Mn5Ga (I4/mmm) as a Rare-Earth-Free Permanent Magnet: A Property Assessment

The estimated µ₀Mₛ from the magnetisation density is ~5.5 T — an eye-catching number if taken at face value. For context, Nd₂Fe₁₄B sits at ~1.6 T and elemental α-Fe at ~2.15 T. That figure warrants skepticism; it likely reflects DFT spin states calculated without full self-consistency or spin-orbit effects, and may not represent a physically realizable ordered moment at any accessible temperature.

Curie Temperature (via Predict the Curie temperature): 267.7 K (−5.4°C)

This is the result that stops the analysis cold. A Curie temperature below room temperature (~293 K) means the material is not ferromagnetically ordered under ambient conditions. Without ferromagnetic order, there is no permanent magnet. Full stop. This alone disqualifies Mn5Ga in this structural configuration from practical permanent magnet applications, regardless of what the saturation moment looks like on paper.

Formation Energy (via Predict formation energy per atom): +0.248 eV/atom

Positive formation energy indicates thermodynamic metastability — this phase sits above the convex hull. That doesn't mean it can't be synthesized (metastable phases are synthesized routinely under non-equilibrium conditions), but it does mean it will tend to decompose toward more stable Mn-Ga configurations given enough thermal energy. Combined with the low Tc, this raises real questions about whether a meaningful synthesis target exists here.

ALIGNN Magnetic Moment (via Predict total magnetic moment per cell): 0.540 µB/cell

This conflicts sharply with the 33.6 µB/cell from the saturation route. The ALIGNN model is trained on DFT-relaxed structures from the JARVIS database and predicts a nearly quenched moment, which is actually more consistent with what's known about Mn-Ga compounds — Mn moments in these systems are strongly geometry-dependent and can cancel antiferromagnetically depending on the local coordination. The discrepancy between the two routes likely reflects the difference between an idealized ferromagnetic DFT spin state (saturation route) and a more realistic model prediction that accounts for competing exchange interactions.

Magnetic Anisotropy Energy: The DFT MAE route (mmoderwell's ABACUS-based route) failed with an SCF convergence error on this structure, so I don't have a K₁ value. This is unfortunate — MAE is actually the most interesting property for the Mn-Ga family, since Mn₃Ga is known to have very high uniaxial anisotropy (K₁ ~ 2 MJ/m³) despite low saturation. That high coercivity potential is the reason this family gets taken seriously at all as a REE-free candidate.

Verdict

Mn5Ga in the I4/mmm structure as modeled here does not look like a viable permanent magnet at room temperature. The Curie temperature prediction of 268 K is the decisive issue. Unless the Tc model is significantly off (possible — Tc is notoriously hard to predict for complex intermetallics), this material would need to be used below −5°C to exhibit ferromagnetic order.

The more promising direction within the Mn-Ga family remains Mn₃Ga, which has a documented Tc around 770 K and high magnetocrystalline anisotropy. The tradeoff there is a low saturation moment (~0.09 T), which limits the energy product (BH)_max. The Mn-Ga design space is essentially a search for compositions and structures that can push Tc upward without destroying the anisotropy.

One thing worth noting: the file is labeled "Mn5Ga" but the description says the expected composition was Mn3Ga and this was a missed prediction. That composition ambiguity is meaningful — the phase diagram in this system is complex (Mn₂Ga, Mn₃Ga, Mn₅Ga₃, etc. are all distinct), and the magnetic properties vary substantially between them. It's possible this structure is physically unrealistic as Mn5Ga and that's partly why the Tc comes out so low.

Worth running the actual Mn3Ga (I4/mmm) against these same routes to see how the numbers compare, and trying the MAE route on a structure where SCF converges — that's where the interesting anisotropy story would live.

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• Done — here's the full assessment: Mn5Ga (I4/mmm) as a Rare-Earth-Free Permanent Magnet Short version: the Curie temperature came out at 268 K (just below room temperature), which is a hard disqualifi

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  @hermes

  #permanent-magnets2h