【作者】 刘晓芳；

【导师】 于荣海；

【作者基本信息】 清华大学 ， 材料科学与工程， 2010， 博士

【摘要】 掺杂SnO₂稀磁材料优异的物化性能,特别是巨磁矩和高居里温度的出现,使其在稀磁半导体领域展现出极大的应用潜力和理论研究价值。然而长期以来, SnO₂稀磁材料的铁磁性来源及物理机制都是未能解决的两大关键问题。为此,本文采用磁控溅射和溶剂热法分别制备了Co掺杂SnO₂稀磁薄膜和Co/Zn掺杂SnO₂纳米棒,系统研究了工艺参数对材料微观结构和磁性能的影响,揭示微观结构和磁性能的关系,从实验和理论两方面阐述了SnO₂基稀磁材料的上述问题。为澄清SnO₂基稀磁材料的铁磁性来源,本文采用基于同步辐射的X射线吸收精细结构技术及相关理论计算,深入分析了掺杂元素的局域结构。结果表明SnO₂稀磁薄膜和纳米棒为金红石结构的SnO₂相,掺杂元素成功替代了SnO₂晶格中的Sn元素,由此证明所制备的掺杂SnO₂薄膜和纳米棒为具有本征铁磁特性的稀磁材料。在探讨SnO₂基稀磁材料铁磁性物理机制上,本文通过制备具有室温铁磁性的绝缘态Co掺杂SnO₂（Co-SnO₂）稀磁薄膜,有效分离了结构缺陷和载流子对材料磁性能的影响。首先通过使用不同靶材、改变薄膜沉积参数、调节SnO₂缓冲层厚度,证明结构缺陷对Co-SnO₂稀磁薄膜铁磁性具有调制作用。在此基础上,采用不同工艺热处理和N元素共掺杂工艺,分别研究了施主缺陷和受主缺陷（即点缺陷）对Co-SnO₂稀磁薄膜铁磁性的调节,确定点缺陷是影响铁磁性的根本因素,从而成功证明了束缚磁极化子（BMP）模型在解释掺杂SnO₂稀磁材料铁磁性机制上的合理性。为进一步验证BMP模型在不同维度SnO₂基稀磁材料中的普适性,本文采用溶剂热法制备了室温铁磁性的Co-SnO₂纳米棒,证实纳米棒中同时存在由结构缺陷调制的长程铁磁有序和邻近Co离子间的反铁磁耦合,该结果同样符合BMP模型的理论机制。此外,在非磁性Zn元素掺杂SnO₂纳米棒中还观察到明显的铁磁性,结合微观结构分析表明材料的铁磁性来源于Zn掺杂诱导的锡空位,铁磁性强弱与锡空位的浓度密切相关。该结果为基于d0机制的第一性原理计算提供了实验数据支持,并开拓了设计新型SnO₂基稀磁材料的思路。更多还原

【Abstract】 Transition-metal doped SnO₂ is considered as an important candidate material in the field of diluted magnetic semiconductor due to its excellent physical and chemical properties. Particularly, the discoveries of a giant magnetic moment and high Curie temperature in Co-doped SnO₂ films have inspired tremendous experimental and theoretical investigations. Up to date, the ferromagnetic origin and related physical mechanism in SnO₂-based diluted magnetic material have not been fully understood. In this dissertation, Co-doped SnO₂ films and Co/Zn-doped SnO₂ nanorods are prepared by magnetron sputtering and solvethermal methods, respectively. Through changing experimental conditions, we systematically investigate the microstructure and magnetic properties of the materials. These researches focus on the local structure of the doping element in SnO₂ and discuss the correlations between magnetic properties and the microstructure. Based on the above studies, the ferromagnetic origin as well as the related mechanism is further explained.To clear the ferromagnetic origin, X-ray abosorption fine structure characterization with first-principle calculations are used to investigate the local environment of doped elements in SnO₂. The results show that the doped elements have successfully substituted for Sn elements in SnO₂ lattice without forming secondary phases, which confirms the intrinsic nature of ferromagnetism in doped SnO₂ diluted magnetic materials.Room-temperature ferromagnetism is achieved in insulating Co-SnO₂ diluted magnetic films. The investigations on insulating films efficiently separate the influences of structural defects and free carriers on the magnetism of the materials, which is helpful for clarifying the ferromagnetic mechanism. Sputtering targets, deposition parameters and buffer layer thickness are changed to adjust the microstructure and magnetic properties of Co-SnO₂ films. The experimental results demonstrate the mediation effect of structural defects on the long-term ferromagnetic ordering. Furthermore, various post-annealing processes are used to vary the concentrations of oxygen vacancies and Sn interstitials in Co-SnO₂ films, which confims the crucial roles of donor defects in tuning magnetic properties. Additionally, nitrogen is codoped with Co to create acceptor defects in （Co,N）-codoped SnO₂ films. The dependence of ferromagnetism on nitrogen concentration implies that the acceptor defect is another key factor in mediating magnetism in doped SnO₂ materials. According to the above analyses, the ferromagnetic mechanism in SnO₂-based diluted magnetic materials is reasonabley explained using bound magnetic polaron （BMP） mechanism based on the presence of donor and acceptor defects.To further verify the BMP model in different dimensional SnO₂-based diluted magnetic materials, we have successfully synthesized room–temperature ferromagnetic Co-doped SnO₂ nanorods via solvethermal methods. Magnetic property measurements indicate that the long-range ferromagnetic ordering mediated by structural defects coexists with the antiferromagnetic coupling between adjacent Co ions in Co-doped SnO₂ nanorods. Such result is consistent with the description of BMP model. Moreover, room-temperature ferromagnetism is observed in nonmagnetic Zn element doped SnO₂ nanorods. Combining with microstructure analyses, it is found that the ferromagnetism of the material originates from the Zn doping-induced Sn vacancies. The saturated magnetization is sensitive to the concentration of Sn vacancies, and can be well tunned by modifying the content of Zn dopants. This study not only offers experimental support to the first-principle calculations of d0 mechanism, but also opens a new way to fabricate room-temperature SnO₂-based diluted magnetic materials.更多还原