X-ray characterisation of highly polarised ferromagnetic pnictides

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.

In the impact summary, academic beneficiaries section and impact plan, we outline the realistic aims of the short proposed project (points 1 & 2 below) but also mention the broader potential impact (3) of the ongoing research effort in Warwick into magnetic pnictide alloys, of which this project is a very important part. Our work on ferromagnetic pnictides is motivated both by fundamental interest in the properties of magnetic alloys and by the potential applications of such materials for spintronic and magneto-opto-electronic devices. These devices could ultimately benefit the digital economy through new information technology (IT) capability, for instance via ultra high density data storage and digital security based on quantum encryption, as well as improve the energy efficiency of information technology through lower power non-volatile memory and logic elements. Who will benefit from this research? 1. The project will directly benefit at least 4 Warwick Ph.D. students. 2. The users of research outputs from the project will, in the short term, be other academic researchers in fields related to magnetism and spintronics. Since this is a new collaboration, our own groups and research partners in Warwick will benefit immediately. 3. In the longer term we expect that this project, as part of our broader spintronic materials research effort, will benefit private sector organisations developing new magneto-electronic devices. How will they benefit from this research? 1. Our own Ph.D. students will benefit from development of their research expertise and transferable skills during the project. 2. Research partners in Warwick (in the Theory Group and in Chemistry), the Midlands Physics Alliance, and at NSLS will benefit through knowledge-sharing and the development of joint methodologies. Other academic groups will benefit from our results (surface magnetism and spin polarisation). Details are discussed in the Academic Beneficiaries section. 3. Knowledge and intellectual property generated by our wider research effort (ongoing research assisted by this particular project) will enable the potential exploitation of these materials in spintronic devices. What will be done to ensure that they have the opportunity to benefit from this research? 1. Development of research-related and transferable skills is formally accredited in the Warwick Postgraduate Certificate in Transferable Skills in Science on which all Physics Ph.D. students are enrolled. 2. We will continue to develop collaborations within Warwick; in particular we will seek further funding from EPSRC and other bodies to expand collaborations with the Theory Group and Chemistry. Knowledge transfer is aided by thriving Condensed Matter Group, Theory Group and Postgraduate weekly seminars. We will supplement these more general forums with regular meetings as outlined in the impact plan. We will attend an NSLS User Meeting to highlight our work there as well as developing collaborations more informally during experiments. High impact publication and conference presentation will ensure a wide audience for our academic output. 3. It would be premature to discuss routes to commercial exploitation of the larger research effort in magnetic alloys, but formal mechanisms, expertise, contacts and advice are readily available via our close links with the Science City initiative through the Advantage West Midlands regional development agency, and through bodies such as the Warwick Ventures technology transfer office.