Dielectric And Magnetic Characteristics Of Lanthanum-Based Double Perovskite Ceramics

In the present thesis, the structure, magnetic and dielectric properties of lanthanum-based double perovskite ceramics were investigated, and the physical nature and structural origin of the dielectric relaxation were discussed thoroughly. Moreover, the possible multiferroic characteristics in these double perovskites were discussed.La2NiMnO6 adopted double perovskite structure, in which Ni2+ and Mn4+ were arranged in a rock-salt configuration. The ordered Ni2+-O-Mn4+ superexchange interactions gave rise to the high ferromagnetic Curie temperature of 275 K. One Debye type dielectric relaxation with an activation energy of 0.17 eV was observed below 300 K. The dielectric relaxation was ascribed to local polar region consisting of Ni2+ and Mn4+. No saturated ferroelectric hysteresis can be achieved in La2NiMnO6 ceramics, which might be caused by low sample resistance and/or weak correlation between dipoles in double perovskite structure.The electrode effect on the dielectric response in La2NiMnO6 ceramics was very limited, and the dielectric response of La2NiMnO6 ceramics primarily originated from the intrinsic polar regions of charge ordering, and the internal barrier layer capacitor effects also contributed to the low frequency dielectric response of the present materials.In double perovskite La2CoMnO6, the ordering of Co2+/Mn4+ and disordering of Co3+/Mn3+ contributed to three magnetic transition temperatures. La2CoMnO6 ceramics demonstrated thermally activated dielectric relaxation with an activation energy of 0.18 eV, which also arose from charge ordering of Co2+ and Mn4+. La2CoMnO6 ceramics exhibited large magnetodielectric effect at 285 K, indicating the strong coupling between magnetism and dielectric relaxation. However, no ferroelectric hysteresis can be detected for La2CoMnO6 ceramics, which might be related to the existence of disordering regions.As a counterpart of La2NiMnO6, La2MgMnO6 crystallized in ordered double perovskite structure with nonmagnetic Mg2+ instead, and exhibited mixed-valence structure of Mn3+ and Mn4+. La2MgMnO6 ceramics demonstrated dielectric relaxation nature with an activation energy of 0.33 eV between 250-400 K, which was attributed to the charge carrier hopping between Mn3+ and Mn4+. Meanwhile, the long range transportation of charge carrier rendered the conductivity to adiabatic small polaron hopping mechanism.In La2Ni(Mn1-xTix)O6 (x=0,0.2,0.4,0.6,0.8,1) system, substituting Ti for Mn did not change the crystal structure of P21/n, in which Ni and Mn/Ti were ordered. The magnetization and Curie temperature of La2Ni(Mn1-xTix)O6 (x=0,0.2,0.4,0.6, 0.8,1) ceramics decreased with Ti content till La2NiTiO6 being antiferromagnetic. Meanwhile, the dielectric constant and relaxation strength deceased with x till no dielectric relaxation can be observed in La2NiTiO6. Ti-substitution reduced the charge ordering regions, which induced the reduction of magnetism and dielectric relaxation simultaneously.