Competing interactions and geometric frustration provide favourable
conditions for exotic states of matter. Such competition often causes multiple
phase transitions as a function of temperature and can lead to magnetic
structures that break inversion symmetry, thereby inducing ferroelectricity
[1-4]. Although this phenomenon is understood phenomenologically [3-4], it is
of great interest to have a conceptually simpler system in which
ferroelectricity appears coincident with a single magnetic phase transition.
Here we report the first such direct transition from a paramagnetic and
paraelectric phase to an incommensurate multiferroic in the triangular lattice
antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric
polarization when the symmetry of the magnetic order changes and
ferroelectricity is only observed when the magnetic structure has chirality and
breaks inversion symmetry. Multiferroic behaviour in RFMO provides a
theoretically tractable example of ferroelectricity from competing spin
interactions. 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 observed experimentally.Comment: 16 pages pdf including 3 figure
