A study of the negative magnetoresistance in pyrocarbons

Abstract

An exhaustive experimental investigation of a series of non-crystalline pyrocarbons has been carried out. In addition to the electrical resistivity, the Hall effect and the positive magnetoresistanoe, the negative magnetoresistance has been especially examined. This effect has been measured as a function of several parameters: temperature (300—1·5 K) magnetic field strength (0·25—60 kgauss) and sample orientation in the magnetic field. It has been shown that the negative component disappears progressively when the crystallinity of the sample increases towards the hexagonal graphite structure, that this component is anisotropic and behaves like a tensor quantity, and that there is no additivity law between the positive and the negative components. A tentative interpretation based on the existence of a pseudo energy gap between the valence and the conduction II bands has not been quantitatively successful. From an empirical analysis, a classification of the different magneto - resistance effects has been presented; all the known models proposed for the negative magnetoresistance are examined; none is able to explain our results. However, a qualitative band model is proposed, which provides a possible explana- tion of the transport properties of these materials.