First-principles Theory of Thermal Effects in Spin Transport

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

TECHNICAL SUMMARY This award supports computational and theoretical research and education aimed at understanding thermal effects in spin-dependent transport. It is well known that spin disorder affects the electronic structure and generates scattering of electrons in magnetic materials, but the specific mechanisms of its influence on the transport properties are poorly understood. The influence of thermal phonons on spin-dependent transport and its interplay with spin fluctuations have also received little attention. The PI will investigate the effects of thermal spin fluctuations and phonons on the electronic structure and transport properties of magnetic materials and heterostructures using first-principles electronic structure theory. This research will pursue several directions: (1) The effect of thermal spin fluctuations on the electronic structure of half-metallic ferromagnets and their interfaces with semiconductors, as well as on the spin injection across these interfaces, will be investigated. The effect of spin-orbit coupling on spin injection from half-metals will also be studied. (2) The mechanisms of temperature dependence of tunneling magnetoresistance in MgO-based magnetic tunnel junctions will be studied, including the effects of thermal spin fluctuations and phonons. (3) The mechanisms of exchange interaction in electron-doped ferromagnetic semiconductor EuO will be studied, along with its spin transport properties at finite temperatures. (4) Spin-disorder resistivity of ferromagnetic metals will be investigated focusing on the quantitative trends in the sequence of heavy rare-earth metals from Gd to Tm, and on the deviations from Matthiessen's rule resulting from the interplay between the spin-disorder and phonon scatterings. The project will have broader impacts by facilitating the design of new and more efficient magnetoelectronic devices, and through the development of new computational tools for the studies of finite-temperature magnetic properties. Research will involve graduate students, who will be educated in modern electronic structure, magnetism and transport theory and gain experience in the use and development of sophisticated electronic-structure codes. NON-TECHNICAL SUMMARY This award supports computational and theoretical research and education aimed at understanding the physical mechanisms that affect the flow of electric current in bulk magnetic materials and tiny magnetic structures of atoms some million times smaller than the size of a human hair. These are materials, in which the electron spin plays an important role. An electron can be thought of as a tiny magnet. Its magnetic properties are related to an intrinsically quantum mechanical property known as spin. The focus of this research is on calculating the temperature dependent current flow through these magnetic materials. A better understanding of how current flows through magnetic materials contributes to electronic device technology for information systems and emerging future electronic device technologies that exploit not only the electron charge as existing devices do now, but also the electron spin. This research will expand our ability to predict the properties of materials starting only from the identities of the constituent atoms. This contributes to the broader vision of being able to design materials with desired properties through computer simulations based on fundamental principles of quantum mechanics. The research involves developing new computational tools for the studies of temperature dependent magnetic properties, which may be shared with the broader computational materials research community. This project will provide educational experiences for graduate students in advanced materials theory and modeling techniques using sophisticated computational tools.