| Manganese oxides with peroveskite structure have been the subject of intense study in recent years due to the complex interaction between electrons, lattices and spins. Specially, the potential technological application related to the colossal manganetoresistance (CMR) effect has activated the interesting on these materials. However, there are many problems in the aspects of the MR at room temperature and temperature stability of MR, which severely limit their practical applications. It is found that when we sum up the experiment study of the manganetoresistance (MR) effect, The low field high manganetoresistance can be most probably achieved in La-( Ca,Ba,Sr )-Mn-0 materials. In order to enhance its MR, Na, K, Ag or vacancy was doped in the matrix compound La0.60Sr0.40MnO3. Their phase structure, magnetic and electrical properties were investigated in detail.In this thesis, La0.60Sr0.40-r-yNax□yMnO3(0≤x≤0.35, symbol "D"represents ion vacancies) , La0.60Sr0.40-r-yNax□yMnO3(0≤x≤0.35)and Agδ-La0.60Sr0.40-xAgx-δ□δ(MnO3 (0≤x≤0.30) samples were prepared by sol-gel method. Their phase structure, magnetic and electrical properties were investigated in detail. Several interesting results have been gotten:1. The phase identification of samples has been carried out by the X-ray diffraction (XRD) with a Rigaku max-RB diffractometer with Cu Kα radiation. The results indicate that the main phase of these three serieses of sample is rhombohedral perovskite structure. A small quantity of Mn3O4 phase appears When x, the Sr2+substituted content by Na+ or K+ ions, is bigger. A small quantity of Ag phase phase appears when Ag was doped in the matrix compound, and x≥0.15.2. Under an applied filed of 1.8T, a maximum room temperature MR of 24.4% is obtained at 292K for the compound La0.60Sr0.07Na0.33MnO3 in all the samples. The MR peak value increased 3 times more than La0.60Sr0.40MnO3 (MRp=6A%, TMR=373K), and the MR peak is moved to room temperature.3. The MR value of the sample La0.60Sr0.25Na0.15MnO3 keeps in a of 3.9%(±0.2%) in the temperature range between 285 K and 345 K under an applied filed of 1.8 T. Up to now, there is no similar report about such good temperature stability in such wide temperature range, had been seen. It will be important for the application of the CMReffect in the future.Moreover, there is a stable MR value of 4.7% (±0.2%) under an applied filed of 1.8 T, for bulk sample Lao.6oSr0.2oKo.i7noo3Mn03 and Lao.6oSro.2oKo.i50o.o5Mn03 in a temperature range between 220 K and 320 K4. Comparing the three series of sample, we found that:The first, for the three series sample, although the radius of Na+ and Ag+ ions is smaller than Sr2+ ions and the radius of K+ ions is bigger than St1+, the crystal cell volume increased approximately linearly with x for x>0.15 and the slope of the curves are increased successively for Na^ K> Ag. Up to now, there is no similar report had been seen.The second, the Curie temperature 7c, metal-semiconducting transition temperature 7mi and the temperature 7mr corresponding maximum value of MR of the three series sample, decreased with the increasing of jc when x>0.\5.And the third, The resistivity peak values change tendency with x are consistent for the three series of sample. The resistivity peak values successive increased for most of the experimental data. Furthermore, the change of MR peak values with x, for the three series of samples, have the same tendency, but the peak values of MR successively decrease for Na> K> Ag.In conclusion, the MR at room temperature and temperature stability of MR can be enhanced by doping Na+;K+ or Ag+ ions and ion vacancies for compound... |
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