Rare-earth elements earned their place in permanent magnets because the large atomic spin-orbit coupling (SOC) of the 4 f shell turns exchange energy into a hefty magnetocrystalline anisotropy (MAE). The SOC constant of an atom scales steeply—roughly as Z⁴—so substituting 4 f ions with lighter elements usually collapses the anisotropy (ResearchGate).
A quieter path has been hiding in plain sight: let heavy p-block elements (Se, Te, Sb, Bi) donate their SOC to nearby 3 d moments.
Because 5 p / 6 p orbitals lie close in energy to 3 d bands, even modest d–p hybridisation can funnel the heavy-atom SOC (0.4–1 eV) into the Fe/Co/Ni sub-lattice, locking magnetic moments along a single axis. Two well-documented cases prove the concept:
Fe₃GeTe₂ – a van-der-Waals ferromagnet whose Te-5 p states account for most of its perpendicular MAE; strain or interface fields can double that anisotropy (Physical Review Links, Frontiers).
MnBi (low-T phase) – starts with in-plane anisotropy at cryogenic temperatures but rises to ≈ 2 MJ m⁻³ near room temperature as Bi-6 p SOC takes over (Nature, PMC).
These successes indicate that heavy-p donors can rival rare-earths when three ingredients coincide:
a short metal–X bond (< 2.8 Å) to maximise d–p overlap;
low site symmetry (hexagonal, trigonal, or layered) so the SOC is not cancelled;
a moderately compliant lattice—soft shear allows minute distortions that convert SOC into an easy axis.
Most large-scale magnet screens still ignore heavy chalcogenides and pnictides, leaving a sizeable slice of phase space uncharted. The families below meet the structural and chemical prerequisites and, critically, contain thousands of known compounds that have never had their MAE measured or even calculated.
Chemical family | Representative phases | Why the odds are good |
|---|---|---|
Layered Fe–Te/Se germanides | Fe₃GeTe₂, Fe₅GeTe₂ | Te-5 p SOC already boosts anisotropy; van-der-Waals gaps let strain, intercalation or stacking tune MAE (American Chemical Society Publications) |
NiAs-type 3 d–Bi/Sb binaries |
(Other niches—interstitially doped Fe-Bi tellurides, Co–Te–X alloys, and Fe-Te/Se heterostructures—look equally healthy but remain virtually untouched by MAE calculations.)
Temperature stability – Unlike Nd₂Fe₁₄B, whose Ku falls by half above 150 °C, MnBi’s anisotropy grows with temperature; Te-bearing tellurides show similar positive slopes, making them attractive for automotive or wind-turbine rotors.
Cost and supply – Se, Te, Sb and even Bi trade at a fraction of rare-earth prices and are geographically diversified.
Tunability – Layered or soft-shear lattices respond strongly to epitaxial strain, chemical pressure or light interstitials, creating knobs to push Ku past the 1 MJ m⁻³ threshold that marks a self-sustaining permanent magnet.
The heavy-p + 3 d motif offers a chemically simple, supply-risk-free route to high anisotropy. Its proof-of-concept compounds are already in the literature, yet systematic exploration has barely begun. A focused survey of Fe/Co/Ni chalcogenides and pnictides—starting with the four families above—could well uncover the first truly commercial-grade, rare-earth-free magnet in decades.
MnBi, FeBi, CoSb
Short M–Bi/Sb bonds plus uniaxial symmetry; MnBi sets the benchmark with 2 MJ m⁻³ at 300 K (Nature) |
Tetragonal Fe(Te,Se) intermetallics | Fe₅Te₂B, Fe₅Te₂P | Hardly explored outside battery research; elastic softness predicted to promote SOC-driven distortion |
Half-Heusler & full-Heusler with heavy Z | Co₂MnSb, Co₂MnBi | Half-metallic band structure supplies high moment; Bi/Sb inject SOC—large perpendicular anisotropy reported at Co₂MnSb/TiCoSb interfaces (SSRN) |
The heavy-p-donor route is scientifically plausible, but its commercial promise hinges on three questions we must answer up-front:
Can we push μ0Ms∙KU above 1 TJ m⁻³?
Can Curie temperatures and processing windows meet EV and wind-turbine specs?
Are Te/Sb/Bi truly “low-risk” when mined at ≪1 % the tonnage of Nd?
Nailing those points will turn an intriguing idea into a credible R&D roadmap.
Critical dimension | Possible weak points | Why it matters |
|---|---|---|
Magnetization (Ms) & energy product (BHmax) | Heavy-p ligands often covalently quench 3d moments or lower the 3d electron count (e.g., Fe₃GeTe₂ has μ0Ms ≈ 0.4 T, well below Nd-Fe-B’s 1.6 T). | High KU is useless if Ms is too small; the figure of merit for permanent magnets is KU · Ms2. |
Curie temperature (TC) | Many layered Fe–Te/Se and half-Heusler Sb/Bi phases order below or only slightly above room temperature. The best-in-class Fe₃GeTe₂ is ≈ 220 K in bulk, and MnBi reaches 630 K but only after tricky doping/anneals. | Traction-motor and generator markets need TC ≫ 450 K. |
Phase stability & processing | MnBi’s low-T phase forms in a narrow window and inverts above 350 °C; Fe–Te/Se intermetallics are brittle and air-sensitive; Co₂MnBi tends to antisite disorder that kills half-metallicity. | Scaling to tonnes requires robust, single-phase sintering or melt-spinning routes comparable to Nd-Fe-B powder metallurgy. |
Saturation-field vs. anisotropy gain | KU from p-SOC grows only as hybridisation × SOC, not SOC alone. If d–p overlap is modest, the gain may top out at a few hundred kJ m⁻³—insufficient for self-biased magnets. | The target is ≳ 1 MJ m⁻³ after microstructural averaging, i.e., bulk KU/3–KU/5, not the single-grain value quoted in DFT. |
Environmental & supply nuances | Tellurium and bismuth are by-products (Cu, Pb refining); world output is <1 % of rare-earth tonnage and concentrated in a few mines. Te is also mildly toxic and oxidises readily. | The cost/supply argument is favorable but not bullet-proof; needs quantification against Nd, Dy, Pr recycling streams. |
Microstructural hardening | Layered or hexagonal lattices can suffer easy domain‐wall motion along vdW gaps or basal planes. Without grain-boundary pinning (Dy₂O₃ in Nd-Fe-B, for example) coercivity may stall well below 0.5 T. | A high KU grain still needs microstructural engineering to turn that into high Hc. |