Quantum spin-torque-driven nonclassical magnetization dynamics and dynamical buildup of long-range entanglement
Date
2021
Authors
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Publisher
University of Delaware
Abstract
The discovery of conventional spin transfer torque (STT) by Slonczewski and Berger in the 1990s ushered in a new era in spintronics technology, with a wide range of applications including microwave oscillators, microwave detectors, spin-wave emitters, and memristors, with STT-MRAM being the most popular one, which provides with zero-leakage power usage compared to the charge based memory alternatives such as SRAMS, DRAMS. In a trilayer spin valve set up with two ferromagnetic (FM) layers (fixed-FM/normal-metal/free-FM), spin polarized current generated by the fixed-FM layer exerts conventional STT on the free-FM layer. The free-FM layer must be non-collinear to the fixed-FM layer, causing spin precession and even magnetization reversal of the free-FM layer. This room temperature spin dynamics can be explained well using classical framework of Landau-Lifshitz-Gilbert (LLG) equation. However, recent spin valve experiment [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017)] at cryogenic temperature, when thermal fluctuation is suppressed, revealed highly non-classical magnetization dynamics signalled by the change of resistance when fixed-FM and free-FM layers are collinear but antiparallel. The goal of this thesis is to delineate the role of STT in current driven magnetization dynamics using fully quantum many-body framework, which is made possible with the recently developed adaptive
time-dependent density matrix renormalization group (tDMRG) algorithm. The effect of entanglement in both electronic and the localized spin subsystems was addressed to differentiate conventional STT and quantum STT driven magnetization dynamics. Lastly, the thesis focused on the exploratory limitations of using quantum-for-electron
and classical-for-localized-spins i.e. quantum classical hybrid approach for a system of localized spins interacting with conduction electrons, in comparison to using fully quantum many-body framework where both conduction electrons and localized spins are treated fully quantum mechanically.
Description
Keywords
Condensed matter physics, Fully quantum, Long-range entanglement, Magnetization dynamics, Spin transfer torque, Spintronics