Browsing by Subject "spintronics"

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    Evaluating Graphene as a Channel Material in Spintronic Logic Devices
    (2016-03) Anugrah, Yoska
    Spintronics, a class of devices that exploit the spin properties of electrons in addition to the charge properties, promises the possibility for nonvolatile logic and memory devices that operate at low power. Graphene is a material in which the spin orientation of electrons can be conserved over a long distance, which makes it an attractive channel material in spintronics devices. In this dissertation, the properties of graphene that are interesting for spintronics applications are explored. A robust fabrication process is described for graphene spin valves using Al2O3 tunnel tunnel barriers and Co ferromagnetic contacts.  Spin transport was characterized in both few-layer exfoliated and single-layer graphene, and spin diffusion lengths and spin relaxation times were extracted using the nonlocal spin valve geometry and Hanle measurements. The effect of input-output asymmetry on the spin transport was investigated.  The effect of an applied drift electric field on spin transport was investigated and the spin diffusion length was found to be tunable by a factor of ~8X (suppressed to 1.6 µm and enhanced to 13 µm from the intrinsic length of 4.6 µm using electric field of ±1800 V/cm). A mechanism to induce asymmetry without excess power dissipation is also described which utilizes a double buried-gate structure to tune the Fermi levels on the input and output sides of a graphene spin logic device independently. It was found that different spin scattering mechanisms were at play in the two halves of a small graphene strip. This suggests that the spin properties of graphene are strongly affected by its local environment, e.g. impurities, surface topography, defects. Finally, two-dimensional materials beyond graphene have been explored as spin channels.  One such material is phosphorene, which has low spin-orbit coupling and high mobility, and the interface properties of ferromagnets (cobalt and permalloy) with this material were explored.  This work could potentially enable spin injection without the need for a physical tunnel barrier to solve the conductivity mismatch problem inherent to graphene.
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    Explorations of constructs for unconventional and topological superconductivities
    (2022-09) Heischmidt, Brett
    In recent years, topology has risen as a prominent topic of study within the physics community. At its core, topology is simply a classification system, where all objects within a particular class (or more formally, space) hold a common property. Physicists tend to find topology interesting for a few reasons. First, the classification system can be extremely neat (clean), as when an integral over a physical space comes out as an integer multiple of some constant. Second, interesting physical manifestations can arise when a system lives in one topological class compared to another. Third, other physical manifestations can arise when crossing between topological classes. This thesis work centers itself around various topologies. The central topology is that related to the phenomenon of Majorana Zero Modes (MZMs), which are superconducting excitations at the split between particles and holes (i.e., zero energy). The topological classes relevant here are arrangements of certain systems that give rise to the MZM. There is a secondary topology associated with MZMs tied to their use in so-called "topological quantum computing." In this type of quantum computing, excitations are moved around one another in such a way that they remember where they have been by accumulation of a particular phase. Due to the physical process and its inherent memory of its path, this process has been dubbed "braiding." Aligned with previous language, the topological classes here, then, are the braids. This work studies two systems within the above motivations, NbSe$_2$ and magnet-semiconductor interfaces. NbSe$_2$ is predicted to be a nodal topological superconductor, wherein classes within the topology are defined on the nodes in the Bogoliubov-de Gennes (BdG) spectrum. (By convention, no nodes is trivial, and presence of nodes gives "nontrivial" classes.) Further, MZMs are predicted to arise when the nodes are present. Another platform for realizing MZMs is a combination of a semiconducting nanowire, s-wave superconductor, and magnetic element. Realizing unambiguous signatures of MZMs has been particularly tricky, however, leading to substantial efforts to understand the interactions of the three elements. The magnet-semiconductor interface studies fit within this context. Chapter 1 introduces some concepts motivating this work. The first concept presented is topology in quantum mechanical systems followed by its tie to superconductivity. The next concepts that are presented are tied to unconventional superconductivity and are central to its use in quantum computing. Chapter 2 presents an experimental analysis of NbSe$_2$. After outlining some history and motivation, device and measurement specifics are described. A main result of two-fold anisotropy in magnetoresistant properties of the superconducting state is presented followed by multiple efforts to rule out trivial causes. With these ruled out, an interpretation is presented describing a competition of superconducting instabilities. Chapter 3 addresses quantum spin transport in InSb nanowires. InAs and InSb nanowires are introduced for their role in experimentally showing MZMs. Experimental work on VLS InSb is sketched, although the focus here is a brief description of simulations relevant to the experimental picture. Progress toward exploring other platforms for this work is then presented. Chapter 4 moves into a computational study of Heusler / III-V semiconductor interfaces, with the motivation of studying the semiconductor-magnet interface. Grounding concepts are presented, followed by computational details for two interfaces, Ti$_2$MnIn-InSb and Ni$_2$MnIn-InAs. Results are finally discussed. Chapter 5 summarizes the work.
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    Spintronics Devices for Advanced Memory and Computing Applications
    (2021-06) Zhao, Zhengyang
    Spintronics, as a beyond-CMOS technology, provides many possibilities for the next-generation information storage and processing. This thesis focuses on the development of novel spintronics devices towards low-energy, high-performance memory and computing applications. In this thesis, we present the manipulation of a magnetic storage unit either with a current-induced spin-orbit torque (SOT) or using a voltage via piezoelectric strain. We also propose a novel in-memory computing architecture based on the SOT storage cell. For the first part, the SOT induced switching is explored for both ferromagnets (FM) and antiferromagnets (AFM) systems. For the study of FM, two fundamental limitations related to the switching of a perpendicular magnetized system are solved. First, this thesis expands the scope of spin torque switchable materials, from interfacial PMA magnets only, to bulk PMA magnets, which have a better thermal stability when scaled down and are regarded as potential candidates in future MRAM. Second, the difficulty of field-free SOT switching is addressed by developing a dipole-coupled composite device. Compared with competitive strategies, the composite device is the most compatible one with existing MRAM technologies and readily applicable for SOT-based memory and logic devices. Beyond the exploration of SOT in FM, this thesis also attempts to tackle the spin torque induced switching in an AFM system, by characterizing the devices with a widely adopted 8-terminal geometry. It is discovered the “saw-tooth” signal, which was previously regarded as the evidence of AFM switching, actually originates from thermal artifacts. Then, the voltage-controlled device is studied utilizing a piezoelectric / magnetic tunnel junction (MTJ) coupled structure for ultra-low power writing of data. Voltage-controlled toggling of MTJ is achieved via the piezoelectric strain generated from a pair of local gates. The local gating design allows efficient manipulation of individual cells and opens the door towards realistic strain-based MRAM. Finally, a new architecture for computational random-access memory (CRAM) is invented based on the 3-terminal SOT-MTJ. Similar to the STT-MTJ based counterpart, the SOT-CRAM allows true in-memory computing and thereby meets the energy and throughput requirements of modern data-intensive processing tasks. Moreover, the excellent features of SOT unit cells would provide a large improvement in speed and energy compared with other in-memory computing paradigms.