La <sub> 0.6 </ Sub> (srt) <sub> 0.4-y </ Sub> Mno <sub> 3 </ Sub>-xag (t = Na, K) The Temperature Stability Of The Bulk Magnetic Resistance Of The Composites Study

Recent years more and more attention has been attracted on the perovskite-mangnese oxides for the colossal mangnetoresistance (CMR) effect discovered in these materials. The potential applications of the CMR effect mainly include magetic reading and magnetic field sensor. Large amount of reseaches have been concentrated on the increasing of the low field magnetoresistance(MR) of these compounds around room temperature at present. Few studies have been developed on MR temperature stability(σ). However, based on the previous work of our group, we investigated the a further around room temperature in this thesis.The powder sample studied in the experiments were prepared by sol-gel method. AgNO₃ were doped into the powder and proceeded to form bulk composite. Three methods were used in this study : the first, AgNO₃ was doped in when the sol of the powder sample were prepared;the second, the powder samples prepared were mixed with AgNO₃, then the mixture was ground, pelletized and sintered;the third, the mixture of the powder samples and AgNO₃ was ground, sintered at 873K for 5 hours and sintered at 1073K for 10 hours, then, pelletized and sintered in the end. All the samples have been investigated on phase structure, electrical and magnetic properties and so on. Some results were achieved as following.1 .The phase identification of samples has been carried out by X-ray diffraction (XRD) with a Rigaku D_max-RB diffractometer with Cu K_α radiation.It was found that the main phases of all samples are rhombohedral structure. For the samples of La_0.6Sr_0.15Na_0.1â–¡_0.15MnO₃+xAgNO₃ which were prepared with the first method, a weak Ag phase appears when x≥0.04. A weak Ag phase and a Mn₃O₄ phase are found in the samples of La_0.6Sr_0.15Na_0.1â–¡_0.15MnO₃-xAg and La_0.6Sr_0.05Na_0.35MnO₃+xAg which were prepared with the second method. Furthermore, no Ag and Mn₃O₄ phases were found in the samples of La_0.6Sr_0.25K_0.15MnO₃+xAg prepared with the third method,2. For the samples of La_0.6Sr_0.15Na_0.1â–¡_0.15MnO₃-xAg prepared with the first and second method, the Curie temperature of the doped sample is higher than the parent samples and it reduces as x increases. But little change is found when the samples of La_0.6Sr_0.05Na_0.35MnO₃+xAg and La_0.6Sr_0.25K_0.15MnO₃+xAg compared with their parent samples.3. For the bulk samples of Ag_x-δ-La_0.6Sr_0.15Na_0.1Agδ□_0.15-δMnO₃, the MR peakvalues of the doped samples are smaller than those of the parent samples. For x=0.04, the MR maintains the value of 5.08%(±0.20%) for the temperature between 207K and 286K.4. For the samples Lao.6Sro.o5Nao.3>Mn03+;tAg with component;c=0.08 and 0.16, the MR are larger 60% and 35% than that of the component x=0.00.5. In the samples of Lao.6Sro.25Ko i5MnO3+xA.g, D4(x=0.08) maintains the value of 5.03%{±0.12%) between the temperature of 282K and 339K. But the parent sample Dl maintains the MR value of 3.83%(±0.15%) between 284K and 343K. Obviously, compared with the parent material, the composite D4 not only increases with a, but also increases with MR value of about 31%.In conclusion, by doping AgNOs into the parent perovskite samples, the MR values increase and the object to improve the o has achieved. It is believed that those improvements will have impacted on the application of the CMR effect.