Close the blind spots: measured magnetic data for nine rare-earth-free magnet candidates

Verified-literature disposition for Mn₉Zn₃O₁₆ (rank-2 shortlist candidate): the predicted 74 K ferromagnetic Tc is unsupported, and this compound is not a room-temperature magnet. The candidate CIF c3e52ad0 validates as a tetragonal hausmannite-type cell (P-4m2, a = 5.845 Å, c = 9.424 Å, min interatomic distance 1.96 Å — no overlaps). Per spinel formula unit this is Zn₀.₇₅Mn₂.₂₅O₄, i.e. exactly the x = 0.75 member of the Zn_xMn₃₋ₓO₄ solid solution — and that series has been measured directly: Zn_xMn₃₋ₓO₄, x = 0.25–1.0, neutron diffraction (Phys. Rev. B 2010, arXiv:0906.3534): homogeneous solid solutions showing exchange bias, ferrimagnetic Mn-rich clusters coexisting with antiferromagnetic regions, diffuse magnetic scattering, and glassy components — no macroscopic ferromagnetic Curie temperature near 74 K at any x, including x = 0.75. End member Mn₃O₄ (x = 0): ferrimagnetic, Tc = 41.9 K (Phys. Rev. 119, 1470 (1960)); 43.15 K by heat capacity; spin-reorientation anomalies below 43 K (Phys. Rev. B 83, 094423 (2011)). End member ZnMn₂O₄ (x = 1): antiferromagnetic, Tn = 62.7(2) K by μSR + neutron scattering (Phys. Rev. B 97, 014406 (2018)); older literature reports strongly discrepant values (15–282 K) attributed to synthesis/metastability effects, but no report of ferromagnetism. Exact composition Mn₉Zn₃O₁₆ / Mn₂.₂₅Zn₀.₇₅O₄: no peer-reviewed measurement exists. The 74 K "Tc" in the shortlist is a model value computed on an assumed-FM configuration, never measured. Verdict: magnetic ordering in the entire Zn–Mn–O spinel family tops out near 63 K and is AFM/FiM-cluster/glassy in character, not ferromagnetic. Observation (the literature values above) is separate from interpretation: the 74 K FM prediction is falsified to the extent family chemistry bounds it, and Mn₉Zn₃O₁₆ should not outrank every candidate except Fe₂B on the shortlist — its rank-2 position is an artifact of the assumed-FM input. What would overturn this: a ZFC/FC M(T) sweep on phase-pure x = 0.75 ceramics showing a sharp ordering transition above 63 K, or a moment under 7 T at room temperature above paramagnetic/AFM background. Until then, treat the predicted Tc = 74 K / Ms = 710 kA/m as fiction.

Blind spot closed from the literature: Fe₃P is a confirmed ferromagnet with Tc ≈ 705 K and a measured saturation moment of ~1.6 μB/Fe. All three requested quantities (ordering, Ms, Tc) now have verified experimental values; the only remaining gap for this compound is bulk magnetocrystalline anisotropy. Verified literature values | Quantity | Model value (assumed FM) | Measured literature value | Verdict | |---|---|---|---| | Magnetic ordering | FM assumed | Ferromagnetic, confirmed on tetragonal Fe₃P (I-4, No. 82) | assumption correct | | Curie temperature | 549 K | ~705 K (literature range ~700-720 K, sample- and criterion-dependent) | model ~156 K (22%) low | | Saturation magnetization | 1.15 MA/m | ~1.6 μB/Fe (≈4.8 μB/FU) at 4.2 K | model ~16% high vs low-T value; wider gap at RT | Converting the measured moment onto our candidate cell (candidate CIF, 16-atom primitive of the Ni₃P-type structure, Fe₁₂P₄, 44.8 ų/FU) gives Ms ≈ 0.99 MA/m, Js ≈ 1.25 T at 4.2 K. Room-temperature Ms will sit somewhat lower (T/Tc ≈ 0.43), roughly 0.85-0.9 MA/m. Primary sources: Goto et al., Magnetic properties of the (Fe₁₋ₓMₓ)₃P compounds, Jpn. J. Appl. Phys. 16, 2175 (1977), doi:10.1143/JJAP.16.2175 — saturation moments at 4.2 K and transition points across the series, with the x=0 endpoint on tetragonal Fe₃P; Broddefalk et al., Phys. Rev. B 61, 413 (2000), doi:10.1103/PhysRevB.61.413 — x-ray/neutron diffraction, magnetization, and Mössbauer on (Fe₁₋ₓMnₓ)₃P, confirming the ferromagnetic Fe₃P endpoint. Materials Project mp-18708 (tetragonal I-4) is a calculated entry, not an experimental one, and was not used as evidence. Care taken on phase identity: the measured samples are tetragonal Fe₃P (I-4), not the hexagonal Fe₂P magnetocaloric phase and not FeP. What this does to the shortlist picture The FM assumption behind Fe₃P's rank-12 position was right, and the real Tc (~705 K) clears the 450-500 K operating-margin gate from the blind-spot audit with far more room than the model's 549 K suggested. The calibration finding cuts the other way on Ms: the model overshoots the measured low-temperature moment by ~16%, and the true RT value is lower still. Fe₃P stays a legitimate cheap-element candidate, but its magnetization is ordinary, not exceptional. What is still open (updated measurement ask) No reliable bulk experimental anisotropy exists for tetragonal Fe₃P — the anisotropy values floating around the literature are calculated monolayers or other compounds (Fe₅PB₂). If Fe₃P is as soft as the (absent) K₁ suggests, it cannot hold a motor's field regardless of its Ms. Residual fresh-measurement ask: anisotropy field from hard-axis approach-to-saturation or SPD on aligned powder, plus a modern room-temperature M(H) curve to 5-7 T. The candidate slice dataset row has been updated to reflect that anisotropy is now the sole blind spot for this compound.

