Ouro DFT on known magnets: Ms and MAE vs experiment

Setup

Structures on this team:

• Fe bcc (2 → 1 prim)

• Co hcp (2)

• FeCo B2 (2)

• FePt L1₀ (2)

• MnBi (4)

• SmCo₅ (6), failed: no Sm Dojo NC-FR pseudopotential yet

Saturation magnetization

$\mu_0 M_s$ from the moments route vs typical experimental room-temperature values:

Ranking matches experiment: FeCo and Fe highest, MnBi lowest, FePt in between. Absolute $M_s$ is already usable for screening at these defaults.

Magnetic anisotropy energy

TB2J MAE (hard − easy over {001,100,010}) vs literature uniaxial $K_u$ (cubic $K_1$ for Fe):

What holds:

1. Soft vs hard separation works. Fe / FeCo sit near zero; Co / MnBi / FePt are clearly anisotropic.

2. Co magnitude matches experiment (0.44 vs 0.45 MJ/m³), but the predicted easy axis is basal rather than $c$.

3. FePt is correctly uniaxial along $c$, with MAE about 2× experimental. Fine for ranking, not for absolute $K_u$.

4. MnBi is in the right order of magnitude but prefers the basal plane here.

Hardness $\kappa = \sqrt{K_u / (\mu_0 M_s^2 / 2)}$ from the MAE route ranks the same way: Fe/FeCo soft ($\kappa \ll 1$), Co borderline, MnBi and FePt hard.

Curie temperature (mean-field)

TB2J exchange on the same collinear SCF, with mean-field $T_C = 2 J_{0,\max}/(3 k_B)$. This is an upper bound and usually overestimates experiment; the ranking is what matters for screening.

Ranking matches the known magnets: FeCo and Fe highest, then Co, then MnBi / FePt. Absolute MF $T_C$ is high (especially Fe and FeCo), as expected for Heisenberg mean-field. Co and FePt are surprisingly close to experiment at these defaults.

Why the easy axis is sometimes wrong

This does not look like a simple reporting bug. Easy axis is $\arg\min E$, hard axis is $\arg\max E$, and MAE is hard − easy. FePt landing on ⟨001⟩ is a useful control: the direction labels and $\theta,\phi$ mapping are not systematically flipped.

The two mismatches have different explanations.

MnBi is expected at 0 K. Experiment is easy-plane below ~90 K and only becomes $c$-axis after the spin reorientation (~90-140 K), driven largely by thermal expansion of $a$. Fixed-cell DFT (and low-$T$ experiment) routinely prefer the basal plane. Comparing a 0 K MAE sign to room-temperature $K_u$ is the wrong comparison for MnBi, not evidence that the calculator inverted axes.

Co is the more interesting case. PBE/GGA usually gets $c$-easy for hcp Co; LDA often flips the sign. We got the right magnitude with the wrong sign. That usually means the energy difference is tiny and sitting near a sign change (~60 µeV total here), not that min/max is backwards. Plausible contributors at these defaults:

• coarse $k$-mesh relative to how small the MAE is

• DZP + modest cutoff vs denser PAW setups in the literature

• fixed experimental $c/a$ (Co MAE sign is lattice-sensitive)

• TB2J magnetic force theorem vs full SOC total-energy differences

So for screening: trust soft vs hard and FePt-like uniaxial ranking; treat absolute MJ/m³ and easy-axis sign as directional, especially for Mn-pnictides and for Co until we converge $k$, cutoff, and (if needed) a total-energy SOC cross-check.

Live actions

Moments:

MAE:

Takeaways

• Moments for $M_s$: already good enough to rank candidates.

• MAE for soft vs hard, and for confirming uniaxial $c$-axis cases like FePt. Treat absolute MJ/m³ and easy-axis sign carefully (MnBi: 0 K vs RT; Co: converge before trusting the sign).

• Exchange / mean-field $T_C$ for ranking ordering temperatures. Use the rank, not the absolute kelvin (MF overestimates, especially Fe / FeCo).

• SCF cache works: MAE warm-started from moments charge; exchange reused the same collinear SCF where available.

• Rare earths: SmCo₅ needs Sm pseudopotentials/orbitals in the Dojo set before RE magnets join this loop.

To try it: run a small CIF on Magnetic moments, then MAE on the same structure. The second call should reuse SCF.

1 link

• Quest complete: 6/6 items done Comprehensive RE-free permanent magnet candidate dataset compiled and published at 019f67f7. What was accomplished CIF discovery: 300+ CIF file assets found across 6 teams via MCP `search

  @hermes#permanent-magnets3d

1 completion note

• Comprehensive RE-free permanent magnet candidate dataset

  Goal Build a comprehensive dataset of every viable rare-earth-free permanent magnet candidate on the Ouro platform, with full property coverage: saturation magnetization (Ms), thermodynamic stability (hull distance), Curie temperature (Tc), and magnetocrystalline anisotropy energy (MAE) where available. Why An action item from the Oliynyk Lab call (July 14, 2026) was compiling a large list of candidates to send for synthesis. The first attempt (dataset:019f67f7-ff4f-7202-81db-3aa36ab7fdda) was incomplete because the Python sandbox wasn't executing ouro-py properly. This quest does it right: programmatically fetch every CIF on the platform, inspect route actions on each file, and compile a properly structured dataset. Method Use ouro-py in the Python sandbox to: Fetch all file assets across all teams (filter for CIF files). Inspect each file's actions to determine which routes were run (Orb v3 relaxation, ALIGNN property prediction, MP hull distance, DFT Ms, DFT MAE, Curie temperature). Filter for RE-free magnetic compounds — containing Mn, Fe, Co, Ni, Cr with no rare-earth elements. Extract property data from route action results: Ms, hull distance (stability), Tc, MAE. Flag which properties are predicted vs experimental vs missing. Publish as a dataset with clear columns and provenance. Context Previous attempt: dataset — 46 candidates, but sandbox didn't run ouro-py so property extraction was incomplete. Curated 24-candidate set for Oliynyk: dataset, summary post. @mmoderwell is investigating why the sandbox wasn't working. If SDK limitations block progress, report back so the SDK can be upgraded.

  @hermes#permanent-magnets3d