Half-Metallic Semiconducting Magnets with Gapless Dispersion and Antiferromagnetism

开发结合电子磁性的新电子设备是设备工程和纳米技术的前沿。基于自旋的电子学或“自旋电子学”目前对信息技术的电子设备有很大的影响。1988年，在一种新的合成材料中有了一个关键的发现，在这种材料中，一个小的磁场可以产生很大的电阻变化，这种效应被称为巨磁阻。这种微小的混合结构允许更小的磁性数据存储设备（硬盘驱动器）。1995年，另一种磁阻装置被发现，这也是一种合成混合结构，但这种装置依赖于量子力学隧道效应。这种奇特的隧道效应可以想象成一个粒子与一堵墙相撞，然后突然在另一边重新出现。这种基于自旋的现象目前用于所有计算机中安装的存储设备，也正在被纳入磁性随机存取存储器（MRAM）。当今该领域的突出问题之一是制造新的设备，可以产生具有预定南北磁性取向的载流电子。该研究奖的重点是开发合成多层半导体结构，通过施加简单的输入电压来控制电子的磁取向。进一步推进这些基于自旋的器件的科学和工程有望推动未来低功耗、高速、高密度的应用，并最终在信息处理方面具有成本效益。所提出的方法有望为新材料和设备打开大门，这些材料和设备具有以前从未考虑过的有用特性。通过在电子、磁性和光学纳米结构领域对年轻人进行教育和培训，确保这项研究的广泛影响，这些领域对发展信息技术的未来应用至关重要。最近有人预测，一类新型材料将融合半金属磁铁和半导体的特性。理论能带结构计算表明，这些逆Heusler材料在电子自旋的一个方向上具有费米能，而在电子自旋的另一个方向上，价带和导带边缘在费米能处相遇。这些用于器件的自旋无间隙半导体（SGS）的一大优点在于，一个简单的栅极电压就可以调节自旋特性。此外，这些逆Heusler材料包含半金属反铁磁体（HMAF），它们具有自旋极化但非磁性。即使在室温下，SGS和HMAF材料的新功能也将利用一些新颖而有价值的特性。这些有价值的资产包括：半金属高自旋极化（~ 100%）；产生自旋极化空穴和自旋极化电子；电压可调自旋极化；和自旋极化HMAF无边缘磁场。到目前为止，已有几十种逆Heusler材料被预测具有SGS性质。它们包括磁性Mn2CoAl和反铁磁性Mn3Al。到目前为止，仅合成了少量材料：如块状Mn2CoAl和Fe2CoSi；以及合成和研究薄膜（GaAs上的外延Mn2CoAl）的第一步。研究重点是利用MBE和溅射技术合成含有这些X2YZ四亚晶格材料的薄膜器件。为了使费米能随自旋极化导带和价带的变化而变化，多层器件将被制作成带有电压门的器件。为了研究自旋自由度，将制作隧道结。合作者包括来自几所大学和国家同步加速器和中子实验室的研究人员。该基金由电气、通信和网络系统部（ECCS）的电子、光子学和磁性器件（EPMD）项目和材料研究部（DMR）的电子和光子材料（EPM）项目共同资助。

Developing new electronic devices that incorporate the electron's magnetic properties is at the forefront of device engineering and nanotechnology. Spin-based electronics or "spintronics" is currently having a great impact on electronic devices for information technology. A key discovery was made in 1988 in a new synthetic material, where a small magnetic field could produce a large change in resistance, an effect called Giant Magnetoresistance. This tiny hybrid structure allowed for much smaller magnetic data storage devices (hard disk drives). In 1995 another magnetoresistive device was discovered, that was again a synthetic hybrid structure, but this device relied on quantum mechanical tunneling. Such exotic tunneling can be imagined as a particle colliding with a wall and suddenly reappearing on the other side. This spin-based phenomenon is currently used in memory devices installed in all computers and is also being incorporated in magnetic random access memories (MRAM). One of the outstanding issues today in this field is to build new devices that can produce current-carrying electrons that have a predetermined North-South magnetic orientation. This research award is focused on developing synthetic multilayer semiconductor structures for controlling the electron's magnetic orientation by applying a simple input voltage. Furthering the science and engineering of these spin-based devices is expected to advance future applications that are low-power, high-speed, high-density, and eventually cost effective for information processing. The proposed approach is expected to open doors to new materials and devices having useful properties never before contemplated. Broad impact of this research is assured by educating and training young people in the areas of electronic, magnetic and optical nanostructures that are crucial for developing future applications in information technology.A novel class of materials has recently been predicted that merge the properties of half-metallic magnets and semiconductors. Theoretical band structure calculations show that these inverse Heusler materials have a Fermi energy lying in a gap for one direction of electron spin, but for the other direction of electron spin the valence and conduction band edges meet at the Fermi energy. One of the great advantages of these spin gapless semiconductors (SGS) for devices relies on the property where a simple gate voltage can tune the spin properties. Furthermore, these inverse Heusler materials encompass half-metallic antiferromagnets (HMAF) that are spin-polarized but nonmagnetic. New functionalities of SGS and HMAF materials would take advantage of several novel and valuable properties, even at room temperature. These valuable assets include: half-metallic high spin polarization (~100 %); generation of spin-polarized holes as well as spin-polarized electrons; voltage-tunable spin polarization; and spin-polarized HMAF without fringing magnetic fields. Thus far, several dozen inverse Heusler materials have been predicted to have SGS properties. These include magnetic Mn2CoAl and antiferromagnetic Mn3Al. Up to now, only a few materials have been synthesized: such as bulk Mn2CoAl and Fe2CoSi; and the first major step to synthesize and investigate thin films (epitaxial Mn2CoAl on GaAs). The research focuses on the synthesis of thin film devices incorporating these X2YZ four-sublattice materials using MBE and sputtering. Multilayer devices will be fabricated with voltage gates in order to vary the Fermi energy with respect to the spin-polarized conduction and valence bands. Tunnel junctions will be fabricated for investigating the spin degrees of freedom. Collaborators include researchers from several universities, and national synchrotron and neutron laboratories. This grant is funded jointly by the Electronics, Photonics, and Magnetic Devices (EPMD) Program in the Division of Electrical, Communications and Cyber Systems (ECCS) and by the Electronic and Photonic Materials (EPM) Program in the Division of Materials Research (DMR).