Half-metallic ferromagnets: materials fundamentals for next-generation spintronics

半导体(如硅)支撑着现代生活的如此多方面，通过电子和数据处理，用于WWW、电信、医药、交通等，因此很难夸大它们的重要性。然而，硅芯片技术正在接近硬物理限制，需要替代技术。一种激进的方法是自旋电子学，电子的“自旋”和电荷都用于数据存储和处理。自旋是电子与磁性有关的一个基本性质：在磁场中，自旋倾向于以两种方式中的一种方式排列，即顺场或逆场。SPIN的充分利用将使革命性的新芯片设计成为可能，这些芯片设计速度快、能效高，并将数据存储与逻辑完全集成在一起。我们将研究半金属铁磁(HMF)材料。HMF是20世纪80年代在理论上发现的一类材料，它结合了半导体和铁磁金属的性质。两个电子自旋排列中只有一个可以很容易地在HMF中移动-它们是“100%自旋极化的”。因此，它们应该是用于自旋电子学的理想材料。然而，尽管在制造HMF器件方面付出了大量的研究努力，但在大多数情况下，HMF的性能并不优于普通的磁性材料(通常是30%-40%的自旋极化)。目前尚不清楚为什么会出现这种情况，这阻碍了HMF被解锁用于先进自旋电子学的潜力。我们建议通过全面而严格地研究实际用于器件的物理形式的HMF来解决这一突出问题，即在氧化物或半导体衬底上的薄膜中。我们将结合我们在四个领域的专业知识：(1)高质量HMF薄膜的生产，(2)精确到原子水平的磁性薄膜的表征，(3)对这些材料的准确理论描述，以及(4)HMF自旋电子器件的制造。这将使我们能够全面地研究HMF性能减弱的最可能的罪魁祸首，即温度、缺陷和HMF/衬底界面。当HMF升温时，自旋极化会坍塌，而对于实际设备来说，这一截止温度必须远远高于室温。我们将明确地衡量这一点，并用最近在沃里克开发的最先进的理论对其进行建模。薄膜中的残留缺陷可以破坏自旋极化，我们将通过原子尺度成像/建模来了解这些缺陷，并调整我们的薄膜生长以将其降至最低。最后，HMF和它的衬底之间必须始终有一个界面，这也会影响自旋极化和功能性能。我们将对界面进行成像和建模，并再次调整我们的增长以优化它们。使用尖端的像差校正电子显微镜(约克和华威都有这样的显微镜，功能互补)，原子尺度的成像和分析是可能的。最后，这项基础性工作将与HMF在原型自旋电子器件中的功能性能相关联。我们将能够使用既定的设计来制造器件，并随后测量实际器件结构上的原子级接口和缺陷。这种从第一原理理论到器件性能的独特能力组合将使首次全面而严格地研究真实薄膜结构中的半金属含量成为可能。我们的目标是从根本上了解HMF在实际结构中的局限性，指导未来的HMF器件设计，并开发HMF薄膜中可能存在的最高室温自旋极化。在York和Warwick之间，我们在三种不同类别的HMF材料(过渡金属磁铁矿、磁铁矿和Heusler合金)方面拥有生长专业知识，这将使我们都能够对HMF有一个全面的了解，并找到用于超高自旋极化薄膜的最佳材料。

Semiconductors (such as silicon) underpin so many aspects of modern life, through electronics and data processing for the WWW, telecoms, medicine, transport, etc., that it is hard to overstate their importance. However, silicon chip technology is approaching hard physical limits and alternatives are needed. One radical approach is spintronics, where the both the "spin" and charge of electrons are used for data storage and processing. Spin is a fundamental property of electrons related to magnetism: in a magnetic field, a spin prefers to align in one of two ways, along or against the field. Full utilisation of spin would enable revolutionary new chip designs, which are fast, energy-efficient and fully integrate data storage with logic. We will study half-metallic ferromagnetic (HMF) materials. HMFs are a class of materials discovered theoretically in the 1980s which combine the properties of a semiconductor and a ferromagnetic metal. Only one of the two electron spin alignments can easily move inside an HMF - they are "100% spin-polarised". They should hence be ideal materials for use in spintronics. However, despite major research efforts to make HMF devices, in most cases HMFs do not outperform ordinary magnetic materials (which are typically 30-40% spin-polarised). There is no clear understanding of why this is the case, which prevents the potential of HMFs being unlocked for advanced spintronics. We propose to solve this outstanding problem with a comprehensive and rigorous study of HMFs in the physical form which is actually used in devices, i.e. in thin-films on an oxide or semiconductor substrate. We will combine our expertise in four areas: (1) production of high quality thin films of HMFs, (2) characterisation of magnetic thin films down to the atomic level, (3) accurate theoretical description of these materials, and (4) fabrication of HMF spintronic devices. This will enable us to study holistically the most likely culprits for weakened HMF performance, namely temperature, defects and the HMF /substrate interface. Spin-polarisation collapses as an HMF heats up, and this cut-off, for a practical device, must be well above room temperature. We will measure this explicitly and model it with state-of-the-art theory developed recently in Warwick. Residual defects in the thin films can destroy spin polarisation and we will both understand these via atomic-scale imaging / modelling and adjust our thin film growth to minimise them. Finally, there must always be an interface between the HMF and its substrate, which also influences the spin polarisation and functional performance. We will image and model the interfaces, and again adjust our growth to optimise them. Atomic-scale imaging and analysis is possible using cutting-edge aberration-corrected electron microscopes (York and Warwick each have such a microscope, with complementary capabilities). Finally, this fundamental work will be correlated with the functional performance of the HMFs in prototypical spintronic devices. We will be able to fabricate devices, using established designs, and subsequently measure the atomic-scale interfaces and defects on the actual device structure. This unique combination of capabilities ranging from first-principles theory to device performance will enable the first comprehensive and rigorous study of half-metallicity in real thin film structures. Our goals are to understand in a fundamental way the limitations of HMFs in real structures, to guide future HMF device design, and also develop the highest possible room temperature spin polarisation in HMF thin films. Between York and Warwick, we have growth expertise in three different classes of HMF material (transition metal pnictides, magnetite and Heusler alloys) which will enable us both to produce a generalised understanding of HMFs and find the best materials for ultra-high spin polarisation films.