Emergent double perovskite phase at LaMnO3/LaNiO3 interfaces: Coupled charge transfer and structural reconstruction

We report ${[{({\mathrm{LaNiO}}₃)}_{\mathrm{n}}/{({\mathrm{LaMnO}}₃)}_{\mathrm{n}}]}_{\mathrm{M}}/{\mathrm{SrTiO}}₃(111)$ superlattices [$n=20$, 10, 5, 2, and 1 unit cells (u.c.)] as ``precursor'' heterostructures for a double perovskite ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$. A charge transfer at the ${\mathrm{LaNiO}}₃$/${\mathrm{LaMnO}}₃$ interfaces and formation of the emergent ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$ phase with ${\mathrm{Ni}}²⁺$ and ${\mathrm{Mn}}⁴⁺$ oxidation states was evidenced by in situ optical ellipsometry and supported by magnetic and Raman measurements. The 10/10 and 5/5 superlattices revealed a complex magnetic behavior with two ferromagnetic phase transitions with ${T}_{\mathrm{C}\text{\ensuremath{-}}\mathrm{low}}=170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{C}\text{\ensuremath{-}}\mathrm{high}}\ensuremath{\sim}250--280\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, originating from the ferromagnetic ${\mathrm{LaMnO}}₃$ and the emergent interfacial ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$ phase, respectively. With decreasing thickness of individual layers, the magnetism of 2/2 and 1/1 superlattices transforms into that of the ordered double perovskite ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$. Furthermore, a structural reconstruction into a monoclinic $({P}₁₂)$ structure of the ordered ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$ at the interface, evidenced by Raman spectroscopy, goes ahead with the charge transfer and magnetic ${\mathrm{La}}₂{\mathrm{NiMnO}}₆$ behavior.