Asymmetric and modulated magnetic profiles in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices*

We have determined the magnetic depth profile of MBE-grown ferromagnetic (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices, where $n$ is nominally equal to 3 and 5. Polarized neutron reflectivity measurements reveal the existence of a modulated magnetic structure that repeats with the superlattice period in both samples. For $n$=5, a moment of $\sim$2.6 $\mu_B$/Mn is measured in the LaMnO$_3$ (LMO) layer, while the moment in the middle of the SrMnO$_3$ (SMO) layer is negligible. The magnetization at the interfaces is found to be asymmetric with an enhanced moment residing at the LMO/SMO interfaces but not at the SMO/LMO interfaces. The origin of the magnetic asymmetry at the interfaces is elucidated from comparison with the structural properties determined by x-ray reflectivity and transmission electron microscopy.