一维磁性纳米结构中的交换偏置效应和自旋玻璃行为研究

Exchange bias (EB) effect in magnetic nanoparticle systems is one of the foreland fields of spintronics. As for the exchange bias effect， not only its potentially technological application in the magnetic devices but also its mechanism has been an attractive issue among topics of interest. Studies show that some antiferromagnetic nanostructures exhibit spin glass behavior and exchange bias effect at low temperatures, and the exchange coupling between the antiferromagnetism and surface spin glass has been suggested to explain the observed exchange bias effect. This project confirms that the exchange bias effect originates from the exchange coupling between the antiferromagnetism and surface spin glass by studying the dynamic behavior of surface spin glass and exchange bias effect in SrMn3O6-x and Li0.44MnO2 nanoribbons. In addition, the project extends Malozemoff random-field model to the one-dimensional (1D) nanostructures, and derives a formula which can give the quantitative interpretation of the exchange bias field in 1D nanostructures, and proposes to estimate the thickness of the spin glass layer on the surface of 1D nanostructures using the random-field model. This topic has the distinctive combining features on theoretical analysis, numerical simulation and experimental research. It can not only enrich the basic theory of mesoscopic physics and spintronics, but also provide experimental and theoretical support for the study of a new generation of spintronics devices.

磁性纳米颗粒系统中的交换偏置效应是当前自旋电子学领域的研究热点问题。交换偏置效应不仅在磁性器件上具有潜在的应有价值，其内在的物理机制也是众多研究人员长期以来讨论的热点。研究发现一些反铁磁纳米结构在低温下表现出自旋玻璃行为和交换偏置效应，反铁磁性与自旋玻璃的耦合作用被建议来解释交换偏置效应。本项目通过对SrMn3O6-x和Li0.44MnO2纳米带中表面自旋玻璃动力学行为和交换偏置效应的研究，证实纳米带的交换偏置效应起源于反铁磁性和表面自旋玻璃的交换耦合作用。此外，本项目将Malozemoff的随机场模型推广到一维纳米带中，得出了定量解释一维纳米带中交换偏置场大小的公式，同时提出利用随机场模型来估计纳米带表面自旋玻璃层的厚度。本课题具有鲜明的理论分析、数值模拟与实验研究相结合的特色。它不仅能够丰富介观物理和自旋电子学的基本理论，而且为研究新一代自旋电子器件提供实验和理论的支撑。

理论上和实验上都已经证实，由于表面无序自旋的存在，反铁磁纳米系统表现出核-壳的磁结构。对于壳层磁行为的本质，不同的研究组给出了不同的结论，如自旋玻璃、弱铁磁性和稀释反铁磁性等。因此，壳层的本质需要进一步的研究。本项目通过测量SrMn3O6-x纳米带交流磁化率随温度的依赖关系,研究其动力学行为,确认了纳米带在低温下存在表面自旋玻璃转变。热剩磁和等温热剩磁的研究表明，SrMn3O6-x纳米带在磁场H<2 T时呈现出自旋玻璃行为；而H>2 T呈现出稀释反铁磁行为。我们认为稀释反铁磁团簇镶嵌到自旋玻璃背景中导致此现象。SrMn3O6-x纳米带的交换偏置场随着温度的上升而迅速下降，并且在自旋玻璃转变温度以上消失。这说明交换偏置与自旋玻璃有着密切的关系。磁锻炼效应的分析表明，SrMn3O6-x纳米带的交换偏置效应起源于表面自旋玻璃与芯部反铁磁的界面耦合作用。此外，我们还研究了CaMn3O6纳米带的磁性。结果表明,CaMn3O6纳米带分别在在125 K和240 K表现出铁磁性和电荷有序。本项目为反铁磁和自旋玻璃的耦合可以导致交换偏置提供了直接的证据。