Effects of substrate-induced strain on transport properties of LaMnO3+δ and CaMnO3 thin films using ferroelectric poling and converse piezoelectric effect

We have investigated the effects of the strain induced by ferroelectric poling (or the converse piezoelectric effect) on the transport properties of LaMnO3+delta and CaMnO3 thin films grown on ferroelectric 0.67Pb(Mg1/3Nb2/3)O-3-0.33PbTiO(3) single-crystal substrates. The ferroelectric poling of the substrate gives rise to a reduction in the in-plane tensile strain of the LaMnO3+delta film, which results in a significant decrease in the resistance and increase in the insulator-to-metal transition temperature T-P of the film. The ferroelectric poling also leads to opposite effects on the magnetoresistance (MR) below and above T-P, namely, MR is reduced for T < T-P while MR is enhanced for T > T-P. These strain effects are explained in terms of coexisting phases whose volume fractions are modified as a result of the reduction in the Jahn-Teller (JT) distortion due to ferroelectric poling. An investigation of the effects of the strain induced by the converse piezoelectric effect on the resistance of the LaMnO3+delta and CaMnO3 films shows that the resistance-strain coefficients (Delta R/R)/(Delta c(film)/c(film)) of the LaMnO3+delta film is much larger than those of the CaMnO3 film. This result may imply that the strain-induced modification of the electronic bandwidth alone cannot account for the large (Delta R/R)/(Delta c(film)/c(film)) observed in the LaMnO3+delta film, and highlights that the strong coupling of charge carriers to JT distortion is crucial for understanding the effects of the substrate-induced strain in manganite thin films.