Perovskite Structure Multiferroic Film Preparation And Performance Research

New optical, mechanical, magnetic and electrical devices tend to miniaturization, thin-film, and multifunctionality with the development of modern electronic information technology. Materials with single functionality are more and more difficult to meet the mentioned requirements, so the multifunctional multiferroic materials attract much attention due to their interesting properties and potential applications.In this thesis, two kinds of multiferroic thin films, i.e. multiferroic perovskite heterostructures and single phase BiFeO3films, were prepared by pulsed-laser deposition and sol-gel method, respectively. For perovskite heterostructures, the morphology, crystalline structures, magnetic properties, electrical properties, epitaxial relationship and effects of applied magnetic and electric fields on the physical properties, and the magnetoelectric coupling effect have been investigated. For single phase BiFeO3films, the effects of annealing temperature, annealing time, bias voltage, and the light illumination on the current leakage behaviors of BiFeO3were investigated by the conductive atomic force microscopy, and the current-voltage characteristics, respectively. The leakage mechanism was analyzed and the method how to control the leakage current was proposed. The main content and conclusions of this thesis are summarized as follows.(1) Epitaxial ferromagnetic films (La0.7Sr0.3MnO3, La0.7Ca0.3MnO3), and ferroelectric films (Pb0.52Zr0.48TiO3, BaTiO3) were successfully grown on single crystalline (001) SrTiO3substrates by pulse laser deposition (PLD), respectively. Subsequently, several ferromagnetic/ferroelectric or ferroelectric/ferromagnetic heterostructures were prepared. Among them, PZT/LSMO heterostructures shows good epitaxial relationships with SrTiO3substrates. The results of magnetic and electric properties show that even under a very small polarized voltage of0.4V, the maximal resistance change of LSMO at90K is as large as45.5%; however, the polarized electric field has little influence on the magnetization, and the magnetic field has also an obvious influence on the ferroelectric behavior of the PZT layer.(2) The polycrystalline BiFeO3(BFO) thin films were grown on Pt/Ti/SiO2/Si substrates by sol-gel method. The current leakage behaviors were investigated by both the conductive atomic force microscopy and the current-voltage characteristic measurements. The results show that with increasing annealing temperature and time, the structure of BFO films changes from the amorphous to crystalline phase and the charge transport pathways appears at the grain boundary. The amorphous phase can act as a barrier layer to hinder the charge transport and the leakage current density of the films with amorphous layer decreases by two orders of magnitude as compared with that of the polycrystalline BFO film. It is found that the local charge transport pathways are mainly located at the grain boundary of the films and the leakage current density can be tuned by the annealing temperature, the annealing time, the bias voltage and the light illumination. A possible leakage mechanism was proposed to interpret the charge transport in polycrystalline BFO films and a method was proposed to decrease the leakage current density of the BFO films.