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(v = 4 GHz), X(v = 9.4 GHz), and Q(v = 34.4 GHz) microwave bands for the external de-magnetic field applied parallel (H parallel to c) and perpendicular (H perpendicular to c) to the sample hexagonal c-axis. The results obtained in the H parallel to c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, He-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. (C) 2002 Elsevier Science Ltd. All rights reserved.