Structural, Magnetic And Electrical Transport Properties Of Perovskite Manganites

The manganite with a perovskite structure has been attracted a lot of attention in condensed matter physics and material physics since the discovery of the colossal magnetoresistance (CMR) phenomenon, not only because of the potential applications of CMR effect in magnetic field sensors, magnetic reading heads and magnetic random access memories, but also because of many intriguing physical properties observed in this type of oxide, such as the insulator-metal transition induced by the applied magnetic field, the electronic/structural phase separation. In this thesis, we have investigated the structural, magnetic and electrical transport properties of manganites through doping on the A- and B-site in the perovskite manganites. The main results are generalized as follows:We have prepared the ferromagnetic insulating manganite Nd_0.85Na_0.15MnO₃ polycrystalline sample by Pechini process. The magnetic researches indicate that the short-range ferromagnetic order and paramagnetic state coexist around the Curie temperature (TC). The electron spin resonance (ESR) lines at various temperatures demonstrate the dynamic process of the phase separation phenomenon with decreasing temperature. In the higher temperature range, the temperature dependence of ESR parameters and electrical resistivity are discussed at the frame of the adiabatic small polaron hopping model, and the activation energies related to the creation of the polarons and activating the hopping of the polarons were obtained.Ac susceptibility measurements have demonstrated that polycrystalline Fe-doped manganite LaMn_0.7Fe_0.3O₃ is a typical cluster-glass-like state in the low temperature range. Combined with all results, we conclude that magnetic clusters, contributed from ferromagnetic interaction between Mn³⁺ and Mn³⁺/Fe³⁺ ions, develop as temperature approaches the ordering temperature, and compete with antiferromagnetic interaction between Fe³⁺ ions in character of cluster-glass-like state in lower temperature. We have also performed a series of measurements to study the low temperature dynamics of LaMn_0.7Fe_0.3O₃ sample. The results, obtained in the zero-field-cooled and field-cooled dc magnetization and magnetic relaxation, demonstrate striking memory effects. It was also found that, in the aging experiments, there are symmetrical response on negative and positive temperature change around 30 K, and there are not symmetrical response around 61 K, representing two different non-equilibrium states of this system.The polycrystalline CaMn1-xFexO3 (0≤x≤0.35) phases adopt an orthorhom- bically distorted perovskite structure. The lattice expansion and weakening antiferro- magnetism are observed with increasing Fe concentration. Magnetization reversal is observed in the field-cooled magnetization curves under the field of 1 kOe for x = 0.80 and 0.10, which can be attributed to a spin-canting arrangement in the antiferro- magnetic matrix. For x≥0.15, the negative magnetization phenomenon disappears, and ferromagnetic component coexists with antiferromagnetic one, but antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show the insulating behavior in the studied temperature range for all compositions. Fe doping, even at a level as low as x = 0.02, causes a marked resistivity increase, which can be ascribed to the disordered arrangement of transition metal cations. Further increasing Fe content causes resistivity to gradually decrease due to the increasing carriers.