Defect induced polar distortion in SrMnO₃ thin films

Defect engineering in perovskite oxides enables novel functionalities by inducing and controlling lattice defects, effectively breaking lattice symmetry, stabilizing polar states and inducing ferroelectricity in non-polar/paraelectric or ferro/antiferromagnetic oxide thin films. A polar state can be stabilized in SrMnO₃ (SMO) thin films by displacing Mn ions. However, conventional epitaxial strain engineering necessitates deposition on diverse single crystalline substrates with varying misfit strains and optimization of thin film growth conditions, posing challenges in achieving polar states in SMO thin films. In this study, polar distortion was achieved by inducing defects in SMO epitaxial thin films grown on Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ substrates La_0.7Sr_0.3MnO₃ (LSMO) electrode. Scanning transmission electron microscopy analysis revealed that Mn ion displacement and the c/a ratio increased on moving from the SMO/LSMO interface to the top surface of the SMO film. Electron energy loss spectroscopy depth profiles revealed variations in oxygen stoichiometry and Mn³⁺/Mn⁴⁺ ratio across the cross section of the SMO film. Consequently, a polar state was stabilized through strain gradient induced by defect chemistry in SMO thin films. Our study demonstrates that defect engineering can be effectively utilized in the realization of electric field-controlled magnetic devices at room temperature.