  Neutron scattering studies and measurements of the dielectric susceptibility
and ferroelectric polarization P have been carried out in various magnetic
fields H for single-crystal samples of the multiferroic system LiVCuO4 with
quasi one-dimensional spin 1/2 Cu2+ chains formed of edge-sharing CuO4 square
planes, and the relationship between the magnetic structure and
ferroelectricity has been studied. The ferroelectric polarization is
significantly suppressed by the magnetic field H above 2 T applied along a and
b axes. The helical magnetic structure with the helical axis parallel to c has
been confirmed in H=0, and for H//a, the spin flop transition takes place at
H=2 T with increasing H, where the helical axis changes to the direction
parallel to H. The ferroelectric polarization along a at H=0 is found to be
proportional to the neutron magnetic scattering intensity, indicating that the
magnetic order is closely related to the appearance of the ferroelectricity.
The relationship between the magnetic structure and ferroelectricity of LiVCuO4
is discussed by considering the existing theories.
