Magnetoresistance And Electrical Transport Properties Of Transition Metal-Carbon Composites And Nano-Sized Thin Films

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