  We studied the microscopic mechanism of multiferroics, in particular with the
"spin current" model (Hosho Katsura, Naoto Nagaosa and Aleander V. Balatsky,
Phys. Rev. Lett. 95, 057205 (2005)). Starting from a system with helical spin
configuration, we solved for the forms of the electron wave functions and
analyzed their characteristics. The relation between ferroelectricity and
Dzyaloshinskii-Moriya interaction (I. Dzyaloshinskii, J. Phys. Chem. Solids 4,
241 (1958) and T. Moriya, Phys. Rev. 120, 91 (1960)) is clearly established.
There is also a simple relation between the electric polarization and the wave
vector of magnetic orders. Finally, we show that the bond-bending exists in
transition metal oxides can enhance ferroelectricity.
