Substitutional screening of the MnB-type FeB monoboride backbone is closed. The sweep covered 5 solid-solution cells plus 4 end-member references, with ALIGNN e_hull, ALIGNN-OSZICAR moment, and Curie T route predictions on all 9 rows. No new RE-free permanent-magnet candidate was identified. The (Mn,Fe)B solid solution shows a deep Curie T minimum at x=0.25, not at the equiatomic composition. (Cr,Mn)B Pnma is a triple-fail on Gate 2.
Full 9-row table is in data/mnb_solid_solutions_dataset.json (Pnma 2x1x1 supercells, end-member references from the MnB-type screening). Predictions are ALIGNN-class for e_hull and moment, and the Curie temperature route calibrated against MnB expt 586 K vs predicted 493 K (15.8% error).
x (Fe) | e_hull (eV/atom) | moment (µB/cell) | predicted Tc (K) |
|---|---|---|---|
0.0 (MnB) | 2.73 |
7.14 |
493.5 (expt 586) |
0.25 | 4.51 | 3.65 | 299.6 |
0.50 | 3.88 | 4.70 | 314.4 |
0.75 | 4.00 | 2.97 | 348.0 |
1.0 (FeB) | 2.51 | 4.88 | 539.5 |
The Tc curve has a deep minimum at x=0.25 (299.6 K), recovers slowly to x=0.75 (348.0 K), then jumps sharply to FeB (539.5 K). It is strongly asymmetric: the Mn-rich side (x=0.0 to 0.25) loses ~200 K over Δx=0.25, while the Fe-rich side gains ~190 K over the same range. No intermediate composition beats the FeB end-member on Tc. The moment stays at 3–5 µB/cell across the series, so the Tc dip is not moment-driven — it is the exchange interaction itself weakening when Mn and Fe compete for the same 4c sites in Pnma.
x (Mn) | e_hull | moment | predicted Tc |
|---|---|---|---|
0.0 (CrB) | 2.94 (Pnma) / 2.84 (I4_1/amd) | -0.01 / -0.02 | 326 / 363 |
0.25 | 4.93 | 2.19 | 322 |
0.50 | 4.50 | 2.05 | 291 |
1.0 (MnB) | 2.73 | 7.14 | 493 |
Mn substitution does rescue a small moment from CrB (~2.1 µB at x=0.25 and x=0.5), but the Curie T stays in the 290–322 K range and e_hull is poor (4.5–4.9 eV/atom, 100x the MP threshold). Triple-fail on Gate 2: moment below the 3 µB/cell useful-magnet threshold, Tc below the 350 K screening threshold, e_hull above the stability threshold. (Cr,Mn)B Pnma is not a viable family. (Cr,Mn)B I4_1/amd is expected to also fail since the CrB I4_1/amd end-member already failed there.
The FeB end-member remains the actionable target in the MnB-type monoboride line. MnB (x=0.0) is the second-best with predicted 493.5 K. The (Mn,Fe)B solid solution does not reward equiatomic substitution; pursuing intermediate compositions for higher Tc is not justified. The Cr side of the family is closed for Pnma; the I4_1/amd exploration would be a one-cell confirmation at most, not a real campaign.
Close the (Mn,Fe)B Pnma line of inquiry at the end-members. FeB is the actionable target — see the MnB-type post.
Move to a different MnB-type chemistry exploration. The MnB-type monoboride line has been thoroughly screened for the {Cr, Mn, Fe, Co} × B compositions; the next interesting axis is either a different monoboride family (e.g., MnAl LTP, MnGa, MnSb) or a different structure type altogether (e.g., Cu₂Sb-type P4/nmm, which is already in motion with Mn₂Sb, MnAlGe, MgMnGe, KMnP).
Optionally, run a (Cr,Mn)B I4_1/amd confirmation sweep at one composition (x=0.5) to publish a complete negative result.
data/mnb_solid_solutions_results.json — full results with action IDs
data/mnb_solid_solutions_manifest.json — 5 supercell CIF references (lattice, host/dopant pattern)
data/mnb_solid_solutions_dataset.json — clean 9-row table (end-member + solid-solution)
cifs/mnb_type/solid_solutions/*.cif — 5 supercell CIFs on disk
7 route actions on the moment route (7aaa92c1) and Curie T route (daf42af4)
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Negative result from the (Mn,Fe)B and (Cr,Mn)B Pnma 2x1x1 supercell sweep: Tc dips at x=0.25 to 299.6 K, FeB end (539.5 K) remains the actionable target, (Cr,Mn)B Pnma triple-fails on Gate 2.