  Control of magnetism by an applied electric field is a desirable technique
for the functionalization of magnetic materials. Motivated by recent
experiments, we study the electric field control of the interfacial magnetism
of CaRuO$_3$/CaMnO$_3$ (CRO/CMO) (001), a prototype interface between a
non-magnetic metal and an antiferromagnetic insulator. Even without the
electric field, the interfacial CMO layer acquires a ferromagnetic moment due
to a spin-canted state, caused by the Anderson-Hasegawa double exchange (DEX)
between the Mn moments and the leaked electrons from the CRO side. An electric
field would alter the carrier density at the interface, leading to the
possibility of controlling the magnetism, since DEX is sensitive to the carrier
density. We study this effect quantitatively usingdensity-functional
calculations in the slab geometry. We find a text-book like dielectric
screening of the electric field, which introduces polarization charges at the
interfaces and the surfaces. The extra charge at the interface enhances the
ferromagnetism via the DEX interaction, while away from the interface the
original AFM state of the Mn layers remains unchanged. The effect could have
potential application in spintronics devices.
