  Ferromagnet-ferroelectric-metal superlattices are proposed to realize the
large room-temperature magnetoelectric effect. Spin dependent electron
screening is the fundamental mechanism at the microscopic level. We also
predict an electric control of magnetization in this structure. The naturally
broken inversion symmetry in our tri-component structure introduces a
magnetoelectric coupling energy of $P M^2$. Such a magnetoelectric coupling
effect is general in ferromagnet-ferroelectric heterostructures, independent of
particular chemical or physical bonding, and will play an important role in the
field of multiferroics.
