Recent work demonstrates that one can transmit spin currents into and out of yttrium iron garnet thin films, as well as use such films to generate spin currents. This demonstration opens a new research field of yttrium iron garnet-based spintronics. This new program will address four issues in this emerging field: (1) spin pumping at yttrium iron garnet/normal metal interfaces; (2) relaxation control in yttrium iron garnet films through interfacial spin scattering and its device applications; (3) spin-wave amplification through interfacial spin scattering and potential device applications; and (4) spin Seebeck effects and potential device applications. Topic (1) addresses the efficiency of spin angular momentum transfer across yttrium iron garnet/normal metal interfaces. Topics (2) and (3) consider the scattering of spin-polarized electrons off the surface of a yttrium iron garnet film and study the effects of such interfacial spin scattering on both uniform and non-uniform modes in the film, and make use of the interfacial spin scattering and yttrium iron garnet thin films to demonstrate a new type of spin torque oscillator as well as low-loss delay lines and phase shifters. Topic (4) studies the spin Seebeck effect in yttrium iron garnet films in a perpendicular configuration in which the temperature gradient is perpendicular to both the film plane and the magnetization. Under Topic (4), work is also planned to demonstrate a new spin torque oscillator that relies on thermally induced interfacial angular momentum transfer. Although several device concepts will be demonstrated, the new program does not include plans for the implementation and integration of spintronic devices such that the workload is reasonable. The new program is supported by preliminary work and collaborations with three leading groups in spintronics. Education activities include teaching spintronics in Advanced Solid State Physics course.
Intellectual Merits: (1) This program will yield the first results on fundamental physics underlying (i) spin angular momentum transfer across yttrium iron garnet/normal metal interfaces, (ii) interactions between spin currents in normal metal layers and excitations in yttrium iron garnet films, and (iii) relaxation control through thermally induced angular momentum transfer. (2) The program will demonstrate new methods for relaxation control in magnetic insulators. Such control is highly desirable because magnetization relaxation not only plays a critical role in the dynamics of spin-based devices but also sets a natural limit to the response time of a device. (3) Two new types of spin torque oscillators will be demonstrated. (4) The program will yield novel approaches for spin-wave amplifications and thereby opens the door to a new class of spin-wave devices.
Broader Impacts: The program will touch on several fundamental issues and will therefore have a significant impact on the advancement of the new research field of yttrium iron garnet spintronics. The technological significance of yttrium iron garnet spintronic devices originates from two features of yttrium iron garnets: (1) extremely small damping and (2) electrically insulating property. The small damping, for example, will allow for the development of new spin torque oscillators that exhibit much narrower spectral linewidth and much larger signal-to-noise ratios than ferromagnetic metal-based oscillators. The advancement of spin wave-based microwave devices and logic devices is bottlenecked by spin-wave damping. The new approaches for spin-wave amplification will promote the development of these devices. The program will provide research opportunities for graduate, undergraduate, and high school students. Outreach to high schools in Colorado will be accomplished through the Colorado State University Little Shop of Physics program. The results from this program will be disseminated broadly through conference presentations, publications, and visits to national laboratories and industry.
