Motional narrowing of the conduction ESR of vapour-grown carbon fibres

Abstract

Conduction electron spin resonance (CESR) spectra have been measured for pristine and boron-doped vapour-grown carbon fibres. Pristine fibres annealed at temperatures between 2300 and 3025 K exhibit broad 'two-dimensional' powder patterns when the fibres axis is perpendicular to the magnetic field. The full g-anisotropy of single crystal graphite is not established until the fibres are annealed above 2800 K. The CESR spectra of fibres heat treated above 2750 K and having diameters of less than 20 mu m show a collapse of the two-dimensional powder pattern into a relatively sharp ESR line at the isotropic value of the pattern, (gc+ga)/2. This narrowing results from the large-scale motion of the electrons in the basal planes during the experiment. Due to the annular morphology of the vapour-grown fibres, the RMS migration of the electrons during the ESR experiment must be of the order of the fibre diameter for motional narrowing to occur. Electron migration is limited by electron-phonon scattering at room temperature. Products of the electron mean free paths and Fermi velocities determined from the narrowing of the ESR signals increase with increasing annealing temperatures. For fibres annealed at all temperatures this product is still slightly less than that estimated for pure graphite. Fibres doped with boron at 2975 K display a reduced anisotropy due to a shift in gc. Boron-doped fibres exhibit motional narrowing behaviour similar to pristine fibres.