The research objectives of this proposal are to engineer a series of spin-transfer devices whose torque originates from thermally generated magnons. It has been predicted that such heat driven spin torque can lead to quantum yield improvements nearly two orders of magnitude greater than present state-of-the-art current-drive spin torque devices. Increasing the usable spin torque with all else constant will have a major impact on spin torque device technologies. Given the great potential, magnonic spin torque devices will be fabricated using a combination of thin film deposition and nanolithography techniques. Device structures will include magnetic oxide-normal metal-metallic ferromagnet spin valves. The operation and characterization of device performance will be entirely optical: ultrafast pump-probe time-resolved Magneto-Optical Kerr Effect to first excite magnons in the oxide, then observe the resulting magnetization dynamics in the metallic ferromagnet. The underlying principle of these devices is the transfer of spin momentum from magnons in a magnetic oxide to the magnetization of a free ferromagnetic layer. Magnons will be generated in the magnetic oxide layer (e.g., a spinel ferromagnet) by an femtosecond laser pulse. These magnons will annihilate at the oxide-normal metal interface (which may contain an additional layer of magnetic atoms), but their spin momentum will be transferred to conduction electrons in the normal metal. Thus, a time dependent accumulation of spin polarized electrons will be generated in the normal metal. The time derivative of this spin accumulation results in a torque on the metallic ferromagnet, which emphasizes the importance of using ultrafast optics. Device performance will be optimized through materials selection, creating and characterizing interfaces amenable to spin transfer, and investigating relevant length scales.
Broader Impacts: The experimental realization of magnonic spin torque devices whose quantum yield is improved by nearly two orders of magnitude beyond the present state-of-the-art will have transformative impact by essentially creating a new class of spin torque devices. Spin-transfer torque devices in general would benefit from replacing the high current densities now necessary for operation, as this causes appreciable heating, and vortex nucleation in the free layer via the unavoidable Oersted field. Magnonic spin torque would address these issues, allowing significant improvements in device performance, fabrication requirements, and reliability.
This program will integrate teaching and training of students by unifying techniques and ideas from physics, electrical engineering, and materials and optical sciences, thereby empowering them with the multidisciplinary talents necessary to become the next generation of leaders in academia and industry. An international collaboration will underscore the importance of global collaborations for modern research. The PIs will continue to build upon their established records of broadening the participation of underrepresented groups through a variety of means, including: mentoring students from USF?s Florida-Georgia Louis Stokes Alliance for Minority Participation Bridge to the Doctorate Program; educating the community about the disproportionate filtering of underrepresented groups by certain admissions policies; organizing summer camps for minority-serving middle schools in collaboration with the Florida Advanced Technological Education Center (FLATE), a NSF-ATE Regional Technological Education Center of Excellence. In collaboration with FLATE and the Hillsborough County School District, additional effort will help develop a science, technology, engineering, and math proficient workforce through training workshops for teachers.
