【作者】 薛庆忠；

【导师】 章晓中；

【作者基本信息】 清华大学 ， 材料科学与工程， 2004， 博士

【摘要】 论文第一部分工作研究了过渡金属—石墨复合材料的巨磁电阻效应。主要成果是:⑴首次观测到了石墨颗粒固体中磁电阻的尺寸效应和温度效应。石墨颗粒固体的磁电阻数值随着平均颗粒尺寸的减小而减小。在温度约为50 K 时,在平均粒径为25.65 μm 的石墨颗粒固体中,观察到了正的线性磁电阻效应;而在温度约为25 K 时,在平均粒径为30.2 nm 的石墨颗粒固体中,观察到了负的线性磁电阻效应。利用正常磁电阻理论、晶粒边界的漫散射理论和电子的弱局域化理论解释了在石墨颗粒固体中发现的有趣的磁电阻规律。这些结果对理解半导体和半金属材料中的正磁电阻具有重要意义。⑵制备了微米过渡金属—石墨复合材料,并研究了其巨磁电阻效应。实验结果表明:过渡金属—石墨复合材料的巨磁电阻随外加磁场的变化规律可以近似用MR ∝Bⁿ来描述,在铁—石墨复合材料中指数n 的数值随着温度的增加和铁含量的减小而增加。Fe_0.2-C_0.8复合材料的巨磁电阻,在外加磁场为5 T、温度为300 K和5 K时,分别高达53.8%和190%。利用正常磁电阻效应和渗流理论解释了过渡金属—石墨复合材料中巨磁电阻随外加磁场的变化规律,给出了出现正线性磁电阻的产生机理。该研究对理解已有的正线性磁电阻现象具有重要指导意义。论文的第二部分工作:采用PLD 方法在Si 基片上制备了非晶碳膜、Fe_x-C_1-x膜和Fe_0.011-C_0.989/Fe 双层膜,并对其微结构、磁电阻和电输运等特性进行了研究。主要结果为:（1）首次观测到27 ℃条件下制备的非晶碳膜/Si 具有明显的电压诱导开关特性,并且其开关电压随温度的升高而逐渐减小。而在300 和500 ℃条件下制备的非晶碳膜却具有完全不同电输运规律。这些非晶碳膜所具有的异常的电输运特性将会在场效应器件、温度传感器、电子开关等领域具有很好的应用前景。利用能带理论探讨了以上在Si 基片上制备的非晶碳膜的电输运和磁电阻现象。（2）对Fe_x-C_1-x/Si 材料的微结构、磁电阻和电输运等特性进行了研究。在一定的温度范围内,Fe_x-C_1-x/Si 的电输运性质与其测量电流密切相关,从而具有有趣的非线性的I-V 特性曲线。Fe_x-C_1-x/Si 还具有异常的磁电阻效应。在T=300 K 和B= 1 T 的条件下,Fe_0.011-C_0.989/Si 的正磁电阻可达33%。（3）建立了一个双通道模型,并利用该模型对Fe_x-C_1-x/Si 的磁电阻和电输运特性进行了解释。该模更多还原

【Abstract】 The size and temperature dependence of magnetoresistance （MR） of bulk graphite were investigated. The MR of bulk graphite decreases with decreasing particle size. The micron-sized graphite exhibits positive linear field dependence of MR at about 50 K, whereas the nano-sized bulk graphite exhibits negative linear field dependence of MR at about 25 K. The possible mechanism for the MR of bulk graphite can be partially understood using ordinary MR theory, weak localization theory and diffuse scattering theory. A large positive MR has been found in micro-sized Fe（Ni）_x-C_1-x composite. Fe0.2-C0.8 composite has the largest MR with the MR value of 53.8% and 190% at room temperature and at 5 K under a magnetic field of 5T, respectively. The magnetic field dependence of the MRs can be described approximately as MR ∝Bn, and the value of n is determined by the Fe concentration and temperature. Every specimen under study has a linear field dependence of the positive MR at different temperature, and therefore we can design different magnetic sensors to satisfy different demands using Fe（Ni）_x-C_1-x composite. Using ordinary MR theory and electrical transport theory,we has interpreted the interesting rule of the MR observed. Amorphous carbon films （a-C film） were deposited on n-Si substrate at different temperatures using pulsed laser deposition. Some anomalous current-voltage characteristics of the a-C films/n-Si are reported. The a-C films/n-Si deposited at 27 ℃has an apparent voltage-induced switch effect, and the value of the switch voltage decreases with increasing temperature. However, the a-C films/n-Si deposited at 300 ℃and 500 ℃are completely different from that of a-C films/n-Si deposited at 27 ℃, which have its own special I-V characteristics. The anomalous I-V characteristics should be of interest for various applications such as field effect devices. Besides, the structure of the a-C films was investigated by Raman and atomic force microscopy and the dependence of resistivity of the a-C films/n-Si on deposition temperatures has been studied in the temperature range of 100-300 K. Finally, using energy band theory we proposed a model to interpret the anomalous current-voltage characteristics observed. Using pulsed laser deposition method we prepared Fex-C1-x films on n-Si （100） substrates. We found that the electrical transport properties of Fex-C1-x/Si are controlled by the measuring current within a given temperature range so that it has an unusual I-V curve. Correspondingly, a colossal electroresistance （ER） of 1500% was found in the Fex-C1-x/Si samples. The switching of the conducting channel from the Fex-C1-x to the Si substrate plays an important role in the electrical transport properties and the ER of Fex-C1-x/Si. The easy control of the resistance of Fex-C1-x/Si by electric current should be of interest for various applications such as field effect devices. A novel type of MR was found in Fex-C1-x films on Si （100） substrates. When temperature T<258 K, the MR of Fe0.011-C0.989/Si substrate is negative and when 258K<T<340 K the MR is positive. And, the MR has different field dependences within the different temperature ranges. A large positive MR of 46% was found at T=300 K and B=1.5 T. A two-channel model is proposed to understand the novel MR and the electrical properties of the Fex-C1-x/Si. Based on this model, an equation of the resistance of Fex-C1-x/Si is developed. The switching of conducting channel from the Fex-C1-x films to the Si substrate plays an important role in the MR and the electrical transport properties of the Fex-C1-x /Si.更多还原