Fixed-spin-induced ESR linewidth and polaron mobility in conducting polymers

The relationship between the polaron mobility and the fixed-spin-induced ESR linewidth is verified experimentally in HCl-doped polyaniline in which a controllable amount of Fe${}^{3+}$ paramagnetic ions have been incorporated. The polaron mobility has been varied by (a) decreasing the temperature, and (b) aging of the material. In both cases, the line broadening has been compared to the dc conductivity, and to the intrachain spin-diffusion rate obtained from NMR measurements. The line broadening appears to be proportional to the spin-diffusion rate. Implications of this line-broadening mechanism are discussed, in particular concerning the temperature dependence of the ESR linewidth.