A Comparative Study Of Cu And Zn Doping In Mn Site Of La_2/3Ca_1/3MnO₃

In this dissertation, the effect of the doping in Mn site with Cu and Zn on the electrical transport and magnetoresistance effect of La2/3Ca1/3MnO3 is carried out. The main investigative results are as follows:The different magnetoresistance material is briefly reviewed. The advance in the study of perovskite manganites has been summarized. More attentions have been paid to the doping effect.Polycrystalline samples of nominal composition La2/3Ca1/3MnO3 were prepared with sol-gel method and solid-state reaction method. Experiments indicate that the sample fabricated by a sol-gel method following sintering treatments at 1450oC has the same electronic transport properties with the sample prepared with solid-state reaction method. As the samples prepared with a sol-gel method, the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with the sintering temperature decreased.Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xCuxO3 were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with Cu doping content increased the sintering temperature decreased. Especially when Cu doping content x exceed 3.5%, the samples have abnormal behavior and the resistivity increased dramatically. Unusual thermal hysteresis is observed in the x=4% samples. The thermal hysteresis disappears gradually with increasing of the sintering temperature.Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xZnxO3() were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the electrical transport of La2/3Ca1/3Mn1-xZnxO3 have a similar feature with the samples of La2/3Ca1/3Mn1-xCuxO3. A significant downward shift of I-M transition and a sharp enhancement in resistivity are present in the sample containing Zn as well as. The study of the conductive mechanism of samples sintered at 1100oC indicated that the Mott's variable range hopping model can simulate the experiment data very well when the doping content x>3.5%.A comparative study of electrical transport and magnetoresistance is performed for the samples of La2/3Ca1/3Mn1-xZnxO3 and La2/3Ca1/3Mn1-xZnxO3(x=4%). Experiments indicate that the MR effect near the insulator-metal (I-M) transition can be substantially enhanced through 4% Cu doping in La2/3Ca1/3MnO3. The large MR value can be obtained in a low magnetic field, which is different from the intrinsic CMR usually observed in perovskite manganites. The maximum MR with a value ~80% of that for H=0.3T is obtained in the sample prepared at 1100oC. Cu doping in La2/3Ca1/3MnO3 also causes a clear enhancement of the Intergrain Magnetoresistance (IMR). The largest IMR, with a value ~40%, for the 0.3T field, is observed in the doped sample sintered at 1000oC. Although the same behavior of transport and MR are observed in two samples, the MR peak near the I-M transition of the sample containing Zn is lower than the sample containing Cu. For the sample containing Cu, the maximum MR appearing near the I-M transition for the 0.3T filed have a value as high as ~80%. This is a sharp contrast with a low value ~30% for the maximum MR obtained in the sample containing Zn.