Tautomeric transition in a Co complex creates a spin crossover and magnetoelectric coupling

Molecular materials offer several intriguing routes to magnetic and magnetoelectric phenomena that are distinct from what is commonly seen in inorganic materials. Co(Cat)(SQ)(4-CNpy)₂ is a cobolt-based molecular material that has a tautomeric transition that leads to magnetoelectric coupling and to a spin crossover (also known as spin-state transition). The tautomeric transition involves a change in valence of a Co ion, where the electron moves to another location on the molecule. The result is that at from low temperature to high temperature, Co³⁺ S = 0 becomes Co²⁺ S = 3/2, and the overall molecule transitions from S = ½ to S = 5/2. At the same time the relocated electron changes the electric dipole of the molecule significantly. The resulting structural distortion of the molecule creates coupling between adjacent molecules, driving a 1^st order phase transition.

Here we explore the resulting coupling between structure, magnetism and electric dipoles at the 1^st order tautomeric transition near 110 K. We perform measurements in DC and pulsed fields to 60 T and as a function of temperature. We investigate the bistable behavior and the dynamics of this 1^st order phase transition, avalanching effects in magnetic fields and the polarization changes across the transition.