【作者】 杨新梅；

【导师】 刘向东； 周兴泰； 夏汇浩；

【作者基本信息】 山东大学 ， 凝聚态物理， 2010， 博士

【摘要】 目前已经有大量实验研究报道碳材料具有铁磁有序,指出碳材料的铁磁性是碳材料的本质特性,与缺陷有关。但是在碳材料中存在的缺陷种类繁多,实验上很难确定到底是哪类缺陷对碳材料铁磁有贡献。并且关于缺陷之间的磁相互作用问题一直都没有一个最直接的解释。所以本论文主要的工作是从实验上对石墨的“铁磁性”进行了研究,探讨石墨的铁磁有序机理。理论研究已经表明空位缺陷、间隙原子缺陷、空位吸附H的缺陷会具有净余磁矩,这些缺陷都可以通过离子注入的方法在石墨中引入。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,研究缺陷与石墨铁磁有序的关系。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,注入在缺陷层中形成铁磁的最大饱和磁矩可以达到9.3emu／g；如果注入剂量达到5×1015cm-2,石墨的铁磁性减弱,主要是因为大剂量注入形成缺陷密度增多使石墨内部结构无序,破坏了石墨缺陷磁矩之间的耦合作用,揭示石墨铁磁有序确实与缺陷有关,通过适当调节注入剂量能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,使缺陷层的缺陷密度尽量均匀分布,发现石墨的铁磁有序还能再继续增强。综上所述,石墨的铁磁有序与缺陷有关,通过调节离子注入的能量与离子注入的剂量能有效调控石墨铁磁有序。离子注入在石墨中形成的缺陷种类繁多。我们采用正电子湮灭技术和SQUID磁性测量结合退火研究石墨铁磁有序与空位缺陷的关系,发现注入70keV的12C+(1×1015cm-2)使石墨形成230nm厚的缺陷层,含有单空位、双空位、四空位、六空位和九空位,同样磁性测量发现离子注入使石墨的铁磁有序增强。将石墨样品在200℃下退火一个小时,发现离子注入形成的空位缺陷和增加的铁磁同时消失,揭示空位缺陷与石墨铁磁有序密切相关。理论计算发现除了空位具有净余磁矩外,空位与H结合的缺陷也有净余磁矩,因此我们结合磁性测量和C元素的近边X射线吸收谱的全电子产额谱(TEY)和荧光电子产额谱(FY)来研究石墨注入前后化学结构变化与石墨铁磁有序的关系,发现15keV的H+注入和50keV的12C+注入都能使石墨缺陷层的π键与。键的比例(Iπ/Iσ)发生变化,离子注入使π键相对于6键增多。15keV的H+注入(1×1015cm-2)使石墨的缺陷层具有了明显的六圆环加H的吸收峰,揭示空位缺陷与H结合形成C-H键。大剂量(1×1016cm-2)注入15keV的H+能够形成更多的C-H键,并且还能够在FY谱中观测到281.6eV的吸收峰,揭示石墨被大剂量H+注入后形成了空位团,导致石墨局域结构无序。50keV 12C+注入后,石墨样品的TEY谱和FY谱在281.6eV也有吸收峰,通过拟合发现吸收峰实际是由两个小峰组成,强度比为2.2099-2.4632,揭示如果281.6eV对应的吸收峰是空位或多空位,那缺陷周围主要有两个位置的碳原子对吸收峰有影响,对应的电子密度比为2.2099-2.4632。磁性测量发现15keV的H+小剂量注入(1×1015cm-2)和50keV的12C+注入(1×1015cm-2)都能在石墨中形成铁磁,揭示空位与H结合的缺陷结构和由空位缺陷形成的无序结构对石墨铁磁有序都有影响。我们的实验结果与H.Ohldag与P. Esquinazi等人结果一致。他们研究了H+注入(6.25×1016cm-2)前后高定向热解石墨的磁圆二色谱,发现280-285eV之间的π共价态具有明显的磁圆二色性,同时还发现C-H键也有磁圆二色性,即化学吸附H的缺陷结构和π的共价态对石墨铁磁有序都有影响。石墨的空位缺陷可以捕获电子成为顺磁中心,只要空位缺陷之间有耦合作用就可以形成铁磁。我们结合电子自旋共振和SQUID磁性测量研究石墨的铁磁机理。原始石墨的电子自旋共振峰是典型的Dyson峰型,揭示石墨的导体特性。原始石墨的各向异性研究和温度演变规律,揭示原始石墨的载流子自旋与石墨晶格有相互作用。70keV的12C+注入使石墨的Dyson峰减弱,但是再次注入同能量的12C+不再使石墨的Dyson峰发生变化,且注入后仍然存在的Dyson峰具有与原始石墨一样的性质,因此这个峰还是与石墨载流子扩散有关,是注入层以下石墨的性质。70keV的12C+注入使石墨在不同于Dyson峰的位置形成了一个类Lorentz峰(L1)。L1峰随着注入剂量增加而增强,揭示L1峰与石墨缺陷密度有关。磁性测量揭示L1峰与石墨的铁磁有序有关。L1峰和D1峰完全不同,L1峰的g因子没有各向异性,也不随温度变化,因此L1峰是由局域缺陷形成,揭示注入改变了石墨表面的能带结构；L1峰的峰宽也不随温度变化,说明未成对电子间的相互作用不随温度变化；L1峰的强度随着温度升高而减弱,与石墨铁磁有序的温度变化一致,因此石墨的铁磁性与局域缺陷有关,石墨铁磁性随着温度升高而减弱的主要原因是热效应导致有效未成对电子数减少。实际上,L1峰是不对称的,揭示了载流自旋与局域自旋之间有强耦合作用。空位缺陷可以捕获电子成为顺磁中心,具有净余磁矩。但是随着温度升高,电子运动越剧烈,就越不容易被空位缺陷捕获,有效未成对电子数会减少。L1峰的性质反映了石墨的铁磁有序是巡游电子和空位缺陷强耦合作用的结果,也揭示了石墨铁磁磁矩随着温度升高而减弱的主要原因是由热效应导致有效未成对电子数减少。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 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.更多还原