Field-induced phase transitions and magnetoferroelectricity in the perfect triangular lattice antiferromagnet RbFe(MoO sub(4)) sub(2) in a vertical magnetic field

We performed electric polarization P, dielectric constant [Greek Lunate Epsilon] and magnetization M measurements on the perfect triangular-lattice antiferromagnet RbFe(MoO sub(4)) sub(2) in high magnetic fields B applied along the c-axis. Whereas M(B) at T=1.5 K monotonically increases with B up to the saturation at View the MathML source Bs=21T, P(B) disappears about 14 T, far below the saturation point. In a field range View the MathML source 13 less than or equal to B less than or equal to 14T, the temperature variation of [Greek Lunate Epsilon] shows a double peak structure below 2.8 K, indicating two successive transitions. These results strongly suggest the presence of a new high-field phase; the magnetic structure changes from a low-field conical state having chirality to a high-field coplanar non-collinear structure without chirality. The obtained phase diagram is in agreement with a theoretical prediction on XY-like Heisenberg spin triangular lattice antiferromagnets.