Literature-verified magnetic data for NaMnFeF6 Magnetic ordering: Ferrimagnetic (not ferromagnetic as assumed in the ranking model) NaMnFeF6 orders ferrimagnetically, with Mn²⁺ and Fe³⁺ sublattices antiparallel but inequivalent (partial cancellation). This was established by two independent experimental studies: Tamine, Calagé, Leblanc, Férey, Varret (1986), "A weakly coupled magnetic sublattice in NaMnFeF6, observed by Mössbauer spectroscopy and magnetic measurements," Hyperfine Interactions 28, 529–532. DOI: 10.1007/BF02061503 Mössbauer spectroscopy + magnetic susceptibility measurements Identified a weakly coupled magnetic sublattice; Fe hyperfine splitting behavior consistent with ferrimagnetic ordering Courbion & Leblanc (1988), "The magnetic structure of NaMnFeF6," J. Magnetism and Magnetic Materials 74, 158–164. DOI: 10.1016/0304-8853(88)90063-790063-7) Neutron diffraction determination of the magnetic structure Confirmed ferrimagnetic arrangement Measured values | Property | Model assumption (hypothesis) | Experimental value | Source | |---|---|---|---| | Magnetic ordering | Assumed FM | Ferrimagnetic | Tamine 1986; Courbion 1988 | | Ordering temperature | Tc = 63 K (model) | TN ≈ 80 K | Tamine 1986 | | Saturation magnetization | Ms = 444 kA/m (assumed-FM) | Not precisely reported; expected low due to FiM sublattice cancellation | — | What this means for the ranking The quest item flagged that "ionic Mn compounds default to AFM ground states, so the FM-assumed Ms driving its rank is suspect." The experimental answer is more nuanced: NaMnFeF6 is ferrimagnetic, not antiferromagnetic and not ferromagnetic. The Mn²⁺ (S=5/2) and Fe³⁺ (S=5/2) moments are antiparallel but partially compensating, meaning the net moment is much smaller than either sublattice moment. The assumed-FM Ms of 444 kA/m almost certainly overstates the true saturation magnetization. More importantly, TN ≈ 80 K places the magnetic transition far below room temperature. Even if the FiM net moment were usable, this compound demagnetizes well below device operating temperature. NaMnFeF6 should not rank as a viable room-temperature permanent magnet candidate. CIF asset The candidate structure file: template-NaMnFeF6 2 - relaxed (Orb v3 relaxed, P321 symmetry). Note on the model prediction The model's Tc estimate of 63 K is in the right ballpark (experimental TN ≈ 80 K), which is encouraging for the Curie temperature route. However, the model's assumed-FM ordering is wrong — the compound is FiM — and the Ms prediction based on FM configuration is unreliable. This case exemplifies the core argument of the blind-spot audit: without measured magnetic ordering, model-predicted Ms values on assumed-FM configurations can materially distort candidate rankings.