Spin-reversal energy barriers of 305 K for Fe${}^{2+}$ $d^6$ ions with linear ligand coordination

A remarkably large magnetic anisotropy energy of 305 K is computed by quantum chemistry methods for divalent Fe${}^{2+}$ $d^6$ substitutes at Li-ion sites with $D_{6h}$ point-group symmetry within the solid-state matrix of Li${}_3$N. This is similar to values calculated by the same approach and confirmed experimentally for linearly coordinated monovalent Fe${}^{1+}$ $d^7$ species, among the largest so far in the research area of single-molecule magnets. Our ab initio results therefore mark a new exciting exploration path in the search for superior single-molecule magnets, rooted in the $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$configurationof$d^6$ transition-metal ions with linear or quasilinear nearest-neighbor coordination. This $d^6$ axial anisotropy may be kept robust even for symmetries lower than $D_{6h}$, provided the ligand and farther-neighbor environment is engineered such that the $d_{xy}^{1.5}{d}_{x^2-y^2}^{1.5}{d}_{z^2}^1{d}_{yz}^1{d}_{zx}^1$ - $d_{xy}^{1} d_{x^2-y^2}^{1} d_{z^2}^2 d_{yz}^1 d_{zx}^1$ splitting remains large enough.