Using first-principles calculations based on the density functional theory, we show a strong strain dependence of magnetic order in (LaMnO$_{ 3}$)$_n$/(SrMnO$_{ 3}$)$_{ 2n}$ (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 SrTiO$_3$ (001) substrate, we find that the ground state is A-type antiferromagnetic and $d_{ x^2-y^2}$ orbital ordered, which is in excellent agreement with recent experiments [S. J. May { \it et al.}, Nature Materials { \bf 8}, 892 (2009)]. Instead, for the case of compressive strain induced by LaAlO$_3$ (001) substrate, we predict that the ground state is C-type antiferromagnetic and $d_{ 3z^2-r^2}$ orbital ordered.
Categories: cond-mat.str-el cond-mat.mtrl-sci