Evidence for internal field in graphite: a conduction electron spin-resonance study

We report on conduction electron spin-resonance measurements performed on highly oriented pyrolitic graphite samples between 10 and 300 K using S(ν=4 GHz), X(ν=9.4 GHz), and Q(ν=34.4 GHz) microwave bands for the external dc-magnetic field applied parallel ( H∥ c) and perpendicular ( H⊥ c) to the sample hexagonal c-axis. The results obtained in the H∥ c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, H int eff( T, H), that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the H int eff( T, H) with the field-induced metal–insulator transition in graphite and discuss its origin in the light of relevant theoretical models.