Multiferroic behavior at a spin state transition

We describe multiferroic behavior at a spin state transition (SST), instead of the more traditional approach using exchange-coupled order like ferromagnetism or spiral antiferromagnetism. SSTs involve large changes in ionic size, bonding and lattice, and so have potential for strong magnetoelectric coupling. We observe a magnetic field-induced electric polarization change at an SST that is within an order of magnitude of the record for any material. SSTs such as the S = 1 to S = 2 transition in Mn³⁺ occur when electrons change their occupation of magnetic partially-filed orbitals. SSTs can be sharp, first-order hysteretic phase transitions and they are an increasingly common functionality in inorganic-organic hybrid materials, persisting up to room temperature in some compounds. We study a Mn-based molecular system in which the high-spin state is Jahn-Teller active, and the JT distortions carry an electric dipole. We show that the magnetic field-induced SST induces ferroelectricity in both DC and pulsed magnetic fields. We use high magnetic fields at the NHMFL to study the magnetic and electric behavior of this system across a significant fraction of its T-H phase space, and compare to a phenomenological theoretical model.