用固相反应烧结法制备了多晶体材料La2/3Ca1/3MnO3(LCMO),La2/3(Ca0.5Ba0.5)1/3MnO3(LCBMO)、La2/3Ba1/3MnO3(LBMO),La2/3(Sr0.5Ba0.5)1/3MnO3(LSBMO)和La2/3Sr1/3MnO3(LSMO),测量了它们在77 K～800 K的电阻和磁电阻特性.实验发现,通过一定的工艺技术制备出的CMR材料有强烈的相界效应,在低温下有大的磁电阻,且在Tc附近存在明显的从金属态向半导体态的转变;用自旋极化电子隧道效应和靠近居里点逾渗模型较好地解释了电阻和磁电阻行为;用非绝热小极化子跃迁模型对高温的电阻特性进行拟合处理,得到了激活能;同时发现退火处理对样品的电阻率有重要的影响.
Polycrystalline samples La2/3Ca1/3MnO3 (LCMO), La2/3(Ca0.5Ba0.5)1/3MnO3 (LCBMO), La2/3Ba1/3MnO3 (LBMO),La2/3(Sr0.5Ba0.5)1/3MnO3 (LSBMO) and La2/3Sr1/3MnO3 (LSMO) have been prepared by a solid state reaction method. We have observed the temperature dependences of the electric resistivity and magnetoresistance from 77 K up to 800 K. Experimental results indicate that the presence of grain boundaries in the colossal magnetoresistance (CMR) materials prepared by the mechanical alloyed method leads to a large magnetoresistance effect at low temperature, and there exists a pronounced metal-semiconductor transition around Tc. According to the models of spin-polarized electron tunneling and percolation, the magnetoresistance as a function of temperature has been explained well. The temperature dependence of resistivity at high temperature is well fit by nonadiabatic small polaron hopping model for LCBMO and LSBMO samples, and their activation energies are obtained. Also it has been found that annealing processing affects the resistivity significantly.
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