Studies Between Defect Structure And The Ferromagnetic Of 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 defects in graphite. However, defects in graphite have many structures. Which kinds of defect are responsible for the magnetic ordering in graphite? In recent years many works have predicted that vacancies, adatoms and the vacancy-hydrogen complexes can induce magnetic moments, which result in the carbon magnetism. We all know ion beam implantation induce the defects which cause the carbon have magnetic moments, at the same time it leads to changes in the internal structure of graphite. But which kinds of structural changes have a direct relationship with graphite ferromagnetism? It still keeps unclear. In this thesis, many methods like XRD measurements and Raman measurements have been used to study the internal structure changes in graphite after ion beam implantation.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 2×1015 cm-2 12C+ implantation to be about 9.3emu/g. When we implanted less than 2×1015 cm-2, the ferromagnetism produced by 12C+ ion increases with dose size. However, when the dose increases to2×1015 cm-2, the saturation magnetic moment decreases substantially. The decrease in ferromagnetism is contributed to amorphous structure, which give rise to weaken the interaction between local magnetic moment induced by defects. The above results prove that the ferromagnetism in graphite has closed relationship with defects. The ferromagnetism in graphite can be adjusted by ion dose size in an accurate way. Mono-energy ion beam implantation can create a damage layer with a narrow Gaussian distributed profile in 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 in HOPG with uniform defects density profile, which give rise to a higher ferromagnetism in HOPG, can be produced by using multi-energy ion beam implantation method. We find that the ferromagnetism of graphite increases with implantation step. It is concluded that 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.According to our study, the vacancy defect caused by carbon ion implantation is the main reason for generating the magnetic in graphite. Annealed the 12C+ ion implanted HOPG sample at 200℃, it's found that vacancies and the induced ferromagnetism by 12C+ ion implantation both disappear. Since the carbon ion implantation produced vacancy defects, then it must also lead to changes in the internal structure of graphite, so we use situ XRD measurement to study the relationship between the internal structure and magnetic changes of graphite. Our experiment show that another peak was found near the (002) lines, it illustrates the atoms interacting weakened, and leads to the interlayer spacing larger, which makes the formation of defects. After annealing at 200℃all the other peaks near the (002) line disappeared. Then we measured XRD patterns at different temperatures, and found that the angle of diffraction peaks decrease with increasing temperature. In other words, the interlayer spacing is gradually increased, showing that changes in the graphite interlayer spacing have an impact on the magnetic properties of graphite.Injecting several times in different energy, we found that the ferromagnetic of graphite becomes stronger with increasing injection. However, when the dose increases to2×1015 cm-2, the saturation magnetic moment decreases substantially. In order to find the relationship between the magnetism and internal structure of graphite, we measured the Ranan spectra of samples after each step of ion implantation. When injected excessive we found a peak around 1140 will be observed, which indicating the beginning of six ring structure destroyed, and also indicates the HOPG began to change into disordered structure.