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

Abstract

Using ferroelectric 0.67Pb(Mg[sub 1/3]Nb[sub 2/3])O₃-0.33PbTiO₃ 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[sub 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[sub C]. The relative changes of the resistance and T[sub C] are proportional to the induced reduction in the in-plane tensile strain (δε[sub xx]) in the film. The reduction in the in-plane tensile strain leads to opposite effects on MR below and above T[sub C], namely, MR is reduced for T<TC while MR is enhanced for T>T[sub 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.