Magnetic Field Dependence of the Hall Effect and Magnetoresistance in Graphite Single Crystals

The Hall coefficient and magnetoresistance in purified (99. 995%) natural graphite single crystals have been measured from 25 to 25 000 gauss at 298^, 77^, and 4. 2^ with the field oriented parallel to the hexagonal axis. Special care was taken in the micromanipulation and strain-free mounting of these small soft crystals. Fast minority carriers due to Fermi-surface warping were discovered by their effect on the low-field Hall coefficient behavior. The compensating effect between the majority electron and hole densities (5-2) 10^18 cm^-3 with n₄{n₇}=1. 0-1. 15 over the above temperature range and mobilities (1. 5-130) 10^4 cm^2/v sec with {₄}{₇}=1. 10-0. 79 makes the Hall coefficient very sensitive to the temperature, impurities, and field where it even changes sign. A quadratic low-field room-temperature magnetoresistance dependence progresses at higher fields to an impurity-insensitive H^1. 78 behavior. The large magnetoresistance ratio of graphite (10^5 at 4. 2^ and 23 kilogauss), along with the appearance of de Haas-van Alphen type oscillations in these properties, demonstrates the small effective masses (0. 03m₀, 0. 06m₀) and long relaxation times (2. 510^-11 sec at 4. 2^). The mobility follows a T^-1. 2 law in the lattice-scattering region 50^. Low-temperature results, showing carrier density differences and mobilities to be most sensitive to impurities, substantiate the relatively high purity of these crystals.