Charge order and phase separation nature of the insulating La_0.67Ca_0.33MnO₃ thin film on NdGaO₃ substrate

Doped manganite systems have been intensively studied due to their rich phase diagrams at different temperature, strain, pressure, and composition conditions. La_0.67Ca_0.33MnO₃ thin film grown on NdGaO₃ substrate has found to exhibit abnormal antiferromagnetic (AFM) insulating ground state, which violates the bulk phase diagram. The puzzle that why the ground state of this d^3.67 system becomes insulating is studied with density functional theory (DFT) calculation. Several potential scheme has been examined, including substrate strain, pinning of Jahn-Teller distortion, and charge order (CO), within different magnetic configurations. We find that without charge order, neither substrate strain nor increased Jahn-Teller distortion are sufficient to induce the insulating state. With Mn⁴⁺/Mn³⁺ valence states ordered in a stripe type pattern along the b lattice vector, the AFM insulating state can be observed. However, an unrealistic large uniaxial strain is required to assist this phase to beat the ferromagnetic (FM) phase. Further study suggests mixture of the CO [Mn⁴⁺]_0.5/[Mn3⁺]_0.5 and pure LaMnO₃ micro regions in the sample.