It is known that spin-polarized current injection may fully compensate for energy dissipation due to damping and result in undamped magnetization precessions in nanomagnets. The frequencies of these undamped magnetization precessions are controlled by injected spin-polarized currents. These microwave spin-torque oscillators have unique properties such as nanoscale dimensions, radiation hardness, wide bandwidth of phase-locking and rapid frequency tuning. For this reason, the microwave spin-torque oscillators have been the focus of considerable experimental and theoretical research lately.
Intellectual Merit: Currently spin-torque oscillators are mostly studied by using the classical spin-wave theory. This spin-wave approach is efficient only for near to generation threshold conditions. The goal of this project is to develop the analysis of microwave spin-torque oscillators based on the nonlinear dynamic system theory and applicable for both near and far from generation threshold conditions. The main objectives of the research work on this project will be 1) stability study of spin-torque nano-oscillators (STNO) with respect to thermally generated spatially non-uniform (spin-wave like) perturbations, 2) noise and spectral density analysis of STNOs by using randomly perturbed magnetization dynamics equations and the theory of stochastic processes on graphs, 3) noise analysis of STNOs by using Poisson-noise perturbations of Landua-Lifshitz-Slonczewski equations, and 4) the study of phase-locking of STNOs in the context of the bifurcation theory. Theoretical results and predictions will be verified through their comparison with numerical simulations and experiments.
Broader Impact: Nanoscience and nanotechnology promise to have a continuing and long-term impact on the US and world economies, and are anticipated to be engines of growth in the near future. The proposed research has potentially transformative technological applications in the area of nano-spintronics. This research will directly support two graduate students, and will also involve undergraduate students via the ECE Department?s MERIT and GEMSTONE programs. They will add to the nation?s pool of talent in this important emerging area of technology. This research will also have a strong international collaboration component and it will serve as a vehicle to expand and strengthen the existing scientific collaborations with Italian colleagues, Drs. G. Bertotti and C. Serpico, as well as with their students.
The project has been concerned with the very active research area of spintronics, in which the spin-polarized current injection in trilayer (spin valve) structures (see Figure 1) is utilized for the design of microwave spin-torque nano-oscillators as well as for magnetization switching on the nanoscale. This area of research has numerous potential applications and magnetic data storage is one example of them. Traditionally, spin-torque phenomena have been studied by using the classical spin-wave theory. In this project, the new approach based on the nonlinear dynamic system theory was advanced to study spin-torque nano-oscillators (STNOs) and spin-torque induced magnetization switching. Our research on the project has involved the following tasks. Study of spin-wave instabilities of spin-torque nano-oscillators; Spectral density analysis of STNOs in the case of magnetization dynamics perturbed by white-noise processes; Analysis of noise in STNOs by using jump-noise perturbed magnetization dynamics; Analysis of phase-locking of STNOs; The development of the averaging technique for the reduction of random magnetization dynamics on the sphere to stochastic magnetic energy dynamics on appropriate graphs which reflect the magnetic energy landscapes; Study of the effects of plasmon resonances on magnetization dynamics and their application to heat-assisted magnetic recording as well as all-optical magnetic recording. Our research has had a strong international collaboration component and it served as a vehicle to expand and strengthen the preexisting scientific collaboration with my Italian colleagues, Dr. Giorgio Bertotti and Dr. Claudio Serpico, as well as with their students. The results of the research have been published in 33 papers (and four more papers are accepted for publication in 2015) in the most prestigious journals such as Physical Review Letters, Physical Review B, Journal of Applied Physics, Physica B: Condensed Matter, European Physical Journal B and IEEE Transactions on Magnetics. During work on the project, we have also published two books: Isaak D. Mayergoyz, "Plasmon Resonances in Nanoparticles" (World Scientific, January 2013) and Isaak D. Mayergoyz and Patrick McAvoy, "Fundamentals of Electric Power Engineering" (World Scientific, January 2015). The pictures of the covers of these books are shown in Figures 2 and 3, respectively. These books reflect the broadening of scope of our research which occurred during the work on the project. Specifically, the book on plasmon resonances reflects the research on the use of plasmon resonances in nanoparticles for heat-assisted magnetization switching and all-optical magnetization switching, which are very promising for a new generation of magnetic storage devices. On the other hand, the book on fundamentals of electric power engineering contains the extensive presentation of magnetics topics related to power engineering. It is conceivable that in the spin-torque area of spintronics may lead to the development of novel principles for design of power (ac-to-dc and dc-to-ac) converters on the nanoscale, which will be fundamentally different from those being currently used in power electronics based on semiconductor technology. During the performance of the research work, two graduate students Ziyu Liu and Andrew Lee as well as one research associate Dr. Patrick McAvoy were supported in this project. They coauthored many papers and gave presentations at international conferences such as MMM and INTERMAG conferences. Ms. Ziyu Liu and Mr. Andrew Lee have completed their PhD dissertations and they will add to the nation's pool of talent in an important emerging area of technology, spintronics. It is believed that the main outcome of this project is the development of rigorous mathematical and physical theory of spin-torque nano-oscillators and spin-torque induced magnetization switching, which will have far-reaching applications in the development of future nanotechnologies.
