Studies Of Microstructures, Magnetic And Electrical Transport Properties In Perovskite Manganites La_1-xAE_xMnO₃（AE=Ca, Sr, Pb）

The La1-xAExMnO3(AE=Ca, Sr, Pb) manganites show a transition of ferromagnetic-paramagnetic near Tc (Tc is a Curie temperature), and is accompanied by a transition of metal-insulator in conductivity. It shows huge change in the resistivity on application of magnetic fields at near Tc, called as colossal magnetoresistance (CMR). Magnetoresistance displays potential applications such as read heads, random access memory and sensor, etc.Close correlation is discovered between magnetic and conductivity properties in the La1-xAExMnO3 compounds and the interaction mechanism are very complex. In this paper, a systematic study was carried out in La1-xAExMnO3, which was substituted in A sites by Ca, Sr or Pb in parent LaMnO3 compound. La1-xAExMnO3 compounds with different content of dopant were prepared by conventional solid-state reaction method and optical floating zone method. The crystal structures, microdefects, electron density and d electron signal of the samples were characterized by means of XRD, positron lifetime and coincidence Doppler broadening techniques. Magnetic properties and electrical properties of the samples at various temperatures were measured by means of a Physical Property Measurement System (PPMS-9). The electronic structures of La1-xAExMnO3 were calculated by the first principles of plane wave ultra-soft pseudo-potential technology based on density function theory. The connection of structure, magnetic properties and electrical properties were systematically studied. The main conclusions are summarized as follows.Investigations of the La1-xCaxMnO3 samples prepared by conventional solid-state reaction method show that ferromagnetism might be the result from the combination effects including the body and surface magnetic phase. The competition between ferromagnetism and antiferromagnetism appear below Tc, separation of the M-T curves measured under the condition of ZFC and FC can be observed in all La1-xCaxMnO3 samples, which is the characteristic of spin-cluster glass. Antiferromagnetic regions increase in a ferromagnetic background of the majority phase in LaMnO3, which leads to the magnetization being unsaturated. Magnetic moment of the LaMnO3 sample is less than that of all La1-xCaxMnO3 samples. With increasing in the content of Ca, the enhancement of the ferromagnetic double-exchange interaction gives rise to the improvement of the saturation of the hysteresis loop. Simultaneous, small magnetic particles formed in the system would lead to the disorder of the magnetic moment, and decrease the magnetic moment significantly below Tc. For samples with x<0.46, the coupling of the grain interface is mainly manifested by the antiferromagnetic order or paramagnetic order, showing the characteristics of antiferromagnetic clusters and insulator of electricity. For sample with x=0.46, the disorder of the coupling of the grain interface is relatively high, and the ferromagnetic coupling play the main role at the grain interface, the sample showing the characteristics of conductor like a metal, and the magnetoresistance of this sample at low temperatures(T<TC) is larger than that of the other doping samples. Positron lifetime spectra of the La1-xCaxMnO3 samples show that the defects in the samples are mainly the La3+ vacancies and the grain boundaries, the open volume of cationic defects and the concentrations of defects decrease with the increase of the content of Ca. The free electron density for the sample with x=0.46 is higher than that of other doping samples, its resistance is less than other samples. Conincidence Doppler spectra of the La1-xCaxMnO3 samples show that with increasing in the contents of Ca in the samples, the localization degree of d electron decrease, while the double-exchange interaction increase. ESR measurements of the La1-xCaxMnO3 samples showed that the g factor and the linewidth T decrease with the increase of the content of Ca. This means that the decrease of spin-lattice interaction will lead to the increase of electron spin magnetic moment. The concentrations of defects in the La1-xCaxMnO3 samples prepared by optical floating zone method are much less than that by solid-state reaction method. For samples with x<0.2, ferromagnetism plays the main role in the system, but there exist antiferromagnetic phase near Tc, and the antiferromagnetic cluster form below Tc. For samples with x≥0.33, ferromagnetic cluster phase exist in the sample near Tc. The reduction of the size of the magnetic particles will result in the increasing of magnetic disorder, and the frozen disorder appears at low temperature. The results of the measurements of the electrical transport properties of the La1-xCaxMnO3 samples showed that the resistivity of the sample prepared by optical floating zone method is less than that prepared by solid-state reaction method, while the intrinsic magnetoresistance of the sample prepared by optical floating zone method is much larger than that prepared by solid-state reaction method, for example, the intrinsic magnetoresistance of the La1-xCaxMnO3 sample prepared by optical floating zone method is 261% measured at a field of IT.Studies of the La1-xPbxMnO3 samples prepared by conventional solid-state reaction method indicate that the double-exchange coupling interaction in the grain and the Curie temperatures of the samples increase with the content of Pb in La1-xPbxMnO3 samples. Meanwhile, with increasing in the content of Pb, the magnetic order of the ferromagnetic particles increase, the non-intrinsic ferromagnetic coupling becomes strong, and the temperature range of ferromagnetic-to-paramagnetic gets wide. The magnetoresistance of the sample is closely related to the intergranular