Oxygen Effect on the Properties of Epitaxial (110) La_0.7Sr_0.3MnO₃ by Defect Engineering

The multiferroic properties of mixed valence perovskites such as lanthanum strontium manganese oxide (La_0.7Sr_0.3MnO₃) (LSMO) demonstrate a unique dependence on oxygen concentration, thickness, strain, and orientation. To better understand the role of each variable, a systematic study has been performed. In this study, epitaxial growth of LSMO (110) thin films with thicknesses ∼15 nm are reported on epitaxial magnesium oxide (111) buffered Al₂O₃ (0001) substrates. Four LSMO films with changing oxygen concentration have been investigated. The oxygen content in the films was controlled by varying the oxygen partial pressure from 1 × 10^-4 to 1 × 10^-1 Torr during deposition and subsequent cooldown. X-ray diffraction established the out-of-plane and in-plane plane matching to be (111)_MgO ∥ (0001)_Al and ⟨11̅0⟩_MgO ∥ ⟨101̅0⟩_Al for the buffer layer with the substrate, and an out-of-plane lattice matching of (110)_LSMO ∥ (111)_MgO for the LSMO layer. For the case of the LSMO growth on MgO, a novel growth mode has been demonstrated, showing that three in-plane matching variants are present: (i) ⟨11̅0⟩_LSMO ∥ ⟨11̅0⟩_MgO, (ii) ⟨11̅0⟩_LSMO ∥ ⟨101̅⟩_MgO, and (iii) ⟨11̅0⟩_LSMO ∥ ⟨01̅1⟩_MgO. The atomic resolution scanning transmission electron microscopy (STEM) images were taken of the interfaces that showed a thin, ∼2 monolayer intermixed phase while high-angle annular dark field (HAADF) cross-section images revealed 4/5 plane matching between the film and the buffer and similar domain sizes between different samples. Magnetic properties were measured for all films and the gradual decrease in saturation magnetization is reported with decreasing oxygen partial pressure during growth. A systematic increase in the interplanar spacing was observed by X-ray diffraction of the films with lower oxygen concentration, indicating the decrease in the lattice constant in the plane due to the point defects. Samples demonstrated an insulating behavior for samples grown under low oxygen partial pressure and semiconducting behavior for the highest oxygen partial pressures. Magnetotransport measurements showed ∼36.2% decrease in electrical resistivity with an applied magnetic field of 10 T at 50 K and ∼1.3% at room temperature for the highly oxygenated sample.