Ferroelectric poling and converse-piezoelectric-effect-induced strain effects in La0.7Ba0.3MnO3 thin films grown on ferroelectric single-crystal substrates

Using ferroelectric 0.67Pb(Mg1/3Nb2/3)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 (MR) of La0.7Ba0.3MnO3 (LBMO) thin films. In situ x-ray diffraction measurements indicate that the ferroelectric poling (or the converse piezoelectric effect) induces a substantial reduction in the in-plane tensile 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 (delta epsilon(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 < T-C while MR is enhanced for T>T-C. We discuss these strain effects within the framework of the Jahn-Teller (JT) electron-lattice coupling and phase separation scenario that are relevant to the induced strain. Similar studies on CaMnO3 thin films, for which there is no JT distortion of MnO6 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.