Tuning the Thermal Properties of Magnetic Tunnel Junctions
Amin, Vivek Pravin
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Due to their ubiquitous presence in hard-disk drives and growing potential as commercially viable memory bits, Magnetic Tunnel Junctions (MTJs) continue to provide impetus for scientific study. The demand for smaller devices and efficient energy consumption mandates further investigation of their thermal properties and possible finite-size effects. Such considerations have prompted a renewed interest in the long-known Seebeck effect, in which a thermal gradient spanning a material induces a voltage. The strength of this induced voltage can change as a function of the device's magnetization configuration - known as the magneto-Seebeck effect or magnetothermopower - in analogy with the Giant (and Tunnel) Magnetoresistance. This thesis presents a theoretical study of this effect in MgO-based MTJs with spin-orbit coupling. We present theoretical calculations of the Tunneling Anisotropic Magneto-Seebeck effect using realistic band structures, and show that the thermal properties of MTJs are tunable via magnetic field. This phenomenon potentially enables the controlled manipulation of temperature gradients, the recycling of wasted heat, and thermal spin-logic. Our calculations employ the Landauer-Buttiker scattering formalism, in conjunction with realistic multi-band tight-binding models fitted to ab-initio calculations. We demonstrate that numerically-unstable transmission resonances, ordinarily described as hot-spots in the literature, more accurately resemble "walls" that weave through each device's two-dimensional Brillouin Zone. We discuss their physical relevance in modern day nanostructures, and argue that their selective removal (via ltering algorithms) aids convergence while preserving each system's essential magnetic-transport properties. Finally, we demonstrate that exploiting spin-orbit coupling in MTJs with a single ferromagnetic contact can actually enhance certain magnetic transport anisotropies, allowing for higher packing densities as well.