  Electric-field controlled exchange bias in a heterostructure composed of the
ferromagnetic manganite La0.7Sr0.3MO3 and the ferroelectric antiferromagnetic
BiFeO3 has recently been demonstrated experimentally. By means of a microscopic
model Hamiltonian we provide a possible explanation of the origin of this
magnetoelectric coupling. We find, in agreement with experimental results, a
net ferromagnetic moment at the BiFeO3 interface. The induced ferromagnetic
moment is the result of the competition between the e_g-electrons double
exchange and the t_2g-spins antiferromagnetic superexchange that dominate in
bulk BiFeO3. The balance of these simultaneous ferromagnetic and
antiferromagnetic tendencies is strongly affected by the interfacial electronic
charge density which, in turn, can be controlled by the BiFeO3 ferroelectric
polarization.
