Electron-spin-resonance studies of pristine and heavily doped polyacenic materials

Electron-spin-resonance (ESR) studies of pristine and heavily doped polyacenic materials prepared by pyrolytic treatment of phenolformaldehyde resin have been carried out. Five pyrolysis temperatures ($T_p$) in the range 500–900 °C were selected to check the magnetic properties of the samples. The ESR line shapes and the spin concentrations of the pristine samples strongly depend on $T_p$, reflecting the structural characteristics in each sample. The ESR measurement of the doped samples, along with the electrical transport study previously reported, leads to the picture that both the p-type and the n-type dopants (${\mathrm{I}}_2$ and Na) essentially have two kinds of roles: (i) scavenging of the magnetic impurities, the energy levels of which exist near the Fermi level, and (ii) generation of conduction carriers giving broadened ESR spectra, particularly in the samples prepared at higher $T_p$. From this observation, it is highly possible that in such doped samples, the spin-orbit coupling plays an important role in the relaxation process, which suggests that the conduction carriers exist on the dopant species with a finite probability.