Electronic control of strong magnetic anisotropy in Co-based single-molecule magnets

Abstract

The observation of large swings in the magnetic anisotropy in ligated ${\mathrm{Co}}_{2}$ dimers has motivated further calculations on single-center Co qubits in quasitetrahedral and quasioctahedral symmetries. In all cases our results indicate that it is the creation of an unquenched orbital moment due to a half-occupied frontier state at the Co center that directly drives large changes in the magnetic anisotropy barriers regardless of whether it is through symmetry breaking, a change in the charge state, or orbital energy reordering. While these observations are computationally demonstrated through our results on both monomers and dimers, the origin of orbital reordering at the Fermi level is not describable in terms of single physical changes. For example, in ligated ${\mathrm{Co}}_{2}$ the appearance of an unquenched orbital moment in one of the Co centers is correlated with a change from antiferromagnetic to ferromagnetic ordering at an energy that is uncharacteristically high for such transitions. Because this raises the spector of electromagnetically switchable quantum devices that operate at higher temperatures relative to that of the first-generation magnetic qubits, we carefully discuss subtle details associated with single-center qubits in realistic atomistic environments.