The objective of this research is to formulate a theoretical framework for spin-polarized transport in ferromagnetic heterojunctions and, by developing modeling tools, to propose and critically assess novel spintronic devices. Information stored in magnetic regions, using different orientations of the magnetization, can be preserved even without supplying any external power, opening possibilities for low-power and non-volatile applications. However, the existing commercial spintronic devices, based on the magnetoresistive effects, utilize only a small fraction of potential spin-based applications. In dilute magnetic semiconductors and in nonmagnetic semiconductor/metallic ferromagnet junctions ferromagnetism can be optically and electrically tuned thus enabling novel classes of spin transistors and multifunctional devices for seamless integration of memory and logic. The approach is to develop a multiscale modeling which combines drift-diffusion equations, generalized Landauer-Buttiker formalism, and first-principles density functional theory, to self-consistently treat largely varying carrier densities, strong deviations from the local charge neutrality, spin-polarized transport, and inhomogeneous ferromagnetism.
Broader Impact The proposed research will be closely integrated with developing physics curricula and an extensive student training with active recruitment of underrepresented minorities through the Louis Stokes Alliance for Minority Participation for undergraduate students. The PI will develop a multidisciplinary course Fundamentals of Spintronics which will span aspects of solid-state physics, engineering, materials science, and nanotechnology. To disseminate the results of this research and provide additional exposure and training of students in spintronics and related disciplines, the PI will co-organize an international workshop on transport in nanostructures in collaborations with the Oak Ridge National Laboratory and the University of Tennessee.
Intelectual Merrit: Conventional electronics relies on the electron’s charge for logic operations in computer microprocessors. In contrast, robust information storage in computer hard drives using another electron’s property: its spin, rather than charge. Spin can be considered roughly to a tiny compass needle which points either "up" or "down," with respect to the magnetic field. These two spin-directions lend themselves to expressing ones and zeroes when storing and processing data. The magnetic moment associated with the electron spin can be interpreted as an elementary magnet, responsible for intriguing magnetic behavior. Permanent magnets (ferromagnets), such as iron, retain their magnetic properties even when the power is switched-off, great for memory applications. The discovery of large resistance changes by an applied magnetic field has enabled a thousand-fold increase in the information storage density of computer hard-drives over the last decade and was recognized by the 2007 Nobel Prize in Physics. However, this is only the tip of the iceberg. A versatile control of spin and magnetism in a wide class of materials and their nanostructures, studied under this Award, may have a much broader impact leading to a new multifunctional devices that seamlessly integrate memory and logic for low-power/high-speed operation. Our main research findings, presented in 25 articles published in refereed journals, 6 book chapters, and handbook comprise: Silicon Spintronics We proposed how silicon, material central to conventional electronics, can be utilized in spintronics and implement spin-injection and detection (image 1). Our proposal for spin-voltaic effect a spin-analog of the photo-voltaic effect, has motivated the experimental demonstration of the spin-photo diode. We have explored how polarization-modulation (we have previously studied in lasers) can lead to silicon spin-interconnects and increase 100 times the information transfer. Spin-Lasers We revealed how adding spin to conventional laser, utilizing magnetic contact or illumination by circularly-polarized light, can significantly improve their operation: reducing the lasing-threshold beyond what was previously thought possible, enhancing the bandwidth (crucial for information transfer) of a laser (image 2) by both polarization and amplitude-modulation, and improving the switching properties. Magnetic Semiconductors We discovered novel methods to control magnetism in semiconductors. A peculiar thermally-enhanced magnetism changes from paramagnetic to ferromagnetic state (image 3). In a simple analogy, an ordered ferromagnetic state, similar to ice, and disordered paramagnetic state, akin to water, the prediction would be that heating water would yield ice. Our work has subsequently motivated a related experimental demonstration. Broader Impacts: Educational and outreach activities were an integral part of this Award. We emphasized two challenges: to reach a broad audience and share with them excitement about science and engineering; to effectively disseminate the results obtained under this award and develop new spintronic resources. The PI has presented several lectures to the lay audiences, including "Putting Spin Into Electronics-Vision for the Future," opening the Symposium on Magnetic Excitations in Semiconductors, 2008. http://mccombe.physics.buffalo.edu/magex-festsymp/program.htm (attendance approximately 400, including many high-school students). Instructional Modules College Physics: an introductory course taught to over 200 students, covering applications of spin-transport and magnetism to magnetic storage and sensing. Since most of these students will never have any additional exposure to physics or engineering courses, reaching out to them was particularly important. The students’ positive feedback to the PIs analogy between a spin-valve and a game of soccer was later used to prepare a figure for a popular article written in Nature. Electrodynamics 2: covered the influence of polarization on the light-emission of a laser and used the analogy of a spin-laser with a bucket (image 4), presented in several publications. Resource Development As a rapidly developing field, resources for newcomers to spintronics are often inadequate. To address this deficiency, the PI has co-authored a comprehensive review article, suitable for students, www.physics.sk/aps/pubs/2007/aps-07-04/aps-07-04.pdf freely accessible to anyone. A comprehensive handbook co-edited with E.Y. Tsymbal (image 5) provides complementary resources: www.amazon.com/Handbook-Transport-Magnetism-Evgeny-Tsymbal/dp/1439803773/ Editorially, we emphasized pedagogical explanations and extensive cross-referencing to make it accessible to a broad audience. An overview was provided by Albert Fert, 2007 Nobel Laureate in Physics. Training Several students supported from this Award were recognized for their accomplishments: C. Gothgen received scholarship awards for International School and Conference Spintech IV, 2007, and Spins in Solids, Summer School, 2006. J. Lee received 2011 Korean-American Scientists and Engineers Association Graduate Scholarship Award. W. Falls (REU student) received the Presidential Fellowship (Buffalo). G. Boeris, 2011 (internship student) received an award from Ecole Polytechnique, Paris, for the project on chirp in spin-lasers completed at Buffalo. The PI fostered a broader training of students in spintronics.He co-organized the Focused Topic: Spin-Dependent Phenomena in Semiconductors, American Physical Society March Meeting, 2007 (5 days, over 150 presentations). He organized and chaired the Invited Symposium: Tunneling Magnetoresistance: Yesterday, Today, and Tomorrow, American Physical Society March Meeting, 2008 (over 150 attendants), as well as the Spintronics Tutorial www.aps.org/meetings/march/events/tutorials/5.cfm American Physical Society March Meeting, 2012 (over 100 participants).
