Abstract
Magnetic eld (B) variation of the electrical polarization Pc (∥ c) of the perfect triangular lattice
antiferromagnet RbFe(MoO4)2 is examined up to the saturation point of the magnetization for
B ? c. Pc is observed only in phases for which chirality is predicted in the in-plane magnetic
structures. No strong anomaly is observed in Pc at the eld at which the spin modulation along the
c axis, and hence the spin helicity, exhibits a discontinuity to the commensurate state. These results
indicate that the ferroelectricity in this compound originates predominantly from the spin chirality,
the explanation of which would require a new mechanism for magnetoferroelectricity. The obtained
eld-temperature phase diagrams of ferroelectricity well agree with those theoretically predicted for
the spin chirality of a Heisenberg spin triangular lattice antiferromagnet.
