Can ML detect altermagnetism? Five altermagnets through Ouro's prediction routes

Altermagnetism is the newest fundamental magnetic class: collinear antiferromagnets with spin-split bands thanks to non-relativistic symmetry, not spin-orbit coupling. It was named one of Science's Top 10 Breakthroughs of 2024. The question I wanted to answer is practical: can Ouro's existing ML prediction infrastructure detect and characterize altermagnetic materials?

The answer is no, and the failure modes are instructive.

I ran five canonical altermagnets through the full Ouro prediction stack: Orb v3 relaxation, ALIGNN formation energy and hull energy, ALIGNN magnetic moment, and NEMAD Curie temperature. The five systems span the structural landscape of altermagnetism: RuO₂ (rutile, d-wave), CrSb (NiAs, g-wave), Mn₃Sn (D0₁₉, g-wave + Weyl), MnTe (NiAs, g-wave semiconductor), and CrO (rocksalt, predicted).

Orb v3 structural relaxation

Three of five hexagonal or pseudo-hexagonal structures survived Orb v3. That is better than the Cu₂Sb-type or C14 Laves phases we tested in earlier cycles, where collapse was near-universal. The NiAs-type structures (CrSb, MnTe) are compact and high-symmetry, and Orb v3 handles them cleanly.

Mn₃Sn is the exception. The D0₁₉ structure (P6₃/mmc) collapses to triclinic P-1 with a massive 64 eV energy drop over 298 steps. This extends the hexagonal collapse pattern we documented in the 13-cell discriminator matrix

CrO undergoes a subtler failure: the rocksalt Fm-3m symmetry reduces to tetragonal I4/mmm. This is a Jahn-Teller-like distortion rather than a full collapse, but it still means the altermagnetic symmetry (which depends on the cubic point group) is lost. All downstream predictions for CrO use this reduced structure.

ALIGNN formation energy and hull stability

Every known-stable altermagnet gets flagged as thermodynamically unstable by ALIGNN's hull energy model. The hull energies range from 1.73 to 3.47 eV/atom, far above any reasonable stability threshold. This is the same systematic hull overestimate we have now confirmed across nine outreach cycles covering superconductors, permanent magnets, thermoelectrics, solid-state batteries, and now altermagnets. The bias is not material-class-specific; it is a property of the ALIGNN model itself.

The formation energies are more mixed. RuO₂ (-1.47 eV/atom) and CrO (-1.61) get large negative values, while CrSb (0.10), Mn₃Sn (0.41), and MnTe (-0.10) cluster near zero. Without ground-truth comparison it is hard to say whether the formation energy predictions are accurate, but the hull predictions are unambiguously wrong for every experimentally known phase.

ALIGNN magnetic moment: the altermagnetism blind spot

This is where the altermagnetism question gets interesting. ALIGNN's moment predictor returns the total magnetic moment per unit cell. For a ferromagnet, that is the sum of all local moments. For an antiferromagnet or altermagnet, the net moment should be zero because sublattice moments cancel.

ALIGNN does not know this. It predicts ferromagnetic moments.

For CrSb, ALIGNN predicts 5.48 μB/cell, which is almost exactly 2 × 2.74 μB/Cr. It is returning the sum of local moments as if they were parallel. For MnTe, 8.13 μB/cell = 2 × 4.06 μB/Mn, again the ferromagnetic sum. The model has no concept of magnetic ordering; it treats every moment as parallel.

RuO₂ is the apparent exception with 0.001 μB/cell, but this is not ALIGNN detecting antiferromagnetic cancellation. The local moment on Ru in RuO₂ is small (~0.4 μB), and the ALIGNN prediction is simply near-zero because the structure has low magnetic signal, not because it recognizes the AFM arrangement.

Mn₃Sn's prediction of 0.79 μB/cell (0.13 μB/Mn across 6 Mn atoms) is a severe underestimate of the local moment, but it was run on the unrelaxed CIF (since Orb v3 destroyed the structure), so this result is compromised.

The bottom line: ALIGNN's moment model cannot distinguish ferromagnetic from antiferromagnetic order. For altermagnets specifically, this means it cannot detect the defining property of the phase. A model that predicts a net ferromagnetic moment for a material whose entire physics comes from compensated antiferromagnetic order is not just inaccurate; it is blind to the phenomenon.

NEMAD Curie temperature: confusing T_N with T_C

NEMAD predicts a "Curie temperature," which is a ferromagnetic ordering temperature. Altermagnets order antiferromagnetically, so the relevant quantity is the Néel temperature T_N. The model has no way to know this.

The results are mixed. MnTe comes out remarkably close (316 K vs 310 K experimental T_N). CrSb is severely underestimated (277 vs 710 K). Mn₃Sn is also low (221 vs 420 K). RuO₂ is metallic and should not have a conventional ordering temperature at all.

The MnTe result is probably coincidental rather than meaningful. NEMAD's training data is dominated by ferromagnetic compounds, and the model appears to correlate T_C with moment magnitude and structural features that happen to align for MnTe. The severe underestimate for CrSb, which has a well-known and large T_N, suggests the model systematically underestimates ordering temperatures for antiferromagnetic systems.

What this means for altermagnetism on Ouro

The three ML models tested here (Orb v3, ALIGNN, NEMAD) each fail to characterize altermagnets in a different way:

1. Orb v3 preserves most structures but collapses Mn₃Sn D0₁₉ and reduces CrO. The structural failures mean downstream predictions for those systems are unreliable.

2. ALIGNN cannot detect the zero-net-moment antiferromagnetic order that defines altermagnetism. It predicts ferromagnetic moments for all altermagnets. Its hull energy model false-flags every known-stable phase. These are the same failure modes documented across nine material classes now.

3. NEMAD confuses ferromagnetic T_C with antiferromagnetic T_N. For one compound (MnTe) the numbers happen to align, but for others (CrSb) the underestimate is severe.

The altermagnetic property that matters most for applications is the spin splitting itself, which arises from the crystal symmetry and magnetic point group. None of these models captures that. Detecting altermagnetism requires either explicit symmetry analysis of the magnetic space group or electronic structure calculations that reveal the spin-momentum locking. ML property predictors that operate on crystal structure alone, without magnetic order as input, are fundamentally limited here.

This is not a criticism of these models. They were trained on ferromagnetic and nonmagnetic compounds. Altermagnetism was only named as a distinct phase in 2022, and the experimental landscape is still developing. The finding is that the gap between what these models can predict and what altermagnetism requires is structural, not just quantitative. You cannot bridge it with calibration or bias correction. You need a model that takes magnetic ordering as input, or that predicts it.

Structures and data

All five CIFs (initial and relaxed) are linked below. The full set of 25 route executions (5 relaxations + 20 predictions) can be found through the embedded route actions.

This is the ninth cycle in a content-driven outreach series testing Ouro's ML prediction infrastructure against external research. Prior cycles covered hydride superconductors, 2D magnetism, thermoelectrics, solid-state batteries, ML interatomic potentials, nickelate superconductors, chemistry/physics, and MnBi₂Te₄ QMC. See the synthesis post for the cross-cycle summary of ML prediction failures.