Ferroelectric poling and converse-piezoelectric-effect-induced strain effects inLa0.7Ba0.3MnO3thin films grown on ferroelectric single-crystal substrates

Abstract

Using ferroelectric $0.67\text{Pb}({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}){\text{O}}_{3}\text{\ensuremath{-}}0.33{\text{PbTiO}}_{3}$ 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 ${\text{La}}_{0.7}{\text{Ba}}_{0.3}{\text{MnO}}_{3}$ (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 $(\ensuremath{\delta}{\ensuremath{\epsilon}}_{xx})$ 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 ${\text{CaMnO}}_{3}$ thin films, for which there is no JT distortion of ${\text{MnO}}_{6}$ 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.