Enhancing stability and magnetism of ThMn12 -type cerium-iron intermetallics by site substitution

There is considerable research interest in discovering new permanent magnetic materials that perform equally as champion neomagnets, with the minimal use of critical rare-earth elements. Recently ${\mathrm{ThMn}}_{12}$-type (1:12) rare-earth iron (Ce-Fe) intermetallic materials have been on the frontline of research as Ce is naturally abundant that drastically lowers the cost of permanent magnets. Here, we investigate the lattice stability and electronic and magnetic properties of Ti- or Zr-substituted ${\mathrm{CeFe}}_{12}$ and ${\mathrm{CeFe}}_{12}\mathrm{N}$ using density functional theory. We find negative formation energy for all compositions in the bulk structure with respect to unaries except for ${\mathrm{CeFe}}_{12}$. The inclusion of nitrogen in the interstitial sites of ${\mathrm{CeFe}}_{12}$ improves its chemical stability by reducing the formation energy. The first time successfully calculated phonon frequencies including $4f$ electrons indicate that all compositions are dynamically stable. With the help of electronic structure calculations, we demonstrate that cerium exhibits the mixed-valence character in 1:12 materials. The mixed-valency sensibly affects the magnetocrystalline anisotropy (MCA) and magnetic moment. Nitrogen improves the net magnetic moment by influencing the spin polarization with extra electrons, although it has the opposite effect in the MCA constant, ${K}_{1}$. The predicted value of ${K}_{1}$ confirms all compounds uniaxial along the crystalline $c$ axis. Especially for ${\mathrm{CeZrFe}}_{11}$, ${K}_{1}$ is the largest in which Ce exhibits ${\mathrm{Ce}}^{3+}$ $(S=1/2)$, and $\mathrm{Ce}(4f)$ spin-density contour is elongated towards the uniaxial direction. The substantially large values of the MCA and magnetic moments suggest that these critical element-free materials qualify for high-performance permanent magnets.