Single-molecule magnets are metal-organic compounds that exhibit magnetic hysteresis in a certain temperature range. As the name suggests, the magnetic properties of single-molecule magnets are of purely molecular origin and do not rely on a long-range ordering of magnetic moments.
Single-molecule magnets are of interest in research as well as application. In research they serve, for example, as test environment for the study of quantum mechanical effects. On the application side, they promise to be useful candidates in the construction of small magnetic memory devices. Responsible for their potential usefulness as smallest magnetic memory unit is the generally large magnetic bi-stability of single-molecule magnets. Consequently, revealing the mechanisms that cause such a large magnetic bi-stability is one of the principal goals on the road to the rational design and discovery of single-molecule magnets.
A first hypotheses speculating that an increase of magnetic bi-stability can be obtained through an increase of the magnetic moment of the compound, has recently been ruled out. Increasing the magnetic moment of polynuclear molecules does not automatically lead to a notable increase in magnetic bi-stability. Consequently, research has focused on ions with large magnetic anisotropies. Lanthanides have been considered as particular promising candidate ions in this regard. Even though the corresponding compounds exhibit a large effective energy barrier towards the relaxation of the magnetic moment, the expected increase in magnetic bi-stability was not found.
Recently, researchers from the University of Stuttgart, the Freie Universität Berlin, the Laboratoire National des Champs Magnétiques Intenses in Grenoble and from the Max Planck Institute for Chemical Energy Conversion, have presented principles for the rational design of effective single-molecule magnets. The researchers combined experimental methods with computational modelling to comprehensively study a mono-nulcear, tetrahedrally coordinated cobalt(II) single-molecule magnet which exhibits a very high effective energy barrier and pronounced magnetic bi-stability. In particular, the researchers obtained information on the magnetic properties from far-infrared, magnetic circular dichroism and high-frequency electron paramagnetic resonance spectroscopy. Correlated calculations (Complete active space self-consistent field, CASSCF, and the N-electron-valence-perturbation theory to second order, NEVPT2) on DFT-optimized geometries were used to gain insight into the ligand field. The researchers relate the favorable properties of the cobalt(II) compound at hand to a strong ligand field in combination with axial distortion. The in-depth understanding gained in this study allows the formulation of design principles for improved materials.