Strain Driven Insulator-Metal Transition at 300 K in Epitaxial La_0.9Sr_0.1MnO3 Films

We report the discovery of an insulator-metal transition at 300 K in La_0.9Sr_0.1MnO₃ ultrathin films epitaxially grown on a (100) SrTiO₃ substrate. This compound has the tolerance factor t=0.96 at which the colossal magnetoresistance is at a maximum and the pressure coefficient of the Curie temperature T_C is the highest amongst the series manganites. The epitaxial growth of the films under compressive strain was revealed by high-resolution transmission electron microscopy measurements. By performing a systematic analysis of the dependence of the resistivity on film thickness, applied magnetic field, and dc current, we conclude that compressive strain enhances the kinetic energy favoring the itinerant ferromagnetic state in its competition with the charge ordering state.