We plan to investigate the influence of thin Mg insert layers on magnetism at buried interfaces in structures that model the lower electrode/barrier interface in technologically-relevant crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions (MTJs). Inserting a thin layer of Mg prior to depositing MgO enhances the performance of annealed MTJ devices: however the reason for this enhancement is not well understood. We propose to use specular and off-specular polarized neutron reflectivity on a series of annealed and unannealed structures with different Mg insert thicknesses in order to gain a basic understanding into how Mg insert layers improve MTJ device performance. Structural and magnetic properties of the buried CoFe(B)/MgO interface will be extracted from magnetization and scattering-length-density profiles, complementing our existing electron microscopy and synchrotron studies.