Studies On The Origin Of Ferromagnetic Ordering In Highly Oriented Pyrolytic Graphite

Carbon magnetism has attracted much attention from the community of science and technology for several years. Ferromagnetism in graphite at room temperature (RT) is believed to be intrinsic, which is attributed to the defects in carbon materials. However, there are a variety of defects in carbon materials. It is difficult to confirm which kind of defect is responsible for the magnetic ordering in graphite. Up to now, the origin of ferromagnetism in graphite still keeps unclear. In this thesis, many methods have been used to study the origin of ferromagnetism in graphite.Theoretical calculations have predicted that vacancies, adatoms and the vacancy-hydrogen complexes can induce magnetic moments. We employed superconducting quantum interferometer (SQUID) device to measure the magnetic moments of highly oriented pyrolytic graphite before and after 70keV 12C+ion implantation. It is found that 12C+ion implantation can produce stable RT ferromagnetism in HOPG. The ferromagnetic ordering in graphite can be tuned by implantation dose or by implanted energy, indicates that ferromagnetic ordering in graphite is closely related with defects produced by ion implantation. We can obtain the maximum magnetization induced by 2×1015cm-212C+implantation to be about 9.3emu/g. For the dose range from 3×1014cm-2 to 2x1015cm-2, the saturation magnetic moment increases with increasing implanted dose. However, when the dose increases to 5×1015cm-2, the saturation magnetic moment decreases substantially. This indicates that further increase of the implantation dose may create too high defect density, leading to the reduction of magnetic moment induced by defects, and the lattice disorder, perhaps even amorphous zones in the lattice, may destroy the band structure and carrier density that are necessary for magnetic coupling. The above results prove that the ferromagnetism in graphite has a close relationship with defects. The ferromagnetism in graphite can be adjusted by a suitable modulation of the ion dose size. Mono-energy ion beam implantation can create a damage layer with a narrow Gaussian distributed profile in the subsurface of the sample. Considering the narrow window of the implantation parameter to induce ferromagnetism, only a small portion of the implanted layer is responsible for magnetic ordering. Our results show that a ferromagnetic layer with uniform defects density profile in HOPG, which can give rise to a higher ferromagnetism in HOPG, can be produced by using multi-energy ion beam implantation. We find that the ferromagnetism of graphite increases with implantation step. It is concluded that the multi-energy and multi-steps 12C+ion beam implantation is an efficient way to enhance the magnetization of HOPG and the ferromagnetism of graphite is closely related with defects produced by 12C+ion implantation.Ion implantation can produce various defects. We employed positron annihilation technique (PAT) and SQUID measurements to study the correlation between the vacancies and the ferromagnetism of graphite. It is found that 70keV 12C+ion produces a defective layer with a thickness of 230nm. There are single vacancies, divacancies, four vacancy cluster, six vacancy cluster and nine vacancy cluster in the defective layers of graphite.70keV 12C+ion to a dose of 1×1015cm-2 induce ferromagnetism. Annealed the 12C+ion implanted HOPG sample at 200℃, it is found that vacancies and the induced ferromagnetism by 12C+ion implantation both disappear, indicate that the vacancies are closely related with ferromagnetic ordering in graphite.Theoretical studies have indicated that the vacancies-hydrogen complexes also have net magnetic moment. So we employed magnetic moment measurements with SQUID and C element near edge x-ray absorption fine structure spectra (C-NEXAFS) to study the correlation between ferromagnetism of graphite and chemical structures before and after ion implantation. The C-NEXAFS spectra with total electron yield (TEY) and fluorescence yield (FY) can detect the chemical structures in the near surface and that in the depth of about 200nm. It is shown that 15keV H+implantation and 50keV C+implantation both induce theπbond increase relative toσbond. From the FY spectra, we can observe the characteristics of vacancy-hydrogen structures in the 15keV H ion (1×1015cm-2) implanted HOPG sample. The absorption spectra corresponding to vacancy-hydrogen structures is more and more distinct in the FY spectra when the HOPG sample is implanted by 15keV H ion implantation to a dose of 1×1016cm-2. In the FY spectra of the 15keV H ion (1×1016cm-2) implanted HOPG sample, there is obvious characteristic of disorder (281.6eV) or vacancy clusters produced by ion implantation. The 50 keV 12C+ion implanted HOPG sample also have disorder or vacancy clusters in the near surface and in the defective layer. Magnetic moments measurement by SQUID indicates that the 15keV H+ion (1×1015cm-2) and 50keV 12C+ion (1×1015cm-2) both produced a ferromagnetic ordering in HOPG sample. The above results indicate that the vacancy-hydrogen complexes and the disorder or vacancy clusters are both related with magnetic ordering in graphite, which is consistent with results of H. Ohldag and P. Esquinazi et al.. They employed X-ray magnetic circular dichroism (XMCD) spectra to investigate the magnetic ordering in proton irradiated HOPG sample, and found thatπ-states at 280-285eV and C-H bond states exhibit a net spin polarization.The vacancies in graphite can be paramagnetic by capturing electrons, which can trigger a ferromagnetic ordering if electron spins of paramagnetic centers couple with each other. We employed electron spin resonance (ESR) spectra and magnetic moment measurements (SQUID) to study the mechanism of coupling between magnetic moments of defects induced by ion implantation. The ESR spectra of virgin graphite is the classical Dysonian peak, indicate that graphite is a conductor. The anisotropy and the evolution of g factor with temperature of virgin graphite indicated that electron spin resonance of graphite is due to the relaxation of the carriers'spins with the lattice of graphite. HOPG sample also has the Dysonian peak (D1) with decreasing intensity at the same resonance field after 70keV 12C+ion implantation. The D1 peak keeps constant when HOPG sample was implanted again with 70keV 12C+ion. The D1 peak has the same anisotropy and the same evolution with temperature as that measured for the virgin HOPG sample. The D1 peak is surely ascribed to the conducting carriers in the HOPG substrate beneath the defective layer. 70keV 12C+ion implantation produces a Lorentz-like resonance peak (L1). The intensity of L1 peak increases with the dose size of implantation, which indicates that L1 peak is induced by defects produced by ion implantation. The L1 peak is related with ferromagnetic ordering of graphite produced by 12C+ion implantation. L1 peak is quite other compared with D1 peak. The g factor of the L1 line is independent of temperature, indicates that the defects with unpaired electrons are localized. And the line width of L1 line is also independent of temperature, suggests that the interaction between magnetic moments of unpaired electrons is constant. The intensity of L1 peak decreases with increasing temperature, which indicates that the effective unpaired electron with localized spins is decreased due to the thermal effect. The L1 line is not symmetric, indicates that there is strong exchange interaction between majority localized spins and minority carrier's spins to form ferromagnetic ordering of graphite. The decrease of ferromagnetism with temperature is mainly due to the decrease of unpaired electrons with localized defects.