Magnetic And Electrical Transport Properties Of Doped Compounds With Perovskite Structure

Perovskite-type compounds (such as double perovskite oxide La2CoMnO6, organic perovskite CH3NH3PbI3) have exhibited rich physical properties. La2CoMnO6,with high Curie temperature, colossal magnetoresistance effect, and magnetic dielectric effect, has a great potentiality in the application of spintronics devices. CH3NH3PbI3 is a novel material applied in solar cells. As an effective method, ions doping and substituting can be used to control and improve the physical properties of perovskite. In this dissertation, we selected several types of ions and studied the doping effects on the magnetic and electrical properties of La2CoMnO6 and CH3NH3PbI3. XRD,SEM and PPMS were used to characterize the physical properties of these materials. The obtained main results are shown as followed.1. Magnetic and electrical transport properties of La2-xCaxCoMnO6. Polycrystalline La2-xCaxCoMnO6 (01 ? x? 0.5) samples were synthesized by the solid-state reaction. The crystal structure has been confirmed to be monoclinic distorted perovskite structure with space group of P21/n at room temperature. Ca2+-substitution could lead the change of the Mn/Co ions valences. Different magnetic phases are coexistent in La2-xCaxCoMnO6 system.The Curie temperature and coercive field are decreased by Ca2+ substitution, while the.exchange bias effect is enhanced. La2-xCaxCoMnO6 can exhibit the negative magnetoresistance effect and semiconducting-like behavior with 3D VRH model. The negative magnetoresistance effect may be caused by the suppression of spin-fluctuations. At low temperature (T < 200 K), small amount of Ca2+-substitution (x ? 0.2) may enhance the magnetoresistance effect in La2-xCaxCoMnO6 system. The negative magnetoresistance effect of La1.8Ca0.2CoMnO6 can be up to 58% measured at 105 K under 7 T. These results may provide the experimental evidence for enhancing the magnetoresistance effect of Lanthanide-Manganese base double perovskite materials.2. Magnetic and electrical transport properties of La2Co1-xMnO6 (B = Fe, Cu).Polycrystalline La2Co1-xFexMnO6 (0 ? x ? 1.0) and La2Co1-xCuxMnO6 (0 ? x ? 0.5) samples were synthesized by the solid-state reaction. The crystal structure of the samples with Fe3+ or Cu2+ substitution is orthorhombic perovskite structure with space group of Pbnm at room temperature. The random distribution of the Fe3+ ions destroys the order of the samples, which results in the decreasing of Curie temperature and coercive field. The substitution of Cu2+ may lead to the multiple valence states of Co/Mn ions, which can further control the magnetic properties of La2Co1-xBxMnO6. The appropriate Cu2+-substitution (x = 0.3) can increase the coercive field. The increasing of Fe3+ or Cu2+ substitution gradually transforms the electrical conduction model from 3D VRH model to TA model. The negative magnetoresistance effect of La2Co1-xBxMnO6 (B = Fe, Cu) is spin-dependent. Below 150 K, the magnetoresistance effect can be enhanced in Fe3+-substitution sample x = 0.1. Under low fields (H < 5 T), small amount of Cu2+-substitution (x ? 0.1) can enhance the magnetoresistance effect. The results provide valuable evidence for controlling and adjusting the magnetic and electrical properties of double perovskite materials by B-site substitution.3. Magnetic properties of CH3NH3Pb1-xBxI3-2xClax(B = Mn/(Mn and Co)),Polycrystalline CH3NH3Pb1-xBxI3-2xCl2x (B = Mn/(Mn and Co)) samples with orthorhombic structure were synthesized by the solvent evaporation method. The samples with magnetic ions substitution exhibit weak ferromagnetism at 5 K,which may result from the direct interactions between Mn2+ and Co2+ ions. The sample with Mn and Co co-doped exhibits exchange bias effect. Our studies can provide experimental evidence for the research on organic perovskite with magnetic ion substitution, and become the evidence for exploring novel magnetic organic perovskite materials in the future.