X-ray characterisation of highly polarised ferromagnetic pnictides

我们操纵微小电流和电荷的能力改变了世界。一个典型的硅芯片上存在数千万个电子组件，所有这些组件都是通过推动电子或利用它们的小负电荷存储电子来工作的。这是通过产生电场的电压来实现的。但电子也有一种称为自旋的性质，它对磁场和电场都很敏感。这是一种量子力学性质，但在外加磁场中，自旋倾向于与磁场平行或相反的方向--朝向北极或南极。这种自转加速或自转减速配置是对数字0或1比特信息的漂亮模拟。使用电子自旋处理和存储信息是一种名为自旋电子学的新兴研究和技术领域。有可能开发高速运行的超低功耗自旋电子晶体管，用于高能效的计算机处理(计算机服务器群，例如在网络上运行金融、搜索和流媒体服务，是巨大的电力消耗)。通过构建磁晶体管，在电源关闭时记住它们的磁状态，可以将处理和记忆结合在一起。甚至有可能使用自旋来执行量子计算，即作为量子比特而不是经典比特。为了能够操纵用于信息处理的自旋，最好将具有预定自旋的电子注入到半导体结构中，例如硅层或砷化镓。正如电子技术依赖于开发半导体材料以及合适的绝缘体和金属触点一样，自旋电子学将取决于对真实材料中自旋行为的理解。从理论上讲，1983年发现的最有希望的一类材料是半金属铁磁体。这些磁性材料只携带电子一个自旋的电流，因此可以用来向自旋电子器件注入仅自旋的电子(比方说)。然而，这些材料的行为还没有被很好地理解，原型设备中自旋上升和下降的电子之间的不平衡不是很高，并且下降到室温[例如，见《自然物理》，第3卷，2007年，第542页]。严格地说，真正的半金属材料只有在绝对零度时才能存在！但并不是所有的半金属都会受到非零温度影响的程度相同。此外，当材料不完美时(例如，晶体结构中含有缺陷或应力)，理论暗示半金属丰度降低。在我们的项目中，我们将使用同步辐射--美国国家同步加速器光源(NSLS)产生的非常强的X射线--来测量两类磁性材料(Heusler合金和二元磁石)的电子和磁性。我们将能够研究温度和材料缺陷的影响。这些实验结果将与理论计算相结合，以最好地理解自旋电子材料的磁性和电学性质。这一结果将提供给其他合作和项目，这些合作和项目寻求在真正的半导体自旋电子应用中开发这些材料。事实上，这个项目的一个关键部分是在这一领域发展新的合作。来自华威的研究团队在技术和材料方面拥有互补的专业知识，我们将进一步发展与NSLS科学家的联合工作，将世界领先的一套方法结合在一起。我们还将开展合作，从理论上更好地描述这些具有挑战性但令人着迷的材料。

Our ability to manipulate tiny electric currents and charges has changed the world. Tens of millions of electronic components exist on a typical silicon chip, all of which work by pushing electrons around or storing them using their small negative charge. This is done using voltages, which produce electric fields. But electrons also have a property called spin , which is sensitive to magnetic as well as electric fields. This is a quantum-mechanical property but in an external magnetic field, the spin tends to align either parallel or opposite ot the field - towards the north or south pole. This spin up or spin down configuration is a beautiful analogue to the digital 0 or 1 bit of information. Processing and storing information using electron spins is a burgeoning field of research and technology called spintronics . It could be possible to develop ultra-low-power spintronic transistors which operate at high speed, for energy efficient computer processing (computer server farms, for example running financial, search and streaming services over the web, are enormous electricity consumers). It may be possible to combine processing and memory by building magnetic transistors which remember their magnetic state when the power is switched off. It may even be possible to use spins to perform quantum computation, i.e. as qubits rather than classical bits. In order to be able to manipulate the spins for information processing, it is preferable to inject electrons with predefined spin into a semiconductor structure such as a layer of silicon or gallium arsenide. Just as the technology of electronics depndended on developing semiconductor materials, as well as suitable insulators and metals contacts, spintronics will depend on understanding the behaviour of spins in real materials. Perhaps the most promising class of materials, discovered theoretically in 1983, is the half-metallic ferromagnet . These magnetic materials only carry electrical current with electrons of one spin, and so could be used to inject just spin up electrons (say) into a spintronic device. However, the behaviour of these materials is not well understood and the imbalance between spin up and spin down electrons in prototype devices is not very high and falls away towards room temperature [see, for example, Nature Physics, vol. 3, 2007, p. 542]. Strictly speaking, truly half-metallic materials cannot exist except at absolute zero! But not all half-metals are expected to suffer from the effects of non-zero temperature to the same degree. Furthermore, when the material is not perfect (for example, containing defects or stress in the crystal structure), theory hints that half-metallicity is reduced. So it is not at all clear which half-metallic (or nearly half-metallic) materials will be best for real spintronics applications.In our project we will use synchrotron radiation - very intense X-rays generated at the National Synchrotron Light Source (NSLS) in the USA - to measure the electronic and magnetic properties of two classes of magnetic materials ( Heusler alloys and binary pnictides ). We will be able to study the effects both of temperature and of imperfections in the materials. These experimental results will be combined with theoretical calculations to provide the best possible understanding of the magnetic and electrical properties of spintronic materials. The results will feed into other collaborations and projects which seek to exploit these materials in real semiconductor spintronic applications. In fact, a crucial part of this project is developing new collaborations in this area. The research team from Warwick have complementary expertise in the techniques and materials and we will further develop joint work with the NSLS scientists to bring together a world-leading suite of methods. We will also develop collaborations to better theoretically describe these challenging but fascinating materials.