Browsing by Subject "Spin Transfer Torque"

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    Magnetic tunnel junction based spintronic logic and memory devices.
    (2011-11) Yao, Xiaofeng
    The development of semiconductor devices is limited by the high power consumption and further physical dimension reduction. Spintronic devices, especially the magnetic tunnel junction (MTJ) based devices, have advantages of non-volatility, reconfigurable capability, fast-switching speed, small-dimension, and compatibility to semiconductor devices, which is a promising candidate for future logic and memory devices. However, the previously proposed MTJ logic devices have been operated independently and therefore are limited to only basic logic operations. Consequently, the MTJ device has only been used as ancillary device in the circuit, rather than the main computation component. In this thesis, study has been done on both spintronic logic and memory devices. In the first part, systematic study has been performed on MTJ based logic devices in order to expand the functionalities and properties of MTJ devices. Basic logic cell with threeinput has been designed and simulated. Nano-magnetic-channel has been proposed, which is the first design to realize the communication between the MTJ logic cells. With basic logic unit as a building block, a spintronic logic circuit has been designed with MTJ as the dominant component. HSPICE simulation has been done for this spintronic logic circuit, which acts as an Arithmetic Logic Unit. acts as an Arithmetic Logic Unit.
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    Transport in Superconducting/Ferromagnetic Heterostructures
    (2019-07) Moen, Evan
    In this thesis I present my research on spin and charge transport in ferromagnet, superconductor (F=S) heterostructures using a self-consistent, clean limit theory. The goal is to characterize realistic samples. The primary focus is on the F1=N=F2=S superconducting spin valve. I also consider the S1=F1=N=F2=S2 ferromagnetic Josephson structures. We solve the Bogoliubov deGennes equations (BdG) using a self-consistent, numerical approach and determine the thermodynamic quantities such as the pair potential. For the charge transport, we use the Blonder-Tinhkam-Kapwijk (BTK) method to determine the conductance G. We study the conductance features and their dependence on the physical parameters such as the layer thicknesses and interfacial quality of the sample. The main results are the dependence of G on the misalignment angle of the magnetizations in F2 relative to F1, which constitutes a 'valve eect'. The valve eect in F=S structures is due to the proximity eect, which is angularly dependent. The critical bias (CB), equal to the gap energy, is non-monotonic with due to this proximity eect. The conductance features are split for incoming spin-up and spin-down electrons, which leads to a subgap (below CB) peak in the total conductance. This subgap peak is dependent on the intermediate F2 layer thickness and ferromagnetic exchange eld h in which the peak position oscillates between zero bias and the CB with a periodicity of =h. These subgap peaks are resistant to high interfacial barriers and lead to a monotonic angular dependence on in the peak maxima. In the S1=F1=N=F2=S2 quasiparticle conductance, there are multiple subgap peaks with similar oscillations in the peak positions. In addition, the conductance peak position oscillates with by a quarter phase between the parallel and antiparallel conguration. We also study the spin transport in the F1=N=F2=S system for realistic parameters. The spin transport quantities are not conserved due to the spin transfer torque (STT) within the ferromagnetic layers, and are spatially dependent. There exists a critical bias feature in which no spin current penetrates the S layer for biases below the CB, and the STT becomes quasilinear for biases above the critical bias.