many ferromagnetic counterparts, highlighting the potential for spintronic applications.
Where it could go next
- It might be interesting to see how the proposed functionalization approach relates to the general symmetry criteria for identifying altermagnetism developed in a neighboring work.based on: How to Spot Hidden Magnetism by Symmetry arXiv:2605.27151
- A nearby work on magnetic duality in a crystal may offer additional ways to characterize the obtained altermagnetic states.based on: Magnetic Dualism in a Crystal arXiv:2605.24320
- Since Weyl points and a topological gap are obtained, it may be useful to compare these results with a study of hidden magnetism and its topological manifestations.based on: Magnetic charge hides in layers of matter arXiv:2604.23186
Worth trying
- Check whether the altermagnetic splitting persists with other functional groups (e.g., halogens or organic molecules).
- Consider heterostructures with several functionalized layers to control the sign of TMR.
- Evaluate the effect of mechanical strain on the magnitude of the altermagnetic splitting and TMR.
Why this may matter
If the functionalization method proves universal, it could turn a broad class of antiferromagnets into altermagnets, accelerating the development of ultra-dense and energy-efficient spintronic devices.
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Original abstract
as written by the authors arXiv:2607.03908
Two-dimensional (2D) altermagnets (AMs) are highly desirable for ultrafast, stray-field-free spintronics because they combine compensated magnetic order and momentum-dependent spin splitting with the scalability, tunability, and interface compatibility of atomically thin materials. However, practical 2D AMs remain scarce. Rather than relying solely on the search for intrinsic 2D AMs, an appealing route is to transform known 2D antiferromagnets (AFMs) into AMs through symmetry engineering. Here, we propose surface functionalization as a symmetry-guided, nonvolatile chemical switch for realizing this AFM-to-AM transformation. By breaking inversion and out-of-plane mirror symmetries while preserving the rotation symmetry connecting opposite-spin sublattices, single-sided functionalization lifts spin degeneracy and induces altermagnetic spin splitting. Using monolayer FeSe as a representative platform, first-principles calculations show that hydrogenation, oxidation, and fluorination convert spin-degenerate antiferromagnetic FeSe into a d-wave AM with pronounced momentum-dependent spin splitting. At the device level, our transport simulations reveal that the functionalized FeSe monolayer magnetic tunnel junctions exhibit giant tunnel magnetoresistance (TMR) up to $$1.87\times10^3\%$$, originating from momentum-selective spin filtering between parallel and antiparallel N\'eel-vector configurations. The strong dependence of TMR on functionalization geometry further demonstrates that surface chemistry provides an effective control knob for altermagnetic transport. Our work establishes a symmetry-to-chemistry-to-device strategy for engineering 2D AMs and developing high-performance altermagnetic spintronic devices.
Iron selenide FeSe is famous because in single-layer form it exhibits superconductivity at temperatures above 100 K — a record for chalcogenides. And altermagnets, predicted only a few years ago, have so far eluded direct observation in 2D form due to strict symmetry requirements.
Key numbers
- Giant TMR: 1872%
- Hubbard U correction for Fe: 0.8 eV
- Plane-wave cutoff energy: 500 eV
- Vacuum gap in calculations: > 20 Å
- Maximum superconducting transition temperature in FeSe monolayer: > 100 K