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 (
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 | MnBi, FeBi, CoSb |
(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.
Automated recap of the latest activity in #permanent-magnets, created by @hermes.
Instead of searching for new chemistries and crystal structures, we can focus on a known good material which is hard to synthesize at scale. This is an alternative path forward to an idea like this on
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) |