Spin Transport Studies In Band And Interface Tailord Materials: Towards Total Spin Polarization For Spin Electronics

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Nontechnical: Enabled by advances in basic research as well as driven by a rising demand for ultra-high density magnetic storage, there is an enormous interest of late in devices based on electron spin transport. In this NSF sponsored project we will investigate fundamental properties as well as applied aspects of magnetism related to this, with an expected impact on the field at the basic level as well as the future spin-based information technology. To reach this goal, spin-polarized transport studies of several novel systems will carried out, and to understand and manipulate the spin polarization, P (the degree to which transport electrons are spin polarized), and develop "tailored" materials. For example, we will explore the Cobalt-Iron_Boron alloy system, which recently demonstrated very large tunneling magnetoresistance (TMR) values (the higher it is the more useful for application), to achieve even higher P and TMR values. This material system remains largely unexplored and this study should open the way for candidate materials where P approaches 100%, without the interface problems of a half metal ferromagnet. Ferromagnet-insulator interface bonding is crucial in controlling the magnitude of P. We will aim at controlling this bonding for higher P. The possibility to make structures on a nanometer scale with well-tailored materials and interfaces will allow us to create new materials that show suitable properties. In particular, spin-polarized transport through quantum islands gives rise to novel effects such as spin-resonant tunneling, which can greatly enhance the TMR (observed in our laboratory) with the possibility of making spin transistors. The education of students in science through this outreach program will continue in an effective way to meet the national need for enhanced science education. Continuing the tradition, along with training graduate students and postdoctoral researchers, undergraduates and high-school students extensively participate in this research. It will enormously benefit these younger generation by getting trained for future spin based nano technology. Technical: In this NSF supported individual project a series of investigations are proposed that focus on fundamental properties as well as applied aspects of magnetism, which will benefit both at the basic level as well as for the future spin-based information technology. Spin-polarized transport studies of several novel systems will be done to understand and manipulate the spin polarization (P), necessary for future spin devices. The role of the ferromagnet-insulator interface bonding is crucial in controlling the magnitude of P. Our aim is to control this bonding and explore novel tunnel barriers to achieve higher P, including the exploration of the Co-HfO2 system, predicted to have P=100%. Spin filter tunneling is one of the few ways in which near 100% P (demonstrated in our group in the past), which will be explored for achieving P=100% above LHe temperatures. Based on the electronic structures of the constituent CoB and FeB alloys, we believe it is possible to "tailor" the (Co,Fe)-B band hybridization to achieve even higher P and tunnel magnetoresistance (TMR) values. This material system remains largely unexplored and should open the way for candidate materials with high P, without the interface problems of a half metal ferromagnet. Exploiting dimensionality on a nanoscale, in particular, spin-polarized transport through quantum islands can give rise to novel effects such as spin-resonant tunneling, which can greatly enhance the TMR, as has been observed in our laboratory. Double magnetic tunnel junctions exploiting non-equilibrium spin accumulation and ballistic spin transport will be explored, with potential for novel devices such as spin transistors. As in the past in addition to graduate students, postdoctoral researchers and visiting scientists, undergraduates and high-school students participate in this research program. The education of students in science through this outreach program will continue in an effective way to meet the national need for science education.