Computational Design of Rare-Earth-Free Magnets with the Ti₃Co₅B₂‑Type Structure

The prolific Ti₃Co₅B₂ structure type has produced exciting materials with tunable magnetic properties, ranging from soft magnetic Ti₂FeRh₅B₂, to semihard magnetic Ti₂FeRu₄RhB₂ and hard magnetic Sc₂FeRu₃Ir₂B₂. Density functional theory (DFT) was employed to investigate their spin–orbit coupling effect, spin exchange, and magnetic dipole–dipole interactions in order to understand their magnetic anisotropy and relate it to their various coercivities, with the objective of being able to predict new materials with large magnetic anisotropy. Our calculations show that the contribution of magnetic dipole–dipole interactions to the magnetocrystalline anisotropy energy (MAE) in Ti₃Co₅B₂-type compounds is much weaker than the spin–orbit coupling effect, and Sc₂FeRu₃Ir₂B₂ has, by far, the largest MAE and strong intrachain and interchain Fe–Fe spin exchange coupling, thus confirming its hard magnetic properties. We then targeted materials containing the more earth-abundant and less expensive Co, instead of Rh, Ru or Ir, so that our study started with Ti₃Co₅B₂, which we found to be nonmagnetic. In the next step, substitutions on the Ti sites in Ti₃Co₅B₂ led to new potential quaternary phases with the general formula T₂T′Co₅B₂ (T = Ti, Hf; T′ = Mn, Fe). For Hf₂MnCo₅B₂, we found a large MAE (+0.96 meV/f.u.) but relatively weak interchain Mn–Mn spin exchange interactions, whereas for Hf₂FeCo₅B₂, there is a relatively smaller MAE (+0.17 meV/f.u.) but strong Fe–Fe interchain and intrachain spin exchange interactions. Therefore, these two Co-rich phases are predicted to be new rare-earth-free, semihard to hard magnetic materials.