【作者】 侯克玉；

【导师】 赵明文； 周兴泰；

【作者基本信息】 山东大学 ， 凝聚态物理， 2011， 硕士

【摘要】 石墨的铁磁有序已经研究了很长一段时间,而且已经有大量的文献指出石墨的铁磁性是石墨的本质特性,与石墨的缺陷有关。但是在石墨中存在的缺陷类型有很多,到底哪一种缺陷使得石墨产生了磁性呢?近年来,有大量的文献指出通过离子注入的方式在石墨中产生的空位缺陷、间隙原子缺陷和空位吸附H等缺陷会具有净余磁矩,从而使得石墨具有铁磁性。我们都知道离子注入方式使得石墨产生了具有净余磁矩的缺陷,但是同时它也会导致石墨内部结构发生变化,到底是何种结构变化和石墨的铁磁性具有直接关系呢?一直也没有一个很好的解释。所以本文从实验上用原位XRD测量方法和测量拉曼光谱方法来分析离子注入后石墨内部结构的变化情况,然后用近边X射线吸收谱方法研究离子注入后石墨内部化学结构的变化。我们主要采用12C+注入高定向热解石墨(HOPG)使石墨具有空位缺陷和间隙原子缺陷,从而使石墨具有磁性。采用超导量子干涉仪装置测量石墨在注入前后的室温磁性变化,发现12C+注入可以调节石墨铁磁性。当注入剂量≤2×1015cm-2时,石墨的铁磁性随着注入剂量增加而增强,注入在缺陷层增加最大铁磁饱和磁矩可以达到9.3emu/g;如果注入剂量超过2×1015cm-2,石墨磁性随着注入剂量增加而减弱,主要是因为此时石墨内部变成无序结构,破坏了石墨缺陷磁矩之间的耦合作用。所以石墨铁磁有序确实与缺陷有关,通过适当调节注入剂量可以调节石墨注入层的缺陷浓度,使石墨缺陷磁矩间的耦合作用增强,从而增强石墨的铁磁性。注入的离子及形成的缺陷在表面缺陷层的分布是属于高斯分布,在平均阻止深度里的缺陷和离子是最多的。所以我们采用多次不同能量注入的方法,使石墨的缺陷层的缺陷密度尽量均匀分布,发现石墨的铁磁还能再次继续增加。因此石墨的铁磁性与缺陷有关,通过改变离子注入能量和离子注入剂量能有效调控石墨铁磁有序。根据我们的研究得知,碳离子注入高定向热解石墨后引起的空位缺陷是使石墨产生磁性的主要原因,而在200℃退火一小时后,发现空位缺陷明显减少使得磁性消失。既然碳离子注入产生了空位缺陷,那么它肯定也会引起石墨内部结构的变化,所以我们用原位XRD测量方法研究了石墨内部结构的变化与石墨磁性变化的系。实验表明在离子注入后的样品谱线(002)峰附近发现有其他的峰形成,说明原子相互作用减弱,从而使得缺陷形成,导致层间距变大。伴随着退火过程,(002)主峰旁边的其它小峰逐渐消失,而经过200℃退火后主峰附近的其它小峰全部消失。而后我们又分别测量了在不同温度下的XRD图谱,发现随着温度的升高衍射峰的峰位角在逐渐的减小,可见石墨产生了缺陷从而使得层间距在逐渐的增大。我们在不同能量多次注入中我们发现在开始的注入过程中随着注入的增多石墨的铁磁性在逐渐的变强,而当注入量过大时,也就是超过2×1015cm-2的时候,石墨的铁磁性又开始变小。我们分别测量了几步注入后样品的拉曼光谱,企图找到石墨磁性变化与石墨内部结构的变化的关系。通过测量我们发现,在注入量过大时,在～1140 cm-1处会观察到一个峰,说明石墨的六环结构开始遭到破坏,也预示着高有序石墨开始向无序结构变化。更多还原

【Abstract】 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.更多还原