Neutron and Synchrotron Radiation Scattering Studies of New Ferromagnetic Semiconductors and their Nanostructures

磁性半导体目前受到极大的关注，因为这些材料有望给计算机和通信技术带来革命性的变化。新一代电子学，通常被称为“自旋电子学”，不仅利用电子电荷，而且利用它的自旋--这一特征在目前使用的半导体芯片中没有得到利用。美国、日本和欧洲的一些团队现在正在竞争寻找合成新的磁性半导体的最佳方法，以适合建造实用的自旋电子学设备。除了材料技术学家正在进行的努力外，凝聚态物理学家还需要付出大量努力来表征新出现的材料的磁性和其他相关性质。中子散射和同步辐射是两种强大的实验工具，使人们能够从原子水平详细了解凝聚态物质系统的磁性。这个项目的目的是使用这两种技术来研究新的自旋电子学材料，特别是强调它们的磁性背后的物理机制。应该强调的是，引起半导体磁性的机制与大多数其他已知的磁性系统(如铁)并不完全相同，而且这种机制的所有细节还没有完全被理解。未来自旋电子学设备的基础将是纳米结构，如超晶格--即由极薄的磁性和非磁性半导体层交替制成的“三明治”。关于这种三明治的一个问题--从设计自旋电子学设备的角度来看是非常重要的--是两个磁层如何在中间的非磁性“间隔物”上“沟通”。中子和同步辐射仪器特别适合研究这些现象。这些研究也是我们项目的重要组成部分。铁磁半导体(FMSC)目前受到极大的关注，因为这种材料对于开发“自旋电子学”--一种不仅可以控制电流大小，而且可以控制其自旋极化的新一代电子学--是必不可少的。该项目的目的是开发中子和同步辐射散射技术的潜力，以阐明有关新合成的FMSC材料及其纳米结构的几个重要问题。应该强调的是，FMSCs在许多方面不同于传统的铁磁材料，后者要么是金属，要么是绝缘体。与金属一样，某些新型FMSC系统(例如，Ga(Mn)As)的磁性是由载流子感应的--然而，不是由电子而是由空穴引起的。这一新的物理机制的细节尚不清楚。非弹性中子散射工具在这类研究中可能有很大帮助，因为它们使人们能够获得表征磁性离子之间相互作用的非常准确的交换参数值。另一个重要的问题是理解由非磁性间隔物隔开的FMSC层之间的交换相互作用转移的机制。中子和同步辐射反射仪是研究这种相互作用的有力工具。这些技术还使人们能够研究磁性/非磁性半导体异质结中界面区域的结构缺陷。这些缺陷可能会对未来自旋电子器件的性能产生重大影响。因此，对这个问题的洞察具有相当重要的意义。

Magnetic semiconductors currently receive a great deal of attention because these materials are expected to revolutionize the computer and communication technologies. The new-generation electronics, usually referred to as "spintronics", exploits not only the electronic charge, but also its spin - a feature not taken advantage of in the presently used semiconductor chips. A number of teams in the US, Japan and Europe are now competing to find the best ways of synthesizing new magnetic semiconductors suitable for building practical spintronics devices.Parallel to the ongoing efforts of material technologists, much effort is also needed from condensed matter physicists to characterize the magnetism and other related properties of the new emerging materials The scattering of neutrons and synchrotron radiation are two powerful experimental tools that allow one to obtain a detailed atomic-level insight into the magnetism of a condensed matter system. The aim of this project is to use these two techniques for investigating new spintronics materials, with particular emphasis on the physical mechanism underlying their magnetism. It should be stressed that the mechanism giving rise to semiconductor magnetism is not exactly the same as in most other known magnetic systems (e.g., iron), and not all details of that mechanism have yet been fully understood. The building blocks of future spintronics devices will be nanostructures such as superlattices - i.e., "sandwiches" made of alternating extremely thin layers of magnetic and non-magnetic semiconductors. One question concerning such sandwiches - very important from the viewpoint of designing spintronics devices - is how two magnetic layers "communicate" across the intervening non-magnetic "spacer". Neutron and synchrotron radiation tools are particularly well suited for investigating these phenomena. Such studies are also an essential part of our project.Ferromagnetic semiconductors (FMSC) currently receive a great deal of attention because such materials are essential for developing "spintronics" - a new-generation electronics in which not only the current magnitude, but also its spin polarization can be controlled. The aim of this project is to exploit the potential of neutron and synchrotron radiation scattering techniques to shed light on several important issues concerning newly synthesized FMSC materials and their nanostructures. It should be stressed that FMSCs differ in many respects from "conventional" ferromagnetic materials, which are either metals or insulators. As in metals, the magnetism of certain novel FMSC systems (e.g., Ga(Mn)As) is induced by carriers - however, not by electrons, but by holes. Details of this new physical mechanism have yet to be understood. Inelastic neutron scattering tools may greatly help in such studies because they enable one to obtain very accurate values of the exchange parameters characterizing the interactions between magnetic ions. Another important issue is understanding the mechanism of exchange interaction transfer between FMSC layers separated by a non-magnetic spacer. Neutron and synchrotron radiation reflectometry are powerful tools for investigating such interactions. These techniques also enable one to study structural defects in the interface regions in heterostructures made of magnetic/nonmagnetic semiconductors. Such defects may significantly influence the performance of future spintronics devices. Therefore, insight into this issue is of considerable importance.