Magnetism and Nanoscale Structural and Compositional Irregularities in MBE-grown La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$~on SrTiO$_{\mathrm{3}}$(001)

Double perovskites (A$_{\mathrm{2}}$BB'O$_{\mathrm{6}})$ are a fascinating class of oxides with considerable potential for applications requiring ferromagnetic and semiconducting properties. We have investigated MBE-grown La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$ and have found that despite the fact that Mn and Ni are present as 4$+$ ($d^{\mathrm{3\thinspace }}$: $t_{2g}^{\mathrm{3}}e_{g}^{\mathrm{0}})$ and 2$+$ ($d^{\mathrm{8\thinspace }}$: $t_{2g}^{\mathrm{6}}e_{g}^{\mathrm{2}})$ respectively, and exhibit suitable XMCD signatures, the volume-averaged moment per formula unit is considerably less than 5 Bohr magnetons. Our electron energy loss spectroscopy (STEM-EELS) and atom probe tomography (APT) results to date reveal that there is considerable disorder in the B-site sublattice for as-deposited films, despite excellent volume-averaged stoichiometry. While air annealing results in substantial ordering, the moment remains low due to the nucleation of NiO inclusions with needle-like shapes revealed only by APT. First principles modeling suggests that even though the double perovskite is quite stable if nucleated in excess O, the presence of O vacancies facilitates structural disorder. In this talk, we will present our latest results on this fascinating material.