Magnetically induced ferroelectricity in Bi2CuO4

The tetragonal copper oxide ${\mathrm{Bi}}₂{\mathrm{CuO}}₄$ has an unusual crystal structure with a three-dimensional network of well separated ${\mathrm{CuO}}₄$ plaquettes. The spin structure of ${\mathrm{Bi}}₂{\mathrm{CuO}}₄$ in the magnetically ordered state below ${T}_{\text{N}}\ensuremath{\sim}43$ K remains controversial. Here we present the results of detailed studies of specific heat, magnetic, and dielectric properties of ${\mathrm{Bi}}₂{\mathrm{CuO}}₄$ single crystals grown by the floating zone technique, combined with the polarized neutron scattering and high-resolution x-ray measurements. Down to 3.5 K our polarized neutron scattering measurements reveal ordered magnetic Cu moments which are aligned within the $ab$ plane. Below the onset of the long range antiferromagnetic ordering we observe an electric polarization induced by an applied magnetic field, which indicates inversion symmetry breaking by the ordered state of Cu spins. For the magnetic field applied perpendicular to the tetragonal axis, the spin-induced ferroelectricity is explained in terms of the linear magnetoelectric effect that occurs in a metastable magnetic state. A relatively small electric polarization induced by the field parallel to the tetragonal axis may indicate a more complex magnetic ordering in ${\mathrm{Bi}}₂{\mathrm{CuO}}₄$.