  A simple but general microscopic mechanism to understand the interplay
between the electric and magnetic degrees of freedom is developed. Within this
mechanism, the magnetic structure generates an electric current which induce an
counterbalance electric current from the spin orbital coupling. When the
magnetic structure is described by a single order parameter, the electric
polarization is determined by the single spin orbital coupling parameter, and
the material is predicted to be a half insulator. This mechanism provides a
simple estimation of the value of ferroelectricity and sets a physical
limitation as well.
