IMF2017

We report on the discovery of ferroelectricity in the well-studied, gapless quantum antiferromagnet NH₄CuCl₃. The ferroelectric transition temperature was found to be 69 K using specific heat and dielectric measurements [1]. NH₄CuCl₃ has been well studied because of its novel magnetization plateaus. It is also unique because it possesses a gapless ground state, in contrast with the isostructural KCuCl₃. In particular, EPR and neutron scattering techniques has shown that the potassium variant has weak interdimer coupling [2,3]. This feature was also thought to be true of the ammonium variant, with symmetry arguments playing a large role. Recent ¹⁴NMR measurements have suggested that interdimer coupling may play a larger role than believed [4]. However, their possible multiferroicity, as both magnets and ferroelectrics, have not been reported. Such a finding would add a new dimension to their utility as memory storage elements. Here we report the discovery of ferroelectricity in NH₄CuCl₃, and several of its mixed lattices with K⁺. Fields of up to 60 T were applied to study the magnetization behavior at 1.3 K, in addition to orientation and temperature dependent high-frequency EPR measurements. In this talk, we present details of our crystal growth and x-ray diffraction studies of lattice authenticity and purity, dielectric with and without applied magnetic fields, specific heat, magnetic susceptibility, pulsed fields, and EPR characterization of these novel materials.