Study On Preparations And Magnetic Properties Of Manganites With Perovskite Structure

The discovery of magnetoresistance in metal multiplayer, magnetic tunneling junction and manganese perovskite oxides has aroused many attentions recent years. Many works focus on the doped rare-earth manganate for their huge value of magnetoresistance and various magnetic and electric properties. However, there are still a lot of problems need to clarify about the magnetoresistance mechanism. In this thesis, the main research works focused on the influence of B-site substituted by various transition metal neighbors with Mn ionic on the magnetic and electrical properties for the perovskite manganate. Also, the effect of A-site doping on the physic magnetic properties is studied. We hope to understand the action mechanism and influence factors on physical properties in the family compounds more clearly through ions doping.We choose La_0.7Ca_0.3MnO₃ as parent compound to investigate the effect on the magnetic and transport properties of Mn replaced by Cr, Ti, Fe and Ni ions. In the case of Cr doping, the magnetization progressively increases with x. It indicates that the Cr³⁺ ion has the same electronic configuration with Mn⁴⁺(t_2g³). In others case of doping Ti⁴⁺, Ni²⁺ and Fe³⁺, there is the same change trend in the magnetic properties although their exchange interaction between Mn ion is difference. It is found that the resistivity were enlarged in all doping ions for La_0.7Ca_0.3MnO₃ (M= Cr, Ti, Fe and Ni). The fact was found that the crucial effect of Mn-site doping is the increase of Mn-site disorder and the dilution of the double exchange by comparing with their difference of electrical and magnetic properties. Therefore, the exchange interaction between doping ions and Mn ion doesn't play an important role.There are few of reports about the n=3 layered perovskite compounds. But because of the changeable in crystal structure and the need of studying magnetic transport mechanism in theory, the magnetic and electrical properties of Bi³⁺ doped Ca_4-xBi_xMn₃O_10-d and A-sit vacancy La_3-xCaMn₃O_10-d family were investigated in detail. It is found that the magnetization increase with x in x=0.2, and then decrease with x in x>0.2. This is the process of the magnetic structure vary from G-type AFM for x=0 to C-type AFM for x=0.4. this can be ascribe to the powerful superexchange between Mn ionics. In the family of La_xCa_4-xMn₃O_{10 -d} (x=0～0.3), it is found that the results is similar between A-site vacancy doping and other bivalent metal doping. Both can lead to ferromagnetic (FM) increase. Meanwhile, there is slight flexibility in three-layer perovskite, so MR do not inceasing with the degree of A-site vacany.We synthesize the La_0.7Sr_0.3MnO₃ nanorod with length 50μm through a simple solid state method. It is found that the Tc decrease from 360K of tradition solid state way to 240K. We obtain the La_0.7Sr_0.3MnO₃ nanoparticle, which is crystal well, by combining the solid state method and molten salts way.