Abnormal enhancement of ferromagnetism for LaMnO3+δ thin films with decreasing oxygen pressure

The compressive LaMnO3+δ thin films with thickness of ∼20 nm are grown on LaAlO3 (001) single crystal substrates under various oxygen environment. With decreasing oxygen pressure, the in-plane compressive strain is enhanced. It is found that the breathing mode (Q1) and the basal plane distortion mode (Q2) of MnO6 octahedron are suppressed, while the octahedral stretching mode (Q3) is promoted. The promoted Q3 switches the orbital order from x2-1/y2-1 to (x2-y2)+(z2-1) type. The ferromagnetic (FM) transition temperature Tc increases from 85 K to 105 K and the low-temperature magnetization increases greatly by 175%. The abnormal enhancement of magnetization with decreasing oxygen pressure is ascribed to the strain enlarged FM domains due to the decreased MnO6 distortion and the switched orbital ordering. This result is advantageous to explain the novel phenomena in LaMnO3-related systems.