Quantum Spin Fluctuations and Ellipticity for a Triangular-Lattice Antiferromagnet...

The effects of quantum spin fluctuations are investigated for the three-sublattice spin config- urations of a geometrically-frustrated triangular-lattice antiferromagnet in a magnetic field with easy-axis anisotropy. In zero field, the spin amplitudes are most strongly suppressed by quantum fluctuations perpendicular to the anisotropy axis, which at least partially explains the elliptical spirals observed in the multiferroic states of MnWO4, TbMnO3, and CuCrO2. Due to the change in spin angles, quantum fluctuations shift the boundaries between the collinear and non-collinear phases and open a gap between the two types of non-collinear phases.