Density functional study of a ferromagnetic ferroelectric LaMnO3/OBaTiO3 superlattice

Using the GGA+U density functional, we have calculated the lattice constants, atomic positions, magnetization, and ferroelectric polarization of a (LaMnO3)(4.5)(BaTiO)(4.5) superlattice containing five LaO and TiO2 planes and four MnO2 and BaO planes. Although LaMnO3 is antiferromagnetic, it is ferromagnetic in the superlattice. An approximation to the ferroelectric polarization, obtained from a comparison of superlattice and bulk crystal atomic displacements and unit cell volumes, is found to agree reasonably well with that obtained from a Berry phase calculation. The electric polarization is polarized along the longer in-plane lattice vector, while the Mn spins, when the spin-orbit interaction and spin noncollinearity are included, have different magnitudes, but all point in essentially the same direction as the electric polarization.