Strain-engineered magnetic order in (LaMnO3)n/(SrMnO3)2n superlattices

Using first-principles calculations based on the density functional theory, we show a strong strain dependence of magnetic order in (LaMnO3)(n)/(SrMnO3)(2n) (001) superlattices with n = 1,2. The epitaxial strain lifts the degeneracy of Mn e(g) orbitals, thus inducing an inherent orbital order, which in turn strongly affects the ferromagnetic double exchange of itinerant e(g) electrons, competing with the antiferromagnetic superexchange of localized t(2g) electrons. For the case of tensile strain induced by SrTiO3 (001) substrate, we find that the ground state is A-type antiferromagnetic and d(x2-y2) orbital ordered, which is in excellent agreement with recent experiments [May et al., Nat. Mater. 8, 892 (2009)]. Instead, for the case of compressive strain induced by LaAlO3 (001) substrate, we predict that the ground state is C-type antiferromagnetic and d(3z2-r2) orbital ordered.