Surface oxygen control retains low-dimensional ferromagnetic insulator in atomically designed oxide heterostructures

The formation of oxygen vacancies in oxide films can dramatically alter their inherent properties, especially in ultrathin films down to monolayer limit. Gaining an effective control of oxygen content in low-dimensional oxides is essential for their nanoelectronic applications. Herein, we demonstrate enhanced ferromagnetism in atomically designed LaMnO₃/SrTiO₃ (LMO/STO) heterostructures, where the surface oxygen content is controlled by the termination conversion utilizing an SrRuO₃ (SRO) buffer layer. X-ray absorption spectroscopy reveals increased Mn oxidation states along with enhanced hybridization between Mn-3 d and O-2 p states as the termination of LMO converts from MnO₂ to LaO atomic plane. Spatially-resolved electron energy loss spectroscopy further clarifies that the oxidation states of outermost Mn ions recover their bulk level after termination switch, although the electron accumulation at the bottom interface with STO remains virtually unaltered. These results are in line with previous first-principles studies where the disappeared ferromagnetism in ultrathin LMO is ascribed to the oxygen vacancies formed at the MnO₂ open surface. Moreover, for LMO films thinner than four unit cells, capping with another SRO monolayer is found to be crucial to restore the oxygen stoichiometry required for a ferromagnetic ground state. Our findings suggest a general strategy to engineer the oxygen stoichiometry in (quasi) two-dimensional oxide materials for developing high-performance nanoelectronic devices.