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.
The focus of this theoretical project was to achieve better understanding of electronic transport in magnetic materials in the presence of thermally-induced spin fluctuations. Similar to lattice vibrations and residual disorder, the fluctuations of spin moments in metals scatter the conduction electrons and contribute to electric resistivity. These fluctuations exist in all magnetic materials and become stronger at elevated temperatures. Apart from bulk materials, this phenomenon is relevant for interfaces, such as a boundary between a magnetic and nonmagnetic metal. The flow of current across such interfaces is a crucial feature of novel devices utilizing the intrinsic magnetic moment of an electron as an information carrier. Such spintronic devices are used in computers for reading the information from hard drives, in some memory architectures, and in emerging future device technologies exploiting the electron spin. The project also involved the development of the so-called first-principles techniques for better understanding and predicting the magnetic 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. Part of this project was focused on the electric resistivity of the rare-earth metal series from gadolinium to thulium, which was studied using first-principles calculations. We found unexpectedly that the previously used interpretation of the experimental measurements was incorrect, and that the interplay between spin-disorder and phonon scattering is crucial for quantitative understanding of the data. This insight contributes to better understanding of these materials and of spin fluctuations in magnets in general. Another problem that we addressed is related to the flow of the electric current and spin (called spin injection) across the boundary between a half-metallic ferromagnet and a nonmagnetic metal. A half-metallic ferromagnet is a kind of magnetic metal in which at absolute-zero temperature the magnetic moments of all current-carrying electrons are oriented in the same direction. The standard theory of spin injection does not apply to such materials at nonzero temperatures. Our contribution was to extend the theory to this situation, which helped understand the limiting factors for the operation of spintronic devices utilizing half-metals as high-polarization magnetic electrodes. We have investigated the properties of several magnetic materials of current interest. We formulated a prototype model of magnetic interaction in ferropnictide materials, which include a new class of high-temperature superconductors that have recently been under intense scrutiny. This model allowed us to reconcile the key features of the low-temperature magnetic excitation spectrum with high-temperature thermodynamic properties of these materials based on a unified form of the magnetic energy, consistent with both experimental data and first-principles calculations. We have described from first principles the magnetic interaction in electron-doped europium oxide, which is a candidate high-polarization material for spintronic devices. We have also studied the influence of alloying and strain on the magnetic properties of nitrogen martensite, which is a nitrogen-doped phase of iron and a promising rare-earth-free permanent magnet. During the course of this project we have developed new computational tools for the studies of temperature-dependent magnetic properties of alloys. These tools have been implemented in a software package that is used by other researchers and is available for the broader computational materials research community. This project provided educational experiences for graduate students in advanced materials theory using sophisticated computational tools. One PhD degree was awarded based on this research.