ferromagnetic coupling effect below Tc, with the increasing in the Pb content, the increasing in the order of intergranular ferromagnetic coupling gives rise to the decreasing of the magnetoresistance of the sample. At near Tc, with the increasing in the Pb content, the enhancement of the double-exchange coupling interaction leads to the decreasing of the magnetoresistance of the sample, and the range of the temperature corresponding to the peak of the magnetoresistance of the sample increases due to the enhancement of the order of the intergranular ferromagnetic coupling. Positron lifetime spectra of the La1-xPbxMnO3 samples show that for samples with x≤0.36, the mean lifetime decrease with the increase of Pb content, that is, the defect concentration decreases with the increase of Pb content. For the sample with x=0.36, the mean positron lifetime reaches a minimum value, which indicated that the eg electron density reach a maximum value in MnO6 octahedron and the electron-phonon coupling effect decrease. For the sample with x=0.46, the open volume of the defect is relatively large, and the structure of the sample is significantly changed. Coincidence Doppler broadening spectra show that the probability of positron annihilation with d electron reaches maximum for the sample with x=0.36, which indicates that the strong p-d hybridization results in the decrease of localized d electron. However, the probability of positron annihilation with d electron reduce for the sample with x=0.46 due to the distortion of the lattice by doping with high content of Pb.Investigations of the La1-xSrxMnO3 samples prepared by conventional solid-state reaction method demonstrate that the Curie temperature and the double-exchange ferromagnetic coupling interaction increase with the content of Sr in the samples. While the influence of extrinsic ferromagnetic coupling on double-exchange coupling decreases with the increasing in the content of Sr. For the sample with x=0.46, the cluster of extrinsic ferromagnetic coupling may be formed, and new magnetic phase presents in the sample, and the system transforms from spin glass to cluster-spin glass. The results of the measurements of the electrical transport properties of the La1-xSrxMnO3 samples show that the double-exchange interaction and the Curie temperature resistivity of the samples with x=0.3-0.36 are higher than that of the other samples, while the resistivities of the samples with x=0.3-0.36 are less than that of the other samples. For the sample with x=0.46, the seepage channel can not be formed in the sample due to the disorder of the interface magnetic moment, and the ferromagnetic insulating phase presents in the sample. The metal-insulator transition temperature (TP) shifts to low temperature, which is much less than the ferromagnetic transition temperature (Tc), and the resistivity of the sample is relatively large. With the increase in the content of Sr, the double-exchange couplings enhance while the intrinsic magnetoresistances of the samples decrease. Large magnetoresistance at low temperature (T<Tc) can be observed due to the increase of disorder of ferromagnetic particles. Positron lifetime spectra of the La1-xSrxMnO3 samples show that positron lifetimes in the defects of samples with x<0.33 are shorter than that of LaMnO3, that is, the J-T distortion become weak due to the doping of Sr. For samples with x>0.33, the formation of the cationic defects in the sample results in the distortion of the crystal, and thus increase the positron lifetime in the defect. Coincidence Doppler broadening spectra of the samples show that the probability of positron annihilation with d electron reaches maximum at x=0.33, which indicate that p-d hybridization enhance and the double-exchange coupling interaction increase, while the localized of the 3d electrons of Mn decrease. In regarding to the La1-xSrxMnO3 samples prepared by optical floating zone method, for samples with x≤0.33, the ferromagnetic coupling is long-range order, and the saturation magnetization of the sample is close to that of the Mn at T=10K, and the ferromagnetic cluster phase presents in the sample near Tc. For samples with x≥0.46, the disorder of ferromagnetic particles increases and the antiferromagnetism clusters present in the sample below Tc, while the saturation magnetization of the sample drastically decreases. The results of the measurements of the electrical transport properties showed that the intrinsic magnetoresistances of the La1-xSrxMnO3 samples prepared by optical floating zone method are much larger than that prepared by conventional solid-state reaction method, its value can reach 15% for Lao.67Sro.33Mn03, while the resistivity of the sample prepared by optical floating zone method is less than that prepared by solid-state reaction method.The calculation of the electronic structures of the La1-xAExMnO3 (AE=Sr, Ca) compounds show that the partial replacement of La by Ca or Sr in the lattice will increase the Mn-eg and O-p hybridization and decrease the localization of the d electrons, and thus enhance the ferromagnetism and conductivity of the compound. The decreasing in the p-d hybridization in LaMnO3 will result in the increasing in the localization of d electron. The bandwidth of the Mn-3d partial density of states in La0.67Sr0.33MnO3 is larger than that in La0.67Ca0.33MnO3. Indicating that the localization of eg electron in the former is weaker than that in the latter, that is, the eg-p hybridization in the former is stronger than that in the latter, which gives rise to the conductivity of the former being higher than that of the latter. The eg-p hybridization in La1-xAExMnO3 with La vacancies is weaker than that without La vacancy, which leads to the ferromagnetic couplings in the former being weaker than that in the latter.