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

TL;DR

This study predicts a record-high magnetic anisotropy energy of 305 K for Fe$^{2+}$ ions with a specific linear ligand coordination, suggesting new avenues for designing advanced single-molecule magnets.

Contribution

It demonstrates, through quantum chemistry calculations, that Fe$^{2+}$ ions with linear coordination can exhibit exceptionally high magnetic anisotropy, opening new paths for single-molecule magnet development.

Findings

Magnetic anisotropy energy of 305 K for Fe$^{2+}$ in Li$_3$N.

Large anisotropy comparable to experimental values for Fe$^{1+}$.

Potential for robust anisotropy in lower symmetry environments.

Abstract

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 $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$ configuration of $d^6$ 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.