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

半导体（如硅）支撑着现代生活的许多方面，通过电子和数据处理的万维网，电信，医药，运输等，很难夸大它们的重要性。然而，硅芯片技术正在接近硬物理极限，需要替代品。一种激进的方法是自旋电子学，其中电子的“自旋”和电荷都用于数据存储和处理。自旋是与磁性相关的电子的基本性质：在磁场中，自旋倾向于以两种方式之一排列，沿着或逆着磁场。充分利用自旋将使革命性的新芯片设计成为可能，这些芯片设计速度快，节能，并将数据存储与逻辑完全集成。我们将研究半金属铁磁（HMF）材料。HMF是在20世纪80年代理论上发现的一类材料，其联合收割机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内部移动-它们是“100%自旋极化”的。因此，它们应该是自旋电子学中使用的理想材料。然而，尽管在制造HMF器件方面进行了大量的研究工作，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）。目前还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。我们建议解决这个突出的问题，全面和严格的研究HMF的物理形式，实际上是在设备中使用，即在氧化物或半导体衬底上的薄膜。我们将联合收割机结合我们在四个领域的专业知识：（1）生产高质量的HMF薄膜，（2）表征磁性薄膜的原子水平，（3）这些材料的精确理论描述，和（4）制造HMF自旋电子器件。这将使我们能够全面研究最有可能削弱HMF性能的罪魁祸首，即温度，缺陷和HMF /基材界面。当HMF加热时，自旋极化崩溃，对于实际设备来说，这个截止温度必须远高于室温。我们将明确地测量这一点，并用沃里克最近开发的最先进的理论对其进行建模。薄膜中的残余缺陷会破坏自旋极化，我们将通过原子级成像/建模来了解这些缺陷，并调整我们的薄膜生长以最大限度地减少它们。最后，在HMF和其基底之间必须始终存在界面，这也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（约克和沃里克都有这样的显微镜，具有互补的功能），原子尺度的成像和分析是可能的。最后，这一基础工作将与原型自旋电子器件中的HMF的功能性能相关。我们将能够使用已建立的设计来制造器件，并随后测量实际器件结构上的原子级界面和缺陷。从第一原理理论到器件性能的这种独特的能力组合将使真实的薄膜结构中的半金属性的第一次全面和严格的研究成为可能。我们的目标是从根本上了解HMF在真实的结构中的局限性，以指导未来的HMF器件设计，并在HMF薄膜中开发尽可能高的室温自旋极化。在约克和沃里克之间，我们拥有三种不同类型的HMF材料（过渡金属磷属元素化物、磁铁矿和Heusler合金）的生长专业知识，这将使我们能够对HMF产生普遍的理解，并找到超高自旋极化膜的最佳材料。

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.