Interplay Between Ferroelectric and Antiferromagnetic Phase Transitions in RbCoBr3

The temperature dependence of the complex dielectric constant of RbCoBr3 along the hexagonal c- axis was measured in the frequency range from 1 kHz to 1 MHz under magnetic fields of up to 12 T applied parallel to the c- axis. On the basis of the observed dielectric behavior, a probable interplay between lattice and spin systems, and appropriate crystal structures for the three phases below the upper Neel point were deduced. A broad peak of the real part epsilon(')(c)(T) at approximately 30 K was found to exhibit frequency dispersion with two kinds of relaxation time. These relaxation times seem to correspond closely to the magnetic relaxation times ascribed to the domain- boundary motion in Ising spin chains. In the presence of magnetic fields above 8 T, this peak split into two subpeaks. This magnetic field dependence of epsilon(')(c)(T) reflects the suppression of antiferromagnetic ordering under the magnetic field. These findings imply the correlation between the ferroelectric and antiferromagnetic phase transitions in RbCoBr3.