同步辐射技术辅助研究MgO单晶的d0铁磁性

The d- or f-orbitals of ions are empty or fully-occupied in materials such as HfO2, MgO and graphite. Without magnetic ions doping, these materials exhibit so-called d0 ferromagnetism. The mechanism and potential applications in spintronic devices of d0 ferromagnetism have been attracting tremendous interesting of communities. It is generally believed that defects-induced local moments will couple each other via exchange mechanism and therefore result in long range ferromagnetic ordering. It is vital important to systematically characterize defect configurations in materials so as to understand the mechanism of d0 ferromagnetism in a correct way. MgO is an important ionic oxide with the same crystal structure as NaCl and has been widely utilized in many fields such as functional devices. Due to its simple crystal structure, MgO single crystal can serve as a prototype material, defects of which can be correctly and systematically studied. In this proposal, MgO single crystal will be electron-irradiated, γ-ray irradiated or annealed in different atmosphere conditions and hence point defect configurations will be modified. Based on many unique and outstanding properties of synchrotron radiation, we will combine synchrotron radiation scattering and absorption with other experimental techniques, such as positron annihilation spectroscopy, electron paramagnetic resonance spectrum, absorption spectrum and photoluminescence spectrum, to systematically characterize the point defect configurations in MgO single crystal. After establishing a direct correlation between ferromagnetic properties and point defect configurations of MgO single crystal, we will carry out theoretical calculations, based on a crystal model involving experimental results on point defect configurations, to elucidate underlying mechanism of d0 ferromagnetism. The possible ways to modulate d0 ferromagnetic properties of MgO single crystal will also be discussed.

在HfO2，MgO和石墨等材料中，组元离子没有未配对的d电子或f电子。在没有磁性离子掺杂的情况下，这类材料表现出d0铁磁性。通常认为d0铁磁性源于材料的结构缺陷导致的局域磁矩，但具体的形成机制尚未达成共识。d0铁磁性在自旋电子学器件中的可能应用引起了人们的极大兴趣。完整地表征材料的缺陷组态对于准确理解d0铁磁性的产生机制是至关重要的。MgO具有岩盐结构，是一类重要的离子氧化物材料，在功能器件等许多领域中有重要的应用，而简单的晶体结构利于人们完整准确地表征MgO的缺陷组态。本项目以MgO单晶作为研究对象，通过辐照、离子轰击和热处理等方式改变其点缺陷组态，综合利用同步辐射吸收和散射技术、正电子湮没技术、电子自旋共振技术、吸收光谱和光致荧光谱等实验手段来表征材料的点缺陷组态，建立点缺陷组态与铁磁性之间的对应关系，结合理论计算，阐明MgO单晶的d0铁磁性机制，探讨调控其铁磁性的可能途径。

在HfO2，MgO和石墨等材料中，组元离子没有未配对的d电子或f电子。在没有磁性离子掺杂的情况下，这类材料表现出d0铁磁性。通常认为d0铁磁性源于材料的结构缺陷导致的局域磁矩，但具体的形成机制尚未达成共识。完整地表征材料的缺陷组态对于准确理解d0铁磁性的产生机制是至关重要的。本项目以MgO单晶作为研究对象，试图阐明MgO单晶的d0铁磁性机制。.对(100)取向的MgO单晶进行了不同剂量的60Co-γ射线辐照，辐照剂量从30 kGy到1750 kGy。利用同步辐射漫散射技术以及紫外-可见吸收光谱研究了辐照样品的点缺陷情况，并将实验测量的漫散射结果与理论计算结果进行比对，以获得点缺陷组态的信息。利用超导量子干涉仪(SQUID)测量了样品在不同温度下的磁性质。漫散射和吸收光谱的实验结果表明，经γ射线辐照的MgO单晶产生了阴离子弗伦克尔缺陷。γ射线辐照的MgO单晶在室温下没有表现出铁磁性，只是在低温下观察到了顺磁信号，且辐照前后样品在零场冷却和场冷却下的M-T曲线没有变化，说明阴离子空位浓度较低，没有导致MgO的d0铁磁性。.对不同取向的MgO单晶进行了不同剂量的中子辐照，辐照剂量从1.0×1016 到 1.0×1020 cm-2。利用HRXRD、RSM和UV-Vis光谱对辐照样品的缺陷组态进行了深入的研究，并且根据黄昆漫散射理论，计算了偶极力点缺陷引起的漫散射强度分布情况，结合倒易空间漫散射强度分布图和吸收光谱的实验结果，证实了中子辐照的MgO单晶中产生了较高浓度的O离子弗伦克尔缺陷，并且随着辐照剂量的增加，缺陷浓度也随之增大。高浓度的O离子缺陷使得MgO单晶在40 K以下时表现出d0铁磁性。