Determination of the magnetic structure of the quantum multiferroic Cs2Cu2Mo3O12

Slow relaxational dynamics of electric polarization are characteristic of ferroelectrics and reveal themselves in dissipative anomalies of the dielectric constant. We have found such anomalies in the nonpolar frustrated quasi-1D quantum ferro-antiferromagnet Cs2Cu2Mo3O12. The dielectric response shows two distinct components: a critical divergence at the field-induced magnetic quantum critical point (QCP), and an unusual dissipative peak at ultra-low temperatures, present at all fields. The former stems from the inverse Dzyaloshinskii-Moriya mechanism in the strongly magnetized state. The latter manifests itself as a strong modulation in the complex permittivity with a marked field and temperature dependence, which also becomes critical at the magnetic transition. This last feature is in full agreement with a model of Debye-like relaxation of electric dipoles mediated by the soft magnon that softens at the QCP, through a newly discovered multiferroic relaxation process. Nonetheless, a microscopic model is lacking. Understanding the lattice symmetry at low temperatures and determining the magnetic structure of Cs2Cu2Mo3O12 will be instrumental in clarifying the origin of this mechanism.