Spin Torques in Magnetic and Superconducting Tunnel Junctions
The main text is presented in four parts. In chapter 2, we develop a phenomenological theory of voltage induced torques in magnetic tunnel junctions. The reciprocal of this effect and spin-transfer torque can pump charge into an attached circuit when the magnet precesses. We calculate the resulting change in impedances due to this pumping as a function of applied magnetic field and thickness of tunneling spacer. Because the impedances due to voltage and current pumping are qualitatively distinct under variation of the magnetic field, we suggest that this measurement could be used as experimental differentiation between these effects.
In chapter 3 we study magnetic Josephson junctions wherein spin polarized Ohmic and supercurrent exert a torque on the magnetic layer. As a result, there is a nonlinear dynamic interplay between the magnetic order parameter and the phase of the superconducting parameter. This results in a modified stability diagram for both the magnet and superconductor. In particular, we find a nonmonotonic dependence of the critical current on the applied magnetic field and current. When the temperature is raised above the superconducting critical temperature, the leads become metallic and the equations of motion coincide with those of chapter 3.
In contrast to the monodomain models studied in other chapters, chapter 4 examines the effects of micromagnetics on the thermal stability of a typical MRAM bit. In addition to noting that a finite stiffness parasitically effects bit stability, we find that domain-wall nucleation and propagation is the dominant mode of thermal bit flipping.
Finally, in chapter 5 we derive a nonequilibrium expression for spin current between two magnetic leads biased by voltage, temperature or spin. The interaction on the dot is left general and the equation for current can be written as a function of the full retarded Green's function on the dot. We apply this methodology to a dot with large on-site Coulomb interaction.