Crystal Structures, Electrical And Magnetic Properties Of Mg-doped Perovskite Manganites

The discovery of magnetoresistance in metal multiplayer, magnetic tunneling junction and manganese perovskite oxides has aroused much attention 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 about the magnetoresistance mechanism which need to be clarified.In this thesis, the main research focused on synthesis, characterization and investigation of the crystal structure, the magnetic and electrical properties of the cubic perovskite, double perovskite and layered perovskite manganate doped by magnesium in A-site or B-site. We hope to understand the action mechanism and influence factors on physical properties in the compounds family more clearly. The Mg-doped perovskite has not been well-studied so far. We took the cubic perovskite as an object to make a study on Mg-site.The result showed that Mg ions could take both A-site and B-site. In the case of A-site, pure La_1-xMg_xMnO₃ could be gained when x<0.24 in atmosphere;MgMn₂O₄ was observed when x>0.33. In the case of B-site, the Mg-doped sample could be got only when x≤0.40 with the temperature above 1350℃.The value of tolerance factor (t) is only 0.79-0.85 in A-site and 0.85-1.07 in B-Site. It seems that thermodynamics plays a leading role in the synthesis process of the Mg-doped sample. The result shows that it is easier for the Mg-doped sample to form stable compounds in A-site which is twelve-liganded than in B-site which is eight-liganded. But when it comes to the layer-structured perovskite with Mg in both A-site and B-site, the Mg-dopde layered structure could not be obtained even at 1450℃.The experiment result of double perovskite is not ideal either, only the impure mixture obtained both under the condition of pure H₂ and H₂+Ar.It is considered that the Mg ions with high charge and small ion radius play an important role in the synthesis process.We substituted Mg for Mn in the LaMnO₃ compound. LaMn_1-xMg_xO₃ single phases can be were.formed up to x=0.5.Structural and electronic properties were studied by means of x-ray diffraction and SQUID. The samples with low Mg content showed a ferromagnetic state while samples with x>0.8 undergo a spin-glass transition at low temperatures. This implies the presence of competitive interaction between FM and AFM clusters. The spin-glass transition developed at lower temperatures as the Mn content decreased in agreement with a diminution of the number and size of magnetic clusters.When x>0.08, doped sample showed spin-glass state. FM cluster and AFM cluster co-existed in the sample at low temperature. The competition of DE influence of FM with SE influence of AFM decided the character of the sample. It was in spin-glass state when Mn⁴⁺/Mn³⁺≥0.125 existed in B- site doped ions.We investigated the IR spectrum of the samples and got the following results: the v₄ (vibration absorb peak ) of the MnO₆ octahedron was strongly changed by Mg-doped,and was split to two peaks when x≥0.24(A) or 0.28(B). the influence of Mg-doped in perovskite is slight to La_1-xMg_xMnO₃ than LaMn_1-xMg_xO₃.Mg dopant doesn't influence the wavenumbers of the v3 peak of the MnO₆ octahedron in La_1-xMg_xMnO₃. The same phenomenon was observed when v3 peak was studied in LaMn_1-xMg_xO₃...