Half metal oxides: In search for 100% spin polarised materials

自旋电子学是一个快速发展的领域，除了电子的电荷外，它还利用电子的自旋来创造结合逻辑、数据存储和传感器应用的新设备。自旋电子学的巨大潜力激发了从自旋输运、自旋注入/积累和自旋操纵到自旋阀和磁隧道结等器件制造的广泛研究。自旋电子学领域的主要挑战之一是寻找/创造兼容的高自旋极化材料(晶格匹配、导电性匹配、热力学稳定性、高居里温度(TC)等)。采用了cmos技术。在这个方案中，磁铁矿(Fe3O4)被认为是最适合于自旋电子学的高自旋极化材料，通过在原子水平上理解这种材料，寻求解决其实现中的挑战。传统的3D铁磁性金属及其合金在费米水平上只有30%-40%的自旋极化，因此具有更好自旋极化的材料系统对于下一代自旋电子器件至关重要。用密度泛函理论(DFT)预测了费米能级上100%自旋极化材料的存在。这种100%自旋极化材料，也被称为半金属，其中一个自旋通道是金属的，而另一个自旋通道是绝缘的。相当多的材料，包括氧化物(Fe_3O_4、CrO_2、锰氧化物)、锡石、硫系化合物和Heusler合金，已被预测为半金属。在这些材料中，磁铁矿(T_c=855 K)是特别有价值的，因为：(I)它的T_c超过了器件应用的阈值温度500K；(Ii)它与用于自旋电子学应用的两种最重要的氧化物--氧化镁和铝酸镁--有很好的晶格匹配；(Iii)它可以与相关的半导体(SC)形成原子锐利的界面，如砷、氮化镓和碳化硅；以及(Iv)它的层状结构将允许在磁铁/氧化物和磁铁/Sc异质结上进行界面原子工程。值得注意的是，没有其他SP材料具有这些特性。例如，CrO2的T_c低于500K，而Heusler/Sc结并不突然，这是因为Heusler需要较高的退火温度才能完全有序地形成半金属L21结构。为了将磁铁矿融入到器件结构中，需要生长磁铁矿薄膜和具有合适的氧化物、金属和半导体(SC)的磁铁矿异质结构。成功应用Fe3O4需要克服的两个主要挑战是：1)在控制化学计量比和结构缺陷的情况下生长薄膜；众所周知，反相域边界(APB)等缺陷可以完全决定磁铁矿薄膜的功能，从而控制APBS的性质和密度是非常重要的；2)设计磁铁矿/氧化物势垒和磁铁矿/SC之间的界面；界面上的自旋输运主要取决于界面的原子结构。这是理解磁铁矿薄膜中自旋相关现象的结构基础的两个关键步骤，以及一些具有重要技术意义的磁铁矿/氧化物和磁铁矿/Sc界面。这一知识将为基于Fe3O4的自旋电子器件的工程提供一条途径和指导。为了实现这一目标，即薄膜的功能与其原子结构的直接关联，在这一应用中，我提出了对半金属磁铁氧化物薄膜的生长以及对自旋电子器件感兴趣的APBS、磁铁矿/氧化物和磁铁矿/Sc界面的原子和电子结构的联合实验和理论研究。薄膜生长将由分子束外延完成，自旋极化计算将由DFT执行，高分辨率透射电子显微镜、高角度环形暗场成像和电子能量损失谱将用于在原子尺度上全面表征这些系统。

Spintronics is a rapidly developing field that utilises the electron's spin in addition to its charge to create new devices combining logic, data storage and sensor applications. The huge potential of spintronics has stimulated a wide range of research from spin transport, spin injection/accumulation and spin manipulation to device fabrication such as spin valves and magnetic tunnel junctions. One of the main challenges in the spintronics field is to find/create highly spin polarised materials that are compatible (lattice match, conductivity match, thermodynamically stable, high Curie temperature (Tc), etc.) with CMOS technology. In this proposal magnetite (Fe3O4) is proposed as the optimum highly spin polarised material for spintronics and by understanding the material at the atomic level seeks to solve the challenges in its implementation.Conventional 3d ferromagnetic metals and their alloys are only 30-40% spin polarised at the Fermi level, thus material systems with better spin polarisation are essential for the next generation of spintronic devices. The existence of 100% spin polarised materials at the Fermi level has been predicted by density functional theory (DFT). Such 100% spin polarised materials, also termed half-metals, have one of the spin channels metallic while the other spin channel is insulating. A rather large number of materials including oxides (Fe3O4, CrO2, manganites), pnictides, chalcogenides, and Heusler alloys have been predicted to be half-metallic. Among these materials magnetite (Tc=855 K) is of special interest since: (i) it has a Tc in excess of 500K, the threshold temperature for device applications; (ii) it has an excellent lattice match with MgO and MgAl2O4, the two most important oxides for spintronic applications; (iii) it can form atomically sharp interfaces with relevant semiconductors (SC) such as GaAs, GaN and SiC; and (iv) its layered structure will allow interface atomic engineering at magnetite/oxide and, magnetite/SC heterojunctions. It is worth noting that no other SP materials have these properties. For example, CrO2 has Tc below 500K and Heusler/SC junctions are not abrupt due to the high annealing temperature required for Heuslers to fully order into a L21 structure that is half-metallic. In order to incorporate magnetite in device structure, growth of thin films of magnetite and heterostructures of magnetite with suitable oxides, metals and semiconductors (SC) is required.The two main challenges to overcome for successful application of Fe3O4 are: 1) growth of thin films with control of stoichiometry and structural defects; it is well known that defects such as antiphase domain boundaries (APBs) can completely determine the functionality of magnetite films, thus controlling the APBs nature and density is highly important;2) engineering the interfaces between magnetite/oxide barriers and magnetite/SC; spin transport across interfaces critically depends on the interfaces' atomic structure.These are the two crucial steps to understand the structural basis of spin-related phenomena in the magnetite films as well as some of technologically important magnetite/oxide and magnetite/SC interfaces. This knowledge would provide a path and guide for the engineering of spintronic devices based on Fe3O4. In order to achieve this goal, the direct correlation of the films' functionality and their atomic structure, in this application I propose a joint experimental and theoretical study on the growth of half-metal magnetite oxide films and the atomic and electronic structure of the film, APBs and magnetite/oxide and magnetite/SC interfaces which are of interest for spintronic devices. Film growth will be done by Molecular Beam Epitaxy, spin polarised calculations will be performed by DFT, and High Resolution Transmission Electron Microscopy, High Angle Annular Dark Field Imaging and Electron Energy Loss Spectroscopy will be used to fully characterise these systems on atomic scale.