Electron Spin Resonance in Polycrystalline Graphite

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Abstract
The electron spin resonance of polycrystalline graphite is shown to be due to charge carriers, as for single crystals. For lampblack‐base graphite, the variation in g value with temperature agrees approximately with that for single crystal graphite if it is assumed that the spins effectively average out the anisotropic interactions by rapid intercrystallite diffusion. The magnitude and temperature dependence of the spin susceptibility are found to be in excellent agreement with McClure's theoretical predictions for the charge carriers in perfect graphite. Complications of measuring these materials are discussed, including effects of skin depth and microwave heating and the dependence upon crystallite size and sample purity. It is found that two types of impurities can affect the resonance; those which become ionized and shift the Fermi level, and large un‐ionized atoms which do not shift the Fermi level but act as efficient scattering centers for shortening the spin—lattice relaxation time.