Magnetoelectric Coupling in a Mn₄Na-Organic Complex under Pulsed Magnetic Fields up to 73 T

Research on the magnetoelectric (ME) effect (or spin-electric coupling) in molecule-based magnetic materials is a relatively nascent but promising topic. Molecule-based magnetic materials have diverse magnetic functionalities that can be coupled to electrical properties. Here we investigate a realization of ME coupling that is fundamental but not heavily studied─the coupling of magnetic spin level crossings to changes in electric polarization. A mixed-valence Mn₄Na complex with a total ground-state spin S = 5/2 under zero magnetic field and S = 17/2 under high magnetic field undergoes a cascade of ground-state level crossings of the S states with increasing magnetic field. Magnetization and electrical polarization measurements under pulsed magnetic fields up to 73 T show that each spin level crossing is accompanied by a significant change in electric polarization that is an even function of the applied magnetic field. A molecular Hamiltonian describing antiferromagnetic exchange in a distorted tetrahedron of three Mn^III and one Mn^II ions matches the data well. We conclude that the ME coupling is caused by magnetostriction within the polar molecule as it distorts to lower its magnetic exchange energy.