The Study Of Magnetism And Superconductivity In Perovskite Oxides

The studies of oxides with perovskite structure started in 1950’s. In recent years, perovskite oxides have been a hotspot and attracted much attention due to the exciting discovery of high temperature superconductivity in cuprates and CMR (colossal magnetoresistance) effect in magnanites. There are fruitful physical properties in perovskite oxides, such as ferroelectricity, ferromagnetism, piezoelectricity and superconductivity, etc. These properties and their coupling effects inspire studies on them and investigating the working mechanism behind. Besides that, perovskite oxides also have great applied potential in many fields, such as storage, sensor and catalysis, etc.In the first chapter, a brief overview of perovskite oxides is introduced. Their structures and some fascinating properties are also mentioned. And then we mainly focus on two kinds of perovskite oxides, the perovskite manganite and the defective layered perovskite cuprates. The effects caused by different kinds of doping are also discussed.In others chapters, we have performed the study of some perovskite oxides. The samples are prepared and their structure, electrical and magnetic properties are investigated. The major results are:1. LaMnO3-PbTiO3 solutions.Single phase Pb1-xLaxTi1-xMnxO3 (PLTM100x, x=0.20,0.40,0.50,0.60, and 0.80) ceramics have been prepared and investigated. LaMnO3 (LMO) is an A-type antiferromagnetic insulator, while PbTiO3 (PTO) is a ferroelectric insulator. It is believed that the introduction of Pb2+ in A-site may bring local Mn4+thus local Mn3+-O2-Mn4+ferromagnetic interaction, whereas the introduction of diamagnetic cation of Ti4+ in B-site may dilute and disorder the magnetic interaction between magnetic cations, both will have effects on transport and magnetic properties.X-ray diffractions show all ceramics are crystallized into perovskite structures with tetragonal symmetry for PLTMO20 and orthorhombic symmetry for the other compositions. And the lattice parameters decrease with the incresing x. X-ray photoelectron spectroscopy results confirm that the Mn cations in PLTMO20 and PLTMO40 have the valence state of +3, whereas in other compositions, the Mn cations have mixed valences states of +3 and+4. With increasing x from 0.20 to 0.80, the materials change from superparamagnetic to coexisting ferromagnetic and antiferromagnetic magnetic behaviour whereas all compositions persist insulating behaviour. Especially, the PLTMO80 has a large negative magnetoresistance of-41.5% at 10 K with the applied filed of 2 T.We believe these properties are attributed to the coexistence of metallic ferromagnetic and insulating non-ferromagnetic phases.2. Some preliminary work on high Tc superconductive cuprate films(1) SrCuO2-modified LausSr0.15CuO4.Single phase ceramics of the compositions expressed as (1-y)La1.85Sr0.15CuO4+y SrCuO2 (y=0.00,0.02,0.04,0.06) are prepared. Based on the works on ceramics, La1.85Sr0.15CuO4 (y=0) and La1.83i5Sr0.1585CuO3.98(y=0.02) films are prepared by pulsed laser deposition (PLD). The XRD results indicate the films are epitaxial growth with good crystalline quality. As we know, the oxygen vacancies play an important role in electrical properties. The films are annealed in flowing oxygen to reduce oxygen vacancies. The XRD results show that the lattice parameters increase slightly by annealing, which indicates that annealing is helpful to reduce oxygen vacancies. However, the electrical properties still need further optimization.(2) BaCuO2-modified YBa2Cu3O7YBa3Cu4O9 and YBa2Cu3O7 films are prepared. The XRD results indicate the films are epitaxial growth with good crystalline quality. And in some annealed samples, we observe the existence of superconductivity. Further works are underway.