Raman study of correlation between defects and ferromagnetism in graphite

The variation of ferromagnetism induced by C-12(+) ion implantation in highly oriented pyrolytic graphite was systematically studied by using Raman spectroscopy in conjunction with magnetic moment measurements and annealing treatments. It was found that the magnetization of the implanted sample was closely correlated with the density of the defects, which was characterized by the Raman spectra, produced by the implantation. It is clear that by using consecutive implantation steps at different energies to increase the vacancy defects in the implanted layer, the magnetization of the sample increases with the number of the implantation steps until the fourth step of implantation, which causes the near surface layer to be highly disordered or amorphous, weakening the magnetic coupling and thus resulting in the decrease in magnetization. The annealing treatments of the sample indicate that the ferromagnetism induced by the implantations is stable at room temperature. However, when the sample is annealed at 473K (the Wigner energy release temperature), the density of vacancies and interstitials is abruptly decreased and the magnetism induced by the implantations is extinguished. This finding gives a clear indication of the key role of the defects produced by C+ ion implantation in graphite.