Magnetoelectric effects in an organometallic quantum magnet

Abstract

Metal-organic materials constitute a new field in which to search for ferroelectricity and coupling between electricity and magnetism. We observe a magnetic field-induced change in the electric polarization, $\ensuremath{\Delta}\mathbf{P}(H)$, that reaches 50 $\ensuremath{\mu}$C/m${}^{2}$ in single crystals of NiCl${}_{2}$--4SC(NH${}_{2}$)${}_{2}$ (DTN). DTN forms a tetragonal structure that breaks inversion symmetry with the electrically polar thiourea molecules [SC(NH${}_{2}$)] all tilted in the same direction along the $c$ axis. The field $H$ induces canted antiferromagnetism of the Ni $S=1$ spins between 2 and 12 T and our measurements show that the electric polarization increases monotonically in this range, saturating above 12 T. By modeling the microscopic origin of this magnetoelectric effect, we find that the leading contribution to $\ensuremath{\Delta}\mathbf{P}$ comes from the change in the crystal electric field, with a smaller contribution from magnetic exchange striction. The finite value of $\ensuremath{\Delta}\mathbf{P}$ induced by magnetostriction results from the polar nature of the thiourea molecules bonded to the Ni atoms, and it is amplified by the softness of these organic molecules.