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有数千万个电子元件，所有这些元件都是通过推动电子或利用它们的小负电荷储存电子来工作的。这是使用产生电场的电压来完成的。但是电子也有一种叫做自旋的特性，它对磁场和电场都很敏感。这是一种量子力学性质，但在外部磁场中，自旋倾向于与磁场平行或相反-朝向北极或南极。这种向上旋转或向下旋转的配置是数字0或1位信息的美丽模拟。利用电子自旋处理和存储信息是一个新兴的研究和技术领域，称为自旋电子学。有可能开发出超低功耗的自旋电子晶体管，高速运行，用于节能的计算机处理（计算机服务器群，例如通过网络运行金融，搜索和流媒体服务，是巨大的电力消费者）。通过制造磁性晶体管，当电源关闭时，它能记住自己的磁性状态，从而将联合收割机处理和存储结合起来。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，优选的是将具有预定义自旋的电子注入到半导体结构（诸如硅或砷化镓层）中。正如电子学技术依赖于半导体材料以及合适的绝缘体和金属触点的发展一样，自旋电子学将依赖于对真实的材料中自旋行为的理解。也许最有希望的一类材料是1983年理论上发现的半金属铁磁体。这些磁性材料只携带一个自旋的电子的电流，因此可以用来将自旋向上的电子注入自旋电子器件。然而，这些材料的行为没有被很好地理解，并且在原型器件中自旋向上和自旋向下电子之间的不平衡不是非常高，并且福尔斯朝向室温下降[参见例如Nature Physics，vol.3，2007，p.542]。严格来说，真正的半金属材料，除非是在绝对零度，否则是不可能存在的！但并不是所有的半金属都会受到非零温度的影响。此外，当材料不完美时（例如，在晶体结构中包含缺陷或应力），理论暗示半金属性会降低。因此，目前还不清楚哪种半金属（或近半金属）材料最适合于真实的自旋电子学应用。在我们的项目中，我们将使用同步辐射--美国国家同步辐射光源（NSLS）产生的非常强的X射线--来测量两类磁性材料（Heusler合金和二元磷属元素化物）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，以提供对自旋电子材料的磁和电特性的最佳理解。研究结果将提供给其他合作和项目，这些合作和项目旨在利用这些材料在真实的半导体自旋电子应用中的应用。事实上，该项目的一个关键部分是在这一领域开展新的合作。来自沃里克的研究团队在技术和材料方面具有互补的专业知识，我们将进一步与NSLS科学家开展联合工作，以汇集一套世界领先的方法。我们还将开展合作，以更好地从理论上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.