Electric polarization along thecaxis in the ferroelectric helimagnetic phase ofCuFe1−xGaxO2

Abstract

A triangular lattice antiferromagnet CuFe 1-x Ga x O 2 has been known to exhibit an elliptic screw-type magnetic ordering with spontaneous electric polarization "perpendicular" to the c axis, specifically along the [110] direction of the crystal. In the present study, we have performed pyroelectric measurements on CuFe 1-x Ga x O 2 (x = 0.035), revealing that the ferroelectric phase also exhibits spontaneous electric polarization "parallel" to the c axis (P c ). While the ferroelectric polarization along the [110] direction is naturally led from the symmetry of the trigonal (or monoclinic) crystal structure with the screw-type magnetic ordering, the emergence of P c is not trivial. To understand the origin of P c , we have refined the crystal structure by means of resonant x-ray diffraction measurements near the K-absorption edges of Fe 3+ and Cu + ions. We have also reanalyzed the magnetic structure in the ferroelectric phase using the results of the previous spherical neutron polarimetry [Nakajima et al., Phys. Rev. B 79, 214423 (2009)]. On the basis of the refined magnetic and crystal structures, we have discussed the microscopic mechanism for the emergence of P c .