| The discovery of intergrain magnetoresistance in Sr2FeMoO6 as high as 10% at room temperature (300K), due to it's potential teclrnological applications in magnetic recording, sensors as well as it's theoretical values, made a wide serial A2B払?6 double perovskite materials attracted considerable interests from the physical fields. Concentrating on the magnetoresistance (MR) phenomenon of double perovskite (DP), this thesis compared dou- ble perovskite materials with the explicitly studied ABO3 materials at first. Some achieved experimental and theoretical results are carefully reviewed and the common points underly- ing them are also discussed. Then La2NiMnO6 was chosen to be studied about it's electric, magnetic and MR properties. hi the first time, new double perovskite Ca2FeMnO66 and Sr2CrFeO66 are synthesized successfully. They both show the room temperature MR as high as 33% and 19.4%. What distinguished them with the former CMR materials is their PM character at the temperature when the MR shows. Main results of this thesis are shown as following: 1. The electronic and magnetic characters of double perovskite La2NiMnO6 Using the Sol-gel method, we prepared fine La2NiMnO6 samples superior than the traditional solid-state method making sample. The resistance of the sample dramatically increases when the temperature decrease which is similar to the semiconductors. Below 80K the MR effect is observed. The sample behaves as ferrimagnet under Tc~350K.The Curie temperature shows a little fluctuation with different samples made in various sintering processes. Their magnetization show time dependent characters somewhat similar to the spin glasses. 2. The MR effect of new DP Ca2FeMnO66 and its novel magnetic behavior Ca2FeMnO6 samples are made by the Sol-gel methods following with different temperature sintering. XRD analysis indicate a single phase of orthorhombic structure, where a0 5.6505 + 0.0006A,b0 7.6338 + 0.0008A,c0 5.3402 ?0.000SAfor the 1373K sintered sample. In the temperature range cooling by liquid Nitrogen, it is at the param- agnetic state, which is confirmed by the M-T curve and the Msbauer spectra. At 300K, the samples have the MR effects as high as 33% ,12% and 9.5%,where certain hysteresis as FM appear. Seemingly it is conflicted with the PM character observed under statistic magnetic measurements. The physical picture of nanoferromagnetic clusters induced by the electric current in PM matrix is given to interpret this confliction. It is supported by the non-linear I-V curve. After disposition under 70K, irreversible phase transition from PM to FM is observed in the 1373K sintered sample. The FM phase has coercive force 32500e and 45000e at 80K and 300K respectively, this coercive force is corespondent to the maxim of resistivity in the MR data obtained before the irreversible phase transition. Some fluctuations of magnetization are also observed in the M-T curve, they are similar to the spin glasses. Msbauer measurement shows the coexistence of different valence state of B site ions, which give the possibility of various valence B and B?ions combination states. Tins leads to many metastable states in Ca2FeMnO66 sample, it is the existence of many metastable states that make a spin glass. Experiments also show the temperature and the magnetic field have strong influences on the transition between these different metastable states. 3 The magneteresistan...
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