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

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

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.