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In a ferroelectric helimagnetic phase of a spin-driven multiferroic CuFeO2, we have found irreversibly additive evolution of electric polarization (P ) induced by sweeping magnetic field (H) under an applied electric field (E), despite a large coercive electric field in the phase. From the unpolarized neutron diffraction experiments with in-situ P measurements under applied E, we have revealed that increment of P is achieved by the variation of an incommensurate magnetic modulation wave number (q) of the helical magnetic ordering in H-sweeping regardless of increasing or decreasing H. Combining this result with H dependence of the magnetic diffraction intensity and a result of off-bench P measurements, we conclude that the H evolution of P is caused by a change in a (ferroelectric) helicity domains volume fraction by driving the helicity domain wall (DW). Taking into account the results of further detailed P measurements, we propose a possible model of the phenomenon that, instead of simply driving the helicity DW, the variation of q causes the neighborhood of the helicity DWs to store an extra exchange energy and to be sensible for E, and then, these regions and the helicity DWs are reconstructed by applied E so as to enlarge the helicity domain favored by the direction of E. The present study demonstrates the magneto-electric cross correlation in driving multiferroic DW: we could activate the frozen ferroelectric DW by means of H-sweeping. This is also an achievement of driving an antiferromagnetic DW, which is difficult in conventional antiferromagnets in principle.

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 .