Neutron Scattering Studies of Magnetic Semiconductor Nanostructures

本研究致力于了解杂化磁性半导体材料中存在的磁性结构和交换作用。由传统的II-VI材料组成的半导体超晶格，其中交替的层是反铁磁性的和非磁性的，表现出惊人的长范围的层间自旋关联，可与金属系统中的自旋关联相媲美。作为半导体掺杂的函数，中子散射被用来探测这些系统中的磁结构、序参数和关联范围。结果表明，除常规超交换外，磁相互作用占主导地位。目前的研究目标之一是详细研究这种交换耦合及其对电子结构、应变效应和掺杂类型的依赖。除了II-VI材料，这项研究还将集中于对新的铁磁III-V系统的详细研究，包括铁磁转变温度与成分的异常关系以及热退火的影响。中子还将提供有关铁磁性团簇或其他纳米尺度的不均匀形成的直接信息，这些信息可能会影响无限范围铁磁性的开始。最后，我们将使用中子反射率来研究有序参数的空间相关性，以及研究混合III-V铁磁半导体与相邻II-VI反铁磁层之间的邻近效应。半导体和磁性材料在许多常见的电子器件中都扮演着基本但不同的角色。例如由半导体制成的计算机存储器和使用磁性材料的硬盘驱动器。最近，利用“自旋电子学”的概念，出现了在单一材料中结合半导体和磁性的可能性。这项研究计划旨在使用强大的中子散射技术来探测半导体和磁性材料的磁性状态。中子可以通过与这些系统中的原子核相互作用而散射，此外，由于中子具有磁矩，它们可以与材料内部的磁场相互作用。这种散射的结果提供了关于材料内部的磁性结构以及各个磁矩耦合在一起的距离的直接信息。这类关于磁性的信息可以与材料的半导体性质的变化相关联，因为该系统是用各种化学元素“掺杂”的。这一结果为同时优化这些新化合物的磁性和半导体方面提供了必要的信息，以用于未来的混合自旋电子器件。

This research is focused on an understanding of the magnetic structure and exchange interactions present in hybrid magnetic semiconductor materials. Semiconductor superlattices consisting of conventional II-VI materials in which alternate layers are antiferromagnetic and non-magnetic exhibit interlayer spin correlations of a surprisingly long range comparable to that in metallic systems. Neutron scattering is used as a probe of the magnetic structure, order parameter, and correlation range in these systems as a function of semiconductor doping. The results indicate that a magnetic interaction other than conventional superexchange is dominant. One of the present research objectives is to investigate in detail this exchange coupling and its dependence on electronic structure, strain effects, and types of doping. In addition to the II-VI materials, the research will focus on a detailed investigation of new ferromagnetic III-V systems, including the unusual dependence of the ferromagnetic transition temperature on composition and on the effect of thermal annealing. Neutrons will also provide direct information on the formation of ferromagnetic clusters or other nanoscale inhomogeneities that may affect the onset of infinite-range ferromagnetism. Finally, neutron reflectivity will be used to investigate the spatial dependence of the order parameter, and to investigate proximity effects between a hybrid III-V ferromagnetic semiconductor and an adjacent II-VI antiferromagnetic layer.Semiconductor and magnetic materials both play essential, but distinct, roles in many common electronic devices. Examples are computer memories made of semiconductors and hard disk drives utilizing magnetic materials. Recently the potential has arisen for a marriage of semiconducting and magnetic properties in a single material utilizing the concept of "spintronics." This research program is designed to use the powerful technique of neutron scattering to probe the magnetic state of materials that are both semiconducting and magnetic. The neutrons can be scattered by interacting with nuclei inside these systems, and in addition, since neutrons possess a magnetic moment, they can interact with magnetic fields inside materials. The result of such scattering provides direct information about the magnetic structure inside a material and over what distances the individual magnetic moments are coupled together. This type of information on the magnetism can be correlated with changes in the semiconducting properties of the materials as the system is "doped" with various chemical elements. The result provides necessary information to simultaneously optimize both the magnetic and the semiconducting aspects of these new compounds for future hybrid spintronic devices.