Extinction of ferromagnetism in highly ordered pyrolytic graphite by annealing

We report that the ferromagnetism of highly ordered pyrolytic graphite (HOPG) samples as measured by hysteretic magnetization loops can be diminished and eventually extinguished with sufficiently long high vacuum anneals at temperatures on the order of 2300 °C. Concomitant with the extinction of ferromagnetism, we observe an anneal-induced increase in grain size (accompanied by possible edge reconstruction) confirmed by X-ray diffraction measurements and improved transport properties, including lower in-plane and out-of-plane resistivity, higher electron and hole mobility and improved charge compensation. The implied reduction of defects and vacancies by annealing suggests that the ferromagnetism of pristine HOPG is correlated with localized states located at zigzag edges, vacancies and related defects.