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The effects of Al ion doping on the Mn site were studied for the colossal magnetoresistance material La0.67Ca0.33MnO3. It is found that the volume of the crystal cell decreases monotonically when the population of Al3 increases across the entire doping range. As the Al3 population increases, the resistance of the material rises rapidly, while the insulator-metal transition temperature TIM decreases linearly. At small Al3 dosage, a thermal activation model properly describes the transport properties at T>TIM, while a metallic model is more suitable at T相似文献
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The transport properties were studied for rare earth manganese oxide La0.67Cao.33Mn1-x FexO3(x=0-O.3) systems. It is found that with increasing Fe^3 -doping content x, the resistance increases and the insulator-metal transition temperature (T1M) shifts to lower temperature. If the doping content is small, the transport properties manifest metallic characteristics in the temperature range of T<T1M, while they will obey a thermal activation model in the temperature range of T>T1M. Such a behavior may be attributed to the Fe^3 -doping and possible Mn ions scattering to electrons. The Fe^3 doping may lead to the formation of Fe^3 -O^2- -Mn^4 channels, which could terminate the double exchange Mn^3 -O^2--Mn^4 channels. The antiferromagnetic clusters of Fe ions may induce the Mn ions to scetter to the electrons. 相似文献
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利用固相烧结的方法制备了La0.67Ca0.33Ti0.01Mn0.99O3磁致冷材料.室温XRD分析表明该材料主要由主相的正交钙钛矿结构锰氧化物和少量La2O3杂相构成.利用VSM测量了样品在磁场下随温度变化的磁化曲线(M-T曲线)和居里温度附近的等温磁化曲线(M-H曲线),通过M-H曲线的一阶导数可得到样品的居里温度Tc(139 K),从不同温度的等温磁化曲线(M-H)计算得到材料在1.5T下的最大磁熵变△SM =0.8 J.(kg·K)-1,由M2-H/M的Arrott曲线表明La0.67Ca0.33 Ti0.01Mn0.99O3相变类型属于二级相变.由于二级相变不伴随晶格体积的变化和潜热的释放,仅是磁性材料磁畴从无序-有序的连续变化,所以二级相变的磁熵变可能较小,但是由于它是个连续的变化过程,发生相变的温区较大,造成的损耗较小.Ti4+离子的掺杂取代的是原有晶格中Mn4+离子(B位)的位置,两者之间不同的离子半径使得晶格发生Jahn-Teller 畸变,同时降低了Mn4+的浓度,使得Mn4+与Mn3+离子之间的双交换作用减弱,铁磁耦合作用减小,导致居里温度降低.该材料体系由于二级相变的温度区间较大,表明材料潜在的致冷能力较强,因此具有较好的应用前景. 相似文献