【作者】 宋成；

【导师】 潘峰；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 博士

【摘要】 稀磁氧化物是一种有重要前景的自旋源,是新一代自旋器件的重要支撑材料。制备具有室温铁磁性的稀磁氧化物,探索其磁性来源,并将其应用于自旋器件是自旋电子学领域的研究热点之一。本文采用磁控溅射、离子注入和脉冲激光沉积技术分别制备了Co掺杂ZnO和LiNbO₃稀磁氧化物材料,系统研究了掺杂浓度、基片类型及其温度、氧分压和沉积速率等工艺参数对稀磁氧化物局域结构和磁学性能的影响,重点探讨了磁性与Co的局域结构之间的关系、稀磁氧化物的磁性起源,并利用获得的稀磁氧化物材料,设计和构造了隧道结原型自旋器件,研究了隧道结的隧道磁电阻和相关的自旋极化、注入与传输现象。研究结果表明,采用磁控溅射技术成功获得了居里温度在700K以上的Co掺杂ZnO稀磁材料,其磁矩与掺杂浓度、基片密切相关。在LiNbO₃铁电基片上的（4%） Co掺杂ZnO绝缘态薄膜中观察到6.1μB/Co的室温巨磁矩,发现磁性薄膜/铁电基片之间的逆磁电耦合能有效提高薄膜的磁性,在束缚磁极子机制的基础上发展了超耦合模型,并对巨磁矩现象进行了合理的解释。通过调控氧分压,实现了稀磁绝缘体与稀磁半导体Co掺杂ZnO单晶薄膜的可控生长。基于绝缘态的Co掺杂ZnO具有室温强磁性,以及分离结构缺陷与载流子浓度对磁性的影响,揭示了结构缺陷是稀磁氧化物室温铁磁性的真正起源,载流子调制机制中的载流子只是结构缺陷的副产物,从而对磁性机制进行了较完整的诠释。利用所获得的Co掺杂ZnO稀磁材料,成功构造了全外延的（Zn,Co）O/ZnO/（Zn,Co）O隧道结。该隧道结在4K下磁场为2T时具有20.8%的正隧道磁电阻（TMR）。由于界面质量的改善与兼容性,TMR能保持到室温,说明实现了室温的自旋注入和传输。实验进一步发现双隧穿层的（Zn,Co）O基隧道结表现出反常的偏压相关隧道磁电阻曲线,不仅对深入认识自旋现象有重要意义,还将拓宽使用隧道结器件的偏压范围。实验还表明（5%） Co掺杂LiNbO₃材料也具有很好的室温稀磁特性,验证了基于缺陷的束缚磁极子机制。Co掺杂LiNbO₃是一种室温单相多铁材料,它的铁磁和铁电居里温度非常接近,分别为540K和610K。更多还原

【Abstract】 Diluted magnetic oxides （DMO） are promising spin sources, which are considered as important supporting materials for a new generation of spintronics. Preparing DMO with room temperature ferromagnetism （RTFM）, exploring the origin of RTFM, and applicating DMO to spintronics are the hot topics in the field of spintronics. The effects of preparation parameters, such as doping concentration, substrate and its temperature, oxygen partial pressure and deposition rate, on the local structure and ferromagnetic ordering of DMO have been systematically studied. This dissertation discusses Co-doped ZnO and LiNbO₃ prepared by magnetron sputtering, as well as ion implantation and pulsed laser deposition, respectively. These researches concentrate on the correlations between magnetic properties and the local structure, the origin of RTFM in DMO. Employing the prepared DMO materials, magnetic tunnel junctions （MTJ） prototype spintronics are designed and fabricated, whose tunnel magnetoresistance （TMR） and the corresponding spin polarization, injection and trasnsport are explored.The results show that the Co-doped ZnO films deposited by magnetron sputtering possess a Curie temperature higher than 700K, and the magnetic moments of Co are intimately correlated to the doping concentration and the substrate. A giant magnetic moment of 6.1μB/Co is observed in （4 at.%） Co-doped ZnO insulating films. Converse magnetoelectric coupling between Zn0.96Co0.04O films and ferroelectric substrates can effectively enhance the ferromagnetic ordering. A super coupling mechanism based on bound magnetic polarons （BMP） is developed to understand the giant magnetic moment. Taking advantage of modulating the oxygen partial pressure, diluted magnetic insulators （DMI） and semiconductors （DMS） in single-crystal Zn0.96Co0.04O films can be controllably synthesized. Based on the insulating Co-doped ZnO films showing robust RTFM, as well as separating the effects of carrier concentrations and structural defects on magnetization, we unambiguously demonstrate that RTFM is profoundly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The origin of RTFM in DMO is then clearly clarified.Taking advantage of the prepared Co-doped ZnO films, fully epitaxial （Zn,Co）O/ZnO/（Zn,Co）O junctions are successfully fabricated. A positive TMR of 20.8% is obtained at 4 K and at 2 Tesla. Due to the improved crystallinity and compatibility of electrode/barrier interfaces, TMR can resist up to room temperature, indicating that spin injection and transport at room temperature are realized in the junctions. （Zn,Co）O-based MTJ with double-barriers show anomalous bias voltage dependent TMR, which are not only significant for deeply exploring spin phenomena, but also expand the range of bias voltage for using MTJ devices.The results also show that （5%） Co-doped LiNbO₃ is a real DMO with RTFM, demonstrating that the bound magnetic polarons based on defects are the origin of RTFM in DMO. Co-doped LiNbO₃ is a room temperature single-phase multiferroic, which possesses very similar ferromagnetic and ferroelectric Curie temperature, i.e., 540 K and 610 K, respectively.更多还原