Preparation And Structure Of Lead/barium-based Perovskite Ferroelectric And Multiferroic Thin Films

Due to the excellent dielectric,piezoelectric and ferroelectric properties,ferroelectric thin films are widely applied in memories,sensors,micro-mechanical systems,etc.The multiferroic films,with ferroelectric and magnetic orders,have expanded the applications in multifunctional devices.In this thesis,two typical perovskites,PbTiO3 and BaTiO3,are chosen as the subjects.Through substitution or strain engineering,the excellent ferroelectric and multiferroic films are deposited The connection of the structure(lattice structure and electronic structure)and the properties(ferroelectric,magnetic and magnetoelectric properties)is studied.Through the substitution of Sm and Fe,the(1-x)PbTiO3-xSmFeO3 films are deposited by sol-gel method.The films have robust ferroelectric polarization with considerable ferromagnetism.Through the synchrotron XAS and XPS,the Fe ions are confirmed to be mostly as Fe3+ with low oxygen vacancy concentration,which benefits the low leakage current and ferroelectric hysteresis loop.The fatigue failure is improved by using the metal oxide LaNiO3 as the bottom electrode.Through the high-temperature PFM,ferroelectric hysteresis loop measurements,XRD,etc,the temperature-dependent behavior of the defect dipoles is investigated.The thermal motion of the defect dipoles could match the velocity of the polarization switching and induces the increasing leakage current beyond 150OC,which is demonstrated by the temperature-dependent ferroelectric hysteresis loops.The high-temperature XRD and Raman indicate no significant impact of the defeet dipoles on the lattice orientation and the vibration,which means that the defect dipoles are not long-range ordered.The defect dipoles tend to reorient from in-plane direction to the out-of-plane direction at around 150? and further induce the reorientation of spontaneous polarization,which is confirmed by the PFM images.This gives the horizon of achieving the reversal of spontaneous polarization through defect engineering without external electric field.Large remanent polarization(100 ?C/cm2)is achieved in BaTiO3-based thin films through the interphase strain,with dramatically increased Curie temperature from 130? to higher than 1000?.The "interphase" structure is observed by STEM,which demonstrates the coherent lattice structures of BaTiO3 and BaO.The interfacial BaTiO3 behaves larger Ti displacement,which guarantees the enhanced polarization of the composite film.The XAS spectra give the evidences of the larger Ti displacement as well as the maintenance of the Ti-O hybridization.Through interphase strain,the BaTiO3:CoFe204 composite films are designed.The XRD and RSM confirm the out-of-plane lattice parameters of BaTiO3 and CoFe2O4 are perfectly coherent,while the in-plane lattice parameters are more relaxed.The coherence of BaTi03 and CoFe204 would enhance the magnetoelectric coupling.The converse magnetoelectric coupling is determined in the assistance of MFM.Moreover,the distribution of Co and Fe in the octahedral and tetrahedral sites is reassigned,which would change the magnetic structure of CoFe2O4.The modified CoFe2O4 structure proves the promising application of interphase strain in the magnetic and multiferroic materials.