Spin Polarized Tunneling Studies in Transition Metals, Alloys and Heavy Fermions

这个项目将从基本物理和自旋电子学的重要技术领域的角度来探索电子自旋的性质。重点将放在构成自旋输运现象基础的表面和界面上。我们的目的也是为了将自旋极化隧穿技术扩展到新的领域：(1)确定其与晶体方向、势垒界面和体磁矩的关系，(2)隧道势垒的解析研究。尽管近三十年来许多理论方法已经解决了这一问题，但隧道效应测量的偏振值的来源和大小仍然是一个悬而未决的问题。这项工作将研究晶体取向上隧穿电子的自旋极化与表面能带结构之间鲜为人知的关系。这包括外延铁磁薄膜的自旋极化隧道效应、界面结合效应和隧道势垒特性。制造技术将操纵平面薄膜磁隧道结中的界面和势垒，以努力最大化自旋极化值。自旋隧穿受到隧道势垒中的三角洲掺杂(有或没有磁矩)的影响--主要是翻转自旋，甚至增加自旋隧穿几率，除了铁掺杂的情况外。这一领域尚待了解，将得到很好的调查。最先进的分子束外延系统使设计具有新型界面材料的薄膜成为可能，从而产生独特的电子和磁性。理论支持来自橡树岭国家实验室的威廉·H·巴特勒教授，他在能带结构计算、界面效应以及铁磁隧道结结构方面具有专长。自旋隧穿研究可能会被启动，这是第一次在重费米子系统中进行此类研究，其中发生了磁跃迁的温度诱导变化和压力诱导变化。所有级别的学生和博士后都将参与这项研究，从而创建一批技术专家，特别是在未来的信息技术领域-自旋电子学，以及普通磁学和薄膜科学。这个凝聚态物理项目通过使用自旋极化隧道技术(一种量子现象)这一独特的工具，拓宽了我们对电子自旋性质(磁学的一个主题)的基本理解。这些成果将有助于为未来可能的基于自旋的信息技术提供所需的知识库。该项目将建立在早先在自旋隧穿加输运方面的成功工作的基础上，并将在该计划的后期阶段，通过研究一类新的磁性材料，即所谓的重费米子金属，向新的前沿推进。一个主要目标是扩大目前致力于分析自旋隧道实验的理论家之间的一种新的、有前途的互动。此外，使用最先进的分子束外延工具的制备方法将操纵材料界面和隧道势垒，以创建新材料并最大化自旋极化值。测量将在存在小磁场或大磁场的情况下在环境和液氦温度下进行。该项目的一些早期成果在实验和理论上都引起了全世界的兴趣，许多大公司参与了非易失性存储元件以及用于超高密度记录的传感器的开发。该项目将研究与高中生、本科生、研究生和博士后研究员的教育和培训相结合。培训将在自旋传输和纳米技术等专业领域进行。受过高等教育和培训的人将为从事基于自旋的信息存储技术领域的教育工作者和工人的职业生涯做好充分准备。

This project will explore electron spin properties, from both the fundamental physics point of view and from the viewpoint of the technologically important area of spintronics. The emphasis will be on surfaces and interfaces that form the basis of spin transport phenomena. The intention is also to extend the technique of spin-polarized tunneling into new areas: (1) Determination of its relation to crystalline direction, barrier interface, and bulk magnetic moment, (2) Analytical study of tunnel barriers. Though having been addressed for nearly thirty years by many theoretical approaches, the origin and the magnitude of the polarization values measured by tunneling remain an open question. This work will investigate the poorly understood relation between spin polarization of tunneling electrons on crystallographic orientation and surface band structure. This includes spin-polarized tunneling from epitaxial ferromagnetic films, interfacial-bonding effects and tunnel barrier properties. Fabrication techniques will manipulate the interfaces and barriers in planar thin film magnetic tunnel junctions in efforts to maximize the spin polarization values. Spin tunneling is influenced by delta dopants (with or without magnetic moment) in the tunnel barriers - mostly flipping the spins and, except for the case of Fe dopants, even enhancing the spin tunneling probability. This area is yet to be understood and will be well investigated. A state of the art MBE system makes it feasible to engineer thin films with new types of interface materials leading to unique electronic and magnetic properties. The theoretical support comes from Prof. William H. Butler of Oak Ridge National Lab whose expertise is in band structure calculations, interface effects as well as ferromagnetic tunnel junction structures. Spin tunneling studies will possibly be initiated, a first of its kind study in heavy Fermion systems, wherein temperature-induced changes and pressure-induced changes of the magnetic transitions occur. Students of all levels and postdocs will be involved in this investigation thus creating a pool of technical experts particularly in the future field of information technology - spintronics, and in general magnetism as well as thin film science.This condensed matter physics project broaden our fundamental understanding of electron spin properties (a topic in magnetism) by using the unique tool of spin polarized tunneling technique (a quantum phenomenon). The results will contribute to the knowledge base needed for possible future spin-based information technologies. The project will build on earlier successful work in spin tunneling plus transport, and at a later stage of the program will push toward the new frontiers by examining a new class of magnetic materials, so-called heavy Fermion metals. A major goals is to expand a new and promising interaction between theorists who are now working to analyze the spin tunneling experiments. In addition, the fabrication methods, using state of the art molecular beam epitaxy tools will manipulate the material interfaces and tunnel barriers to create novel materials and to maximize the spin polarization values. Measurements will be made in ambient as well as liquid helium temperatures in the presence of a small or large magnetic field. Some of the earlier results of this project have generated worldwide interest, both experimentally and theoretically, with many major companies involved in developing nonvolatile memory elements as well as sensors for ultrahigh-density recording. The project integrates research with the education and training of high-school students, undergraduate students, graduate students and post-doctoral fellows. The training will be in spin transport and in specialized areas such as nanotechnology. The highly educated and trained people will be well prepared for careers as educators and workers in the area of spin-based information storage technology.