Direct transition from a disordered to a multiferroic phase

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in a triangular lattice antiferromagnet. Ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. A magnetic field extinguishes the electric polarization through a change in the magnetic symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that of the observed electric polarization. The observed multiferroic behavior provides a theoretically tractable example of ferroelectricity from competing spin interactions.