Abstract
Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, , has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of , an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group and the propagation vector takes place. Below 14 K, a proper screw ordering (ICM2) and appears as a minor phase which coexists with ICM1 and the ground state down to the lowest measured temperature 5.5 K. No ferroelectric polarization associated with the ICM2 phase was observed in agreement with its nonpolar point group . Finally, a spiral order with cycloid and proper screw components (ICM3), and emerges below 11 K as the ground state of the system. Based on the deduced magnetic point group , we conclude that the ferroelectric polarization in ICM3 is parallel to the axis and is caused by the inverse Dzyloshinskii-Moriya effect with . Unlike the rhombohedral polytype, the additional contribution to the macroscopic polarization is not allowed in the present case due to the symmetry constraints imposed by the hexagonal lattice of .
1 More- Received 17 February 2015
DOI:https://doi.org/10.1103/PhysRevB.91.094434
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