Spin Resonance in Neutron-Irradiated Graphite

Abstract

The carrier spin-resonance line of neutron-irradiated single crystals of graphite at 300\ifmmode^\circ\else\textdegree\fi{}K has been observed as a function of the thermal neutron flux up to a dose of 9.6\ifmmode\times\else\texttimes\fi{}${10}^{18}$ nvt. From the intensity increase and the $g$ shift for $H$ parallel to the $c$ axis, it is concluded that on the average 30 holes become mobile per nvt per ${\mathrm{cm}}^{3}$. This is in agreement with earlier work of Hennig and Hove which was based on electrical measurements. It is shown that the line they reported in spin-resonance experiments on polycrystalline graphite was due to mobile charge carriers and not to paramagnetic carbon centers as they assumed. The number of holes created is compared to a recent electron transmission microscopy investigation of Bollmann where the damage has been observed directly. It is estimated that about one hole per displaced carbon atom is freed.For the unirradiated graphite the linewidth was found to be anisotropic, being 4.6 gauss for $H$ parallel and 3.0 gauss for $H$ perpendicular to the $c$ axis. This shows for the first time an incomplete "motional" narrowing for mobile carrier spin resonance. The anisotropy as well as the width decreases monotonically with irradiation, and at the highest dose investigated the linewidth is isotropic and equal to 1.3 gauss. The change in linewidth with irradiation and temperature is interpreted as due to a change in spin lattice relaxation time ${T}_{1}$ which is caused by carrier scattering via spin-orbit interaction.