Ferromagnetic resonant tunneling diodes : physics and applications

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Ganguly, Swaroop

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This dissertation is an attempt to explore the physics and the applications of the interplay of quantum transport and magnetism in a ferromagnetic resonant tunneling diode: that is, a resonant tunneling diode with a dilute ferromagnetic semiconductor well. The cornerstone of the work is a prediction, following from simple quantum transport theory arguments, that such a structure would exhibit a new physical effect – the Curie temperature should decrease to about half its equilibrium value when the downstream chemical potential is lowered below the quantum well resonance by an applied bias, and then further decrease to a small value when the resonance is lowered below the upstream band-edge. This result is derived analytically and corroborated with self-consistent ballistic quantum transport simulations, in the single-band effective mass approximation. Another effect that is expected to occur as a corollary is a spin hysteresis in these devices, analogous to the charge hysteresis in traditional resonant tunneling diodes. Scattering is included in the quantum transport simulation within the phenomenological Buttiker probe formalism. It is found that the aforementioned effect would be limited by the broadening of the resonance energy level due to scattering, but that for realistic scattering strengths should be observable. Further, it is shown that the predicted effect might be controlled to realize a two-level magnetization switch when the two aforementioned transitions in the Curie temperature are designed to occur at nearly the same voltage. This switching might be effected by tuning the two parameters that enter the analytic theory – the equilibrium chemical potential and the quantum well resonance energy level. The corresponding device parameters that need to be chosen suitably are the doping level in the contacts and the well width respectively. Finally, a three-terminal novel device structure based on the effects predicted called the Ferromagnetic Resonant Tunneling Transistor is proposed. This structure is compatible with next-generation metal-oxide-semiconductor transistor technology. It uses an exchange-biased spin analyzer that offers a high or low resistance to the in-plane current from the quantum well region depending on its spin-polarization, and thereby detects the magnetization state.