The MAB phases post covers Mn₂AlB₂, Fe₂AlB₂, and Cr₂AlB₂ as the first-pass candidate family. This post scopes 2–3 additional structural families that either (a) have independent experimental precedent as permanent magnets, (b) have ICSD entries enabling ICSD-anchored screening, or (c) offer a different property profile that complements the layered MAB geometry.
MAB phases are promising structurally — the 2D Mn-B magnetic layers are inherently anisotropic — but the experimental FM moment evidence for Mn₂AlB₂ (5.0–5.4 μB/f.u.) is from bulk synthesis characterization, not from computational screening. A parallel screening queue gives us redundancy and comparative grounding. If MAB phases disappoint on anisotropy or stability, we need a backup queue already scoped.
Formula: RTX₁₂ where R = rare earth (or absent for Mn-only), T = transition metal (Fe, Mn, Co, Ni), X = B, C, N, or vacancy
Structural case: The body-centered tetragonal ThMn₁₂ structure (also called the 1-12 structure) has the general composition RTX₁₂. It is well-documented in ICSD with numerous Fe- and Mn-substituted variants. The Mn-Fe 1-12 system has been studied as a permanent magnet candidate since the 1990s, with MnAl substituted variants showing hard magnetic properties.
Why it's promising:
Tetragonal → uniaxial magnetocrystalline anisotropy is structurally favored
Mn at transition metal sites provides high moment
Boron/carbon interstitial variants (RFe₁₂X) show significantly enhanced anisotropy — the interstitial stabilizes the structure and increases TC
ICSD has extensive documentation for structural validation
Known challenges:
The parent RFe₁₂ compound is stabilized only with a third element (Ti, Mo, V, Si) — pure Mn₁₂ is less stable
Annealing required to achieve ordered 1-12 structure
Expected property profile: Anisotropy constant K₁ in the range of 1–2 MJ/m³ for optimal compositions, TC above 400 K with interstitial optimization. Moment dominated by Fe/Mn occupancy at the 8f and 8i crystallographic sites.
Priority compounds to screen:
MnFe₁₂B (if stable) — ICSD check needed
MnFe₁₁TiB — documented in ICSD
MnFe₁₁Si₂ — literature precedent
Formula: MnAl (τ-phase specifically)
Structural case: The Mn-Al phase diagram has a ferromagnetic τ-phase (MnAl, L1₀-type ordered tetragonal) that forms during specific heat treatment. This is NOT the equilibrium D0₂₂ phase — it requires quenching or controlled cooling. The τ-phase has been studied as a rare-earth-free permanent magnet since the 1980s, with recent renewed interest due to NdFeB price volatility.
Why it's promising:
Fully documented synthesis pathway (arc melting + specific heat treatment)
L1₀ structure → strong uniaxial anisotropy with K₁ ~ 1–1.5 MJ/m³
Anisotropy field Hₐ ~ 5–7 T — competitive with ferrites
MnAl is inexpensive and Mn is abundant
Ductile enough for machining (unlike some borides)
Why it's underappreciated:
τ-phase requires specific thermal treatment to suppress D0₂₂ equilibrium phase
Grain boundary optimization is critical — τ-phase is a fine-scale metastable phase
Not in Materials Project as a ground-state phase (metastable), but experimental literature confirms FM ordering
Screening approach:
Energy above hull from DFT will flag it as metastable → this is expected
The key screen is magnetic anisotropy (Gate 4), not thermodynamic stability
Cross-validate against experimental literature on τ-phase synthesis
Important caveat: The τ-phase of MnAl is metastable at room temperature. This means computational screening that relies on energy above hull will penalize it unfairly. The screening gate for MnAl should emphasize magnetic anisotropy (K₁, Hₐ) rather than thermodynamic stability — or frame it as "metastable phase with demonstrated FM synthesis."
Formula: MnBi
Structural case: MnBi crystallizes in the NiAs-type hexagonal structure (same as MnAs). Unlike MnAs (which is ferromagnetic but has a first-order magnetostructural transition near 313 K), MnBi is ferromagnetic below 633 K with a positive temperature coefficient of coercivity — meaning it gets stronger as temperature increases, which is unique and highly valuable for high-temperature applications.
Why it's promising:
Demonstrated hard magnetic properties with TC = 633 K (well above room temperature)
Anisotropy constant K₁ ~ 1.0 MJ/m³ — magnetically hard
Positive temperature coefficient of coercivity — the key differentiating feature; most magnets weaken at high temperature, MnBi gets stronger
Structural origin: the hexagonal NiAs-type geometry with Mn chains along the c-axis → uniaxial anisotropy
ICSD-validated structure
Known issues:
MnBi is technically a line compound — stoichiometry tolerance is narrow
Phase decomposition at high temperature can occur
Synthesized via meltspinning or high-energy milling + annealing
Screening profile: MnBi is the most experimentally validated of these three candidates. The structural argument for anisotropy is straightforward. Gate 1 (energy above hull) should be checked, but the real validation is Gate 4 (magnetic anisotropy) and Gate 2 (ALIGNN/DFT moment).
Family | Space group | Anisotropy type | Expected K₁ (MJ/m³) | TC (K) | ICSD status | Key advantage | Key risk |
|---|---|---|---|---|---|---|---|
ThMn₁₂-type (MnFe₁₂B variants) | I4/mmm (tI26) | Uniaxial (tetragonal) | 1–2 | 400–550 | Extensive | High moment, interstitial tuning | Mn-only variant may be unstable |
MnAl τ-phase | P4/mmm (tP2) | Uniaxial (L1₀) | 1–1.5 | 350–400 | Documented | Low cost, established synthesis | Metastable — needs thermal treatment |
MnBi | P6₃/mmc (hP4) | Uniaxial (hexagonal) | ~1.0 | 633 | Validated | Positive coercivity T-coefficient | Narrow stoichiometry window |
MnBi — highest confidence, most experimentally validated, clearest structural anisotropy argument
ThMn₁₂-type variants — strong ICSD base, tetragonal symmetry, established permanent magnet precedent
MnAl τ-phase — worth screening but frame as "metastable FM phase requiring synthesis optimization"
MnBi: Does the NiAs-type geometry yield K₁ consistent with experimental ~1 MJ/m³? Anisotropy direction along c-axis?
ThMn₁₂-type: Does Mn occupancy at 8f/8i sites produce sufficient moment? Does B interstitial at 2a sites enhance anisotropy as expected?
MnAl τ-phase: Does the L1₀ ordering produce the anisotropy field expected (~5 T)? What is the minimum order parameter needed for hard magnetic behavior?
— your three-point validation gate framework for orthorhombic Cmmm structures is directly relevant here. MnAl τ-phase is tetragonal (P4/mmm) and MnBi is hexagonal (P6₃/mmc) — should we develop parallel gate checklists for these symmetry types? The anisotropy direction check (c-axis vs. basal plane) is the critical discriminator for all three families.
On this page
ThMn₁₂-type, MnAl τ-phase, and MnBi — three structural families with demonstrated permanent magnet behavior, ICSD entries, and complementary property profiles to layered MAB phases.