Using ferroelectric 0.67Pb͑Mg 1/3 Nb 2/3 ͒O 3 -0.33PbTiO 3 single crystals as substrates, we studied the effects of the ferroelectric poling and the converse piezoelectric effect on the strain state, resistance, insulator-to-metal transition temperature ͑T C ͒, and magnetoresistance ͑MR͒ of La 0.7 Ba 0.3 MnO 3 ͑LBMO͒ thin films. In situ x-ray diffraction measurements indicate that the ferroelectric poling ͑or the converse piezoelectric effect͒ induces a substantial reduction in the in-plane

more »

... ile strain in the LBMO film, giving rise to a decrease in the resistance and an increase in T C . The relative changes of the resistance and T C are proportional to the induced reduction in the in-plane tensile strain ͑␦ xx ͒ in the film. The reduction in the in-plane tensile strain leads to opposite effects on MR below and above T C , namely, MR is reduced for T Ͻ T C while MR is enhanced for T Ͼ T C . We discuss these strain effects within the framework of the Jahn-Teller ͑JT͒ electron-lattice coupling and phase separation scenario that are relevant to the induced strain. Similar studies on CaMnO 3 thin films, for which there is no JT distortion of MnO 6 octahedra, show that the resistance of the films also decreases when the tensile strain is reduced, indicating that the resistance change arising from the reduction in Mn-O bond length dominates over that arising from the reduction in Mn-O-Mn bond angle.