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，因为这些材料有望彻底改变计算机和通信技术。新一代电子学，通常被称为“自旋电子学”，不仅利用了电子电荷，而且利用了它的自旋——这是目前使用的半导体芯片没有利用的一个特点。美国、日本和欧洲的一些团队正在竞相寻找合成适合制造实用自旋电子学设备的新型磁性半导体的最佳方法。在材料技术人员不断努力的同时，凝聚态物理学家也需要付出很大的努力来表征新材料的磁性和其他相关特性。中子散射和同步辐射是两种强大的实验工具，使人们能够获得凝聚态物质系统磁性的详细原子级洞察力。该项目的目的是利用这两种技术来研究新的自旋电子学材料，特别强调其磁性的物理机制。应该强调的是，产生半导体磁性的机制与大多数其他已知的磁性系统（例如铁）并不完全相同，并且该机制的所有细节尚未完全了解。未来自旋电子学设备的基石将是纳米结构，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”。从设计自旋电子学设备的角度来看，有关这种三明治的一个非常重要的问题是，两个磁性层如何在中间的非磁性“间隔层”之间“通信”。中子和同步辐射工具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，因为这种材料对于发展“自旋电子学”是必不可少的，自旋电子学是新一代电子学，不仅可以控制电流的大小，而且可以控制其自旋极化。本项目的目的是利用中子和同步辐射散射技术的潜力，揭示有关新合成的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）的磁性是由载流子诱导的——然而，不是电子，而是空穴。这种新的物理机制的细节还有待了解。非弹性中子散射工具可以极大地帮助这类研究，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精确的交换参数值。另一个重要的问题是了解由非磁性间隔层分隔的FMSC层之间交换相互作用转移的机制。中子和同步辐射反射计是研究这种相互作用的有力工具。这些技术也使人们能够研究磁性/非磁性半导体异质结构界面区域的结构缺陷。这些缺陷可能会严重影响未来自旋电子学器件的性能。因此，深入了解这个问题是相当重要的。

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.