Structure of Ordered and Disordered Systems

我主要感兴趣的是由外延生长在半导体上的铁磁/非铁磁性层设计的磁性纳米结构的结构。对于这类系统中的磁性行为-磁各向异性和层间耦合-的正确解释强烈依赖于层之间界面的性质、层中的应变以及层和衬底之间的任何相互扩散或反应的程度。同样的因素对于改进自旋电子器件也很重要，这取决于电子通过金属-半导体界面携带自旋的效率。我们使用同步辐射结构技术在两个分子束外延(MBE)设施中原位应用这些因素，这两个设施是我们在PNC/XOR(太平洋西北联盟/X射线光学和研究)光束线上建造的，高级光子源，伊利诺伊州。我们研究金属薄膜，因为它们是一层一层地外延生长的。我们的结构技术包括XAFS(X射线吸收精细结构)、镜面和漫反射X射线反射率、X射线驻波和表面X射线衍射。我们已经广泛地研究了Fe/GaAs、Pd/Fe/GaAs和Fe/Pd/Fe/GaAs，并将继续研究相关的系统。但是，一个长期存在的问题是Fe-GaAs界面的单轴平面内磁性各向异性的根源。我们发现2ML的Fe薄膜存在面内结构各向异性，而4ML及更厚的薄膜则没有。我们将确定面内结构各向异性是否是一个普遍的特征，独立于GaAs的表面重建。在亚单分子层水平上，我们将尝试确定是否在界面上形成Fe-GaAs络合物，以及它的“晶体”结构是什么。我们还将在具有高电子自旋注入效率的Fe/AlGaAs/GaAs中寻找Fe/AlGaAs界面上的Fe络合物。在亚纳秒时间分辨实验中，我们用PNC/XOR 200飞秒激光脉动样品温度，并使用x射线磁性圆脱色来跟踪Gd和Co亚晶格重新定向的磁化强度。

My principal interest is the structure of magnetic nanostructures engineered from ferromagnetic/ non-ferro- magnetic layers epitaxially grown on semiconductors.  The correct interpretation of the magnetic behaviour in such systems- the magnetic anisotropy and interlayer coupling- is strongly dependent on the nature of the interface between layers, the strain in the layers and the extent of any interdiffusion or reaction between layers and substrate. The same factors are important to improving spintronic devices which depend upon the efficiency with which an electron carries its spin through a metal-semiconductor interface.   We examine these factors with synchrotron radiation structural techniques applied in situ in two molecular beam epitaxy (MBE) facilities which we constructed at the PNC/XOR (Pacific Northwest Consortium/ X-ray Optics and Research) beamlines, Advanced Photon Source, IL. We study metallic films as they are grown epitaxially, layer by layer. Our structural techniques include XAFS (x-ray absorption fine structure), specular and diffuse x-ray reflectivity, x-ray standing waves and surface x-ray diffraction. We have examined extensively Fe/GaAs, Pd/Fe/GaAs and Fe/Pd/Fe/GaAs and will continue with related systems.    A persistent problem is the origin of the uniaxial  in-plane magnetic crystalline anisotropy at the  Fe-GaAs interface. We have found an in-plane structural anisotropy for 2 ML Fe but not for 4 ML and thicker films. We will determine if the in-plane structural anisotropy is a universal feature, independent of the surface reconstruction of GaAs. At the submonolayer level we will attempt to determine if a Fe-GaAs complex forms at the interface and what is its "crystal" structure. We will also look for Fe complexes at the Fe/AlGaAs interface in Fe/AlGaAs/GaAs which has a high electron spin-injection efficiency.    We are examining spin dynamics in ferrimagnetic amorphous GdCo alloys. In a sub-nanosecond  time- resolved experiment we pulse the sample temperature with the PNC/XOR 200 femtosecond laser and use x-ray magnetic circular dischroism to follow the magnetization as the Gd and Co sub-lattices re-orient.