| In this thesis,the charge ordering(CO), magnetic-electronic correlation of the perovskite manganites, and electron paramagnetic resonance spectrum(EPR), following are the main results:Cr-doping system with peroviskite structure have been synthesized by solid-state reaction method. By the magnetization measurement, it is found that, without Cr doping sample,experiences a transition from antiferromagnetic state to paramagnetic state and another transition from charge ordering state to out of order state, corresponding temperature are 57K and 255K. The Arrott curve shows that the sample have magnetic prdering under the temperature 60K and 150K. The sample doped with Cr reveals that just a little Cr element can induce remarkable ferromagnetism, the M-H curve under the temperature 150K shows that low ferromagnetism coexist with paramagnetism property in the sample, this phenomena illuminates two states coexist in the sample. Under the temperature 10K, very strong ferromagnetism exist in the sample, from this we can conclude that below the temperature 57K, ferromagnetism coexist with insulation in the sample. Also the CMR of the system increases enormously by doping.Also, La0.75Ca0.25Mn1-xCrxO3(x=0,0.02,0.05,0.1) ceramic samples are prepared by the conventional solid-state reaction method. The microstructure is analyzed by X-ray diffraction, and we measured its magnetic property. The X-ray diffraction shows that the sample has a tetragonal preovskite structure. The ESR indicates that doping has no effect on the value of Lande factor, and the majority of the overall magnetic dipole moment is determined by the spin magnetic dipole moment. The magnetization curves reveal that the sample has the spin canting state, and doping can weaken or destroy this state. The M-T curves prove that high doping in B site can lead to the distortion of lattice, which reduces the magnetization. |
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