Controlling the Magnetic Properties of a Molecule

We have developed a new magnetic molecule, a Mn4 cluster, whose magnetic properties can be controlled by changing its chemical composition. Specifically, we have synthesized three variants of this cluster, characterized their magnetic properties via high-field magnetization experiments in the HFML, and interpreted the results based on theoretical calculations. We started with a Mn4 cluster that was hard to magnetize, and only the maximum magnetic field of that we can generate in our lab (31 Tesla) was enough to saturate its magnetization. By chemical modification we made two variants of this cluster and were able to saturate the magnetization at 25 T respectively 15 T. Theoretical considerations provided us with the same decreasing trend in antiferromagnetic interactions, present in the clusters. This work is an ongoing collaboration within our Institute of Molecules and Materials between Molecular Materials, Theory of Condensed Matter and the High Field Magnet Laboratory, which combines organic synthesis, density-functional calculations and high-field magnetization experiments.

The cluster’s cubic core consists of four manganese ions that are connected with each other via four oxygens. The Mn ions are magnetic due to their electrons that aren’t used for the chemical bonds with the oxygens. These Mn-O-Mn bonds provide a magnetic interaction path for those unpaired electrons, coupling their magnetic moments parallel or antiparallel, ferromagnetically respectively antiferromagnetically, making it easier respectively more difficult to magnetize the cluster. Furthermore, two additional ligands that bridge the Mn ions on the top and at the bottom of this core provide extra magnetic exchange interactions paths. Depending on the electronic properties of these axial ligands, we can change the magnetic interactions between the Mn ions via the paths they provide for them.

Key differences between the magnetic properties of our three types of Mn4 clusters are visualized in their low-temperature magnetization curves. All curves start with a finite slope and slowly approach saturation at the highest magnetic fields; a behaviour that is typical for a molecular magnet dominated by antiferromagnetic inter-ion exchange. All twenty unpaired electrons in the Mn4 cluster then have their spins aligned parallel to the magnetic field. The acetate-bridged cluster has the largest intramolecular antiferromagnetic interactions, resulting in the smallest slope at low magnetic fields and saturation of the magnetization at high magnetic fields (31 T). When electron density is withdrawn from the [Mn4O4] core by the benzoate ligands, this coupling is decreased, as demonstrated in the increased slope for the magnetization of this cluster. Even stronger electron density withdrawal by the trifluoroacetate ligands, results in a largely increased slope at low magnetic fields and saturation at 15 T. Based on density-functional theory calculations we were able to quantify the exchange parameters underlying this behaviour, with three independent exchange constants.

Our results constitute a new starting point for user-designed molecular magnets, where magnetic properties can be predicted and controlled through the electron-withdrawing character of the interchangeable ligands.


Left: Overview of the magnetization of our three manganese clusters at low temperature. Right: Schematic representation of the distorted cubane-like core consisting of four manganese ions, four phenolate oxygens and four carboxylate ligands.

This work was published in:

Erik Kampert, Femke F.B.J. Janssen, Danil W. Boukhvalov, Jaap C. Russcher, Jan M.M. Smits, René de Gelder, Bas de Bruin, Peter C.M. Christianen, Uli Zeitler, Mikhail I. Katsnelson, Jan C. Maan and Alan E. Rowan
Ligand-Controlled Magnetic Interactions in Mn4 Clusters
Inorganic Chemistry, 2009, 48, 11903–11908