Comprehensive study on ferroelectricity induced by a proper-screw-type magnetic ordering in multiferroic CuFeO2: Nonmagnetic impurity effect on magnetic and ferroelectric order

We refined the magnetic structure of a ferroelectric (FE) phase of multiferroic CuFe1-x Gax O2 with x=0.035 by complementary use of spherical neutron polarimetry and a four-circle neutron-diffraction measurement, revealing that the proper-screw-type magnetic structure in the ferroelectric phase has a finite ellipticity of ∼0.9. By means of polarized neutron-diffraction and in-situ pyroelectric measurements, we also investigated the quantitative relationship between the macroscopic ferroelectric polarization (P) and the asymmetry in volume fractions with left-handed and right-handed helical magnetic order in CuFe1-x Alx O2 with x=0.0155 and CuFe1-x Gax O2 with x=0.035. These measurements revealed that the substitution of a small amount of nonmagnetic Ga3+ or Al3+ ions does not significantly change the magnitude of the local ferroelectric polarization but does reduce the sensitivity of P to the poling electric field (Ep). This implies that the mobility of the magnetic domain walls, which is sensitive to magnetic defects due to nonmagnetic substitution, determines the sensitivity of P to Ep because of a one-to-one correspondence between the magnetic and ferroelectric domains.