Thermodynamics of nanomagnetic devices driven by spin currents

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Electron spin is a promising medium for the transmission, processing, and storage of information in future electronic devices. In some of the pursued implementations, information is carried by coherent spin dynamics in magnetic materials through the spin waves, which can be generated by injecting spin current into nanomagnets. It is now well known that many different spin wave modes are simultaneously produced by spin current injection, but their spectral distribution, or the mechanisms controlling it, are not well known, hindering the progress in achieving coherent spin-based device operation. The proposed Project will develop new experimental approaches enabling the characterization of spectral distribution of spin waves generated by spin current, and establish the methods to control it. The possibility that the spin waves form a quasi-equilibrium distribution, described by the effective thermodynamic parameters - temperature and chemical potential will be tested. This will allow the proposed research to identify the fundamental mechanisms underlying the formation of dynamical states in nanomagnetic systems driven by spin currents. The resulting ability to achieve highly coherent magnetization dynamics will contribute to the progress in the implementation of efficient spin-based devices. The project will contribute to the burgeoning Engineering Sciences degree at Emory University, by developing training modules for the new hands-on experimental Materials Science course, and to the highly successful Atlanta Science Festival, by developing educational demos for the general public.The main goal of the Project is to establish the relation between the dynamical and the thermodynamic characteristics of nanomagnetic systems driven by spin current, which will enable efficient engineering and optimization of these characteristics for nanodevice applications. Magneto-optical micro-focus Brillouin Light spectroscopy technique will be utilized to determine the spectral distribution of spin wave quanta known as the magnons. To achieve a broad spectral sensitivity of the technique, momentum-space squeezing and plasmonic effects will be utilized to concentrate the probing light into deep sub-wavelength regions. The obtained results will be used to quantitatively test the hypothesis that nanomagnetic systems driven by spin current can form a quasi-equilibrium state characterized by the effective thermodynamic parameters such as chemical potential and temperature. This will allow the project to establish the relationship between the previously achieved coherent spin current-induced dynamics and Bose-Einstein condensation, a coherent dynamical state spontaneously formed when the chemical potential becomes equal to the lowest magnon energy. By establishing this relation, number of fundamentally and practically important questions will be addressed, such as the role of different dynamical spectral modes play in a magnetic systems in the formation of coherent states, what nonlinear magnon-magnon interactions prevent or facilitate the formation of these states, can they be controlled by engineering the dynamical spectrum, is there a possibility to achieve Bose-Einstein condensation driven by spin current and can the condensate be formed in a large volume of the magnetic system, or is it always localized in nanoscale regions. By addressing these questions, an unprecedented level of understanding of dynamical magnetization states, and the ability to control them for spin-based device applications, will be achieved.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.