Spin information propagation in metallic and insulating ferromagnet based devices

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Since the discovery of the giant magnetoresistive effect in 1988, tremendous advances in spintronics theory and experiment have been made. In particular, numerous novel physics concepts related to the generation and detection of spin accumulation and spin current have been proposed and confirmed. A wide range of plausible spin-based devices have subsequently been proposed. However, aside from the colossal success of hard disk drives and random access memories which are based on spin valves and magnetic tunnel junctions, other proposed spin devices have not been even remotely promising. From the view point of current theory and simulation, the inability to transfer new spin phenomena such as spin Hall conductance, spin pumping and spin electromotive force to promising device application is attributed to too small signals inherent in these phenomena and too difficulty in practical device fabrication.Intellectual merit:The present proposal aims to advance the spintronics field by investigating non-equilibrium spin-wave propagation in various externally controllable conditions. In the traditional approach of describing magnetization dynamics, the magnetization vector has been treated classically via generalized Landau-Lifshitz-Gilbert equations in the presence of the current and the magnetic field. The new concepts and equations proposed here involve quantum nature of the conduction electrons and magnetization: the excitations of electrons and of magnetization are quantum objects which obey the equation of the non-equilibrium quantum distributions. Specifically, both conduction electrons and spin waves enable to propagate spin information in magnetic media although they satisfy different quantum statistics. The generalized spin current, including the spin current of electrons and of spin waves, will be investigated in various materials and devices. The central goal is to find possible spin devices based on the quantum spin propagation beyond classical magnetization propagation.Broader impact: The spintronics research has fundamentally advanced our knowledge at the boundary between classical magnetization and quantum electron transport. The proposed research would take the traditional classical modeling for the magnetization to a fundamentally new region. The quantum non-equilibrium distribution for spin waves can generate unprecedented interesting phenomena. The possible device concepts and proposed prototype device structures are high risks but a potential breakthrough in superior spin-based devices is a possibility. The educational components of the proposal include strong graduate student participation in research, training, and visiting industrial research laboratories for the graduate and undergraduate students, as well as for PI to develop a spintronics course and textbook related to this research project.