Strain controlled orbital state and magnetization in insulating LaMnO_3+δ films

LaMnO_3+δ films with various thicknesses were grown on LaAlO₃ (001) single crystal substrate to investigate the effect of in-plane compressive strain (∼-0.57%) on magnetic properties. All films exhibit a blocking temperature T_b at which the zero field cooled magnetization reaches a maximum, indicating the ferromagnetic (FM) nanoclusters are embedded in the background of antiferromagnetic (AFM) matrix. The onset temperature of FM transition T_c and T_b is increased by 24% and 89%, respectively, with the thickness decreasing from 82.4 nm to 9.2 nm. Simultaneously, the saturation magnetization greatly increases by 309%, which is ascribed to the strain-induced transition of AFM to FM phase due to the orbital order structure switching from x²- 1/y²-1 [A-type] to (x²- y²) + (z²- 1) [F-type].