Nonlinear Theory Of Microwave Spin Torque Nano-Oscillators

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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.