Electronic, Magnetic Properties And Infrared Absorption Of Perovskite Manganites La_0.67-xR_xCa_0.33MnO₃

In this thesis, perovskite manganites La_0.67-xRxCa_0.33MnO₃ (R=Pr,Y,Ho,Gd,Dy) with colossal magnetoresistance effect have been investigated systematically. The main results are generalized as follows:In the polycrystalline samples La_0.67-xPr_xCa_0.33MnO₃ (x=0⁰.67), we studied the temperature dependence of magnetization and resistivity in zero-field-cooling (ZFC) and field-cooling (FC), respectively. It was found that the coexistence of ferromagnetism, charge-ordering and antiferromagnetism in the compounds with increasing the Pr doping content, and the corresponding transfer temperature varies regularly. The samples occur insulator-metal transition with decreasing temperature as x<0.4, on the other hand, when the doping content increases up to 0.5 or more, the resistivity of the samples increases monotonously. It was also found that an abrupt change of the resistivity occurs near TCO, and the field will suppress the occurrence of charge-ordering, pull the I-M transition to higher temperature. It was found that the different model should be used to simulate the resistivity in different temperature scope, and the results of simulation and measurement match each other very well.Base on the analysis of Infrared absorption of the samples, the absorption peak at ⁵30cm-1 was found and it should be some relations with the ferromagnetic features of the compounds. The stretch-mode absorption peak near～620cm^-1 was investigated thoroughly, and found that the peak split into two peaks with the gap about～45cm-1 due to phase separation in the compounds. And the effective carrier number obtained from the infrared absorption spectra changes with temperature, according to the transition of FMM-PMI.With the same radii, Ho³⁺ and Y³⁺ were chosen to substitute La³⁺ in La_0.67Ca_0.33MnO₃. Many novel features of electronics and magnetism, such as the phase separation of FM and AFM, were found. It was also found that the magnetization of Y-doping compounds is lower than that of Ho-doping, and on the contrary, the resistivity of Y-doping is higher than that of Ho-doping. These features should ascribe to the coupling of the ion with moment. The STM peak in infrared spectra also split into two peaks due the phase separation.By the investigation on the Dy/Gd-doped La_0.67Ca_0.33MnO₃, it was found that the coexistence of spin glass cluster with the ferrimagnetism. And the compensation temperature was found in the M-T curves when x=0.2, 0.3, we has interpreted with the sublattice model. The metal-insulator transition occurs in the samples with x<0.2, and the variable range hopping (VRH) model and the polaron model can be used to simulate the resistivity in different temperature scope. Infrared spectra analysis indicates that the STM peak keep its shape, position stably without splitting and the peak at～530cm^-1 weakened and even disappeared in these compounds.