Charge-Spin Conversion and Electronic Transport in Two-Dimensional Materials and van der Waals Heterostructures

Applications related to artificial intelligence (AI), 5G communication, cloud computing, Internet of Things (IoT) will necessitate wide range of data collection, communication and processing. Current charge-based technology using conventional materials suffers adverse effects with down-scaling the device size and has limited efficiency in meeting the future demands for computation and data storage. The exploration of alternative device technology along with new materials is important to enhance computing performance and energy efficiency. In this thesis, I investigated new materials for future memory and logic technologies.\ua0 Recently developed 2D materials such as graphene, semiconductors, and semimetals exhibit remarkable new properties that promise faster and energy efficient non-volatile memory and logic functionalities. For non-volatile memory technologies, increasing efforts are being directed towards exploiting charge-spin conversion phenomena in high spin-orbit coupling (SOC) materials to realize all-electric magnetic memory. Interestingly, magnetic memory devices have been demonstrated on an industrial scale; however, the moderate efficiency and fundamental limitations of the conventional materials employed limit their use in consumer electronics. This thesis addresses some of these critical challenges and presents charge-spin conversion mechanisms in layered high SOC materials such as topological insulators, semimetals, and two-dimensional (2D) materials heterostructures. At the same time, this thesis contributes in the direction of integrating memory and logic devices by investigating 2D semiconductor devices with sub-20 nm narrow channel width and memristive switching in field-effect transistors using 2D semiconductors with graphene contacts. Such 2D semiconductors have enormous prospects for next-generation high-performance and energy-efficient nanoscale field-effect transistors and integration with memory technologies. These studies of charge and spin transport in 2D materials and heterostructures can open the door for nanometer-scale memory, logic and sensing technologies

Two-Dimensional Spintronic Circuit Architectures on Large Scale Graphene

Solid-state electronics based on utilizing the electron spin degree of freedom for storing and processing information can pave the way for next-generation spin-based computing. However, the realization of spin communication between multiple devices in complex spin circuit geometries, essential for practical applications, is still lacking. Here, we demonstrate the spin current propagation in two-dimensional (2D) circuit architectures consisting of multiple devices and configurations using a large area CVD graphene on SiO2/Si substrate at room temperature. Taking advantage of the significant spin transport distance reaching 34 {\mu}m in commercially available wafer-scale graphene grown on Cu foil, we demonstrate that the spin current can be effectively communicated between the magnetic memory elements in graphene channels within 2D circuits of Y-junction and Hexa-arm architectures. We further show that by designing graphene channels and ferromagnetic elements at different geometrical angles, the symmetric and antisymmetric components of the Hanle spin precession signal can be remarkably controlled. These findings lay the foundation for the design of complex 2D spintronic circuits, which can be integrated into efficient electronics based on the transport of pure spin currents

Gate-tunable Spin-Galvanic Effect in Graphene Topological insulator van der Waals Heterostructures at Room Temperature

Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a nonequilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin-galvanic signal. These findings provide an efficient route for realizing all-electrical and gate-tunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits

Charge-spin conversion signal in WTe2 van der Waals hybrid devices with a geometrical design

The efficient generation and control of spin polarization via charge-spin conversion in topological semimetals are desirable for future spintronic and quantum technologies. Here, we report the charge-spin conversion (CSC) signals measured in a Weyl semimetal candidate WTe2 based hybrid graphene device with a geometrical design. Notably, the geometrical angle of WTe2 on the graphene spin-valve channel yields contributions to symmetric and anti-symmetric CSC signal components. The spin precession measurements of CSC signal at different gate voltages and ferromagnet magnetization shows the robustness of the CSC in WTe2 at room temperature. These results can be useful for the design of heterostructure devices and in the architectures of two-dimensional spintronic circuits

Charge to Spin Conversion in van der Waals Metal NbSe2

Quantum materials with a large charge current-induced spin polarization are promising for next-generation all-electrical spintronic science and technology. Van der Waals metals with high spin-orbit coupling and novel spin textures have attracted significant attention for an efficient charge to spin conversion process. Here, we demonstrate the electrical generation of spin polarization in NbSe2 up to room temperature. To probe the current-induced spin polarization in NbSe2, we used a graphene-based non-local spin-valve device, where the spin-polarization in NbSe2 is efficiently injected and detected using non-local spin-switch and Hanle spin precession measurements. A significantly higher charge-spin conversion in NbSe2 is observed at a lower temperature, below the superconducting transition temperature Tc ~ 7 K of NbSe2. However, the charge-spin conversion signal could only be observed with a higher bias current above the superconducting critical current, limiting the observation of the signal only to the non-superconducting state of NbSe2. Systematic measurements provide the possible origins of the spin polarization to be predominantly due to the spin Hall effect or Rashba-Edelstein effect in NbSe2, considering different symmetry allowed charge-spin conversion processes

Strong perpendicular anisotropic ferromagnet Fe3GeTe2/graphene van der Waals heterostructure

Two-dimensional magnets offer a new platform for exploring fundamental properties in van der Waals (vdW) heterostructures and their device applications. Here, we investigated heterostructure devices of itinerant metallic vdW ferromagnet Fe3GeTe2 (FGT) with monolayer chemical vapor deposited graphene. The anomalous Hall effect measurements of FGT Hall-bar devices exhibit robust ferromagnetism with strong perpendicular anisotropy at low temperatures. The electrical transport properties measured in FGT/graphene heterostructure devices exhibit a tunneling transport with weak temperature dependence. We assessed the suitability of such FGT/graphene heterostructures for spin injection and detection and investigated the presence of FGT on possible spin absorption and spin relaxation in the graphene channel. These findings will be useful for engineering spintronic devices based on vdW heterostructures

Development of Torque Sensor Based Electrically Assisted Hybrid Rickshaw

This thesis report is submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Electrical and Electronic Engineering, 2012.Cataloged from PDF version of thesis report.Includes bibliographical references (page 60-61).Cycle rickshaws are the most popular form of transportation inside the cities of Bangladesh, especially for its route and time-flexibility and door-to-door services. Considering the fact that a significant portion of underprivileged population of Bangladesh is directly or indirectly dependent upon the rickshaw-pulling profession, the necessity of scientifically thinking about its improvement and modernization was apparent. This paper describes a research and development project of Control and Applications Research Group, BRAC University, aiming to modernize these green fuelfree transports of Bangladesh using power-assistive technology. This involves design and implementation of an intelligent control system that would make the rickshaw pulling task easier-to-feel by assisting the human power with a motor, turning it into a hybrid vehicle. The motivation of the project was to relieve the rickshaw pullers from the excessive physical exhaustion associated with the task, which mainly occurs while initiating the momentum from rest or low speed to a moderate speed. A motor helping the pullers only during this particular time eradicates this exhaustion to a significant extent, at the same time saves energy by limiting over-use of the motor. A torque-sensor was involved to determine the need-of-assistance a puller feels at a particular time, and an external controller in addition to the motor controller was designed and tested. The model was developed so as to save energy, limit overuse, and keeping the identity and driving mechanism of old rickshaws. An idea of battery charging infrastructure using Solar- Battery-Charging-Station is also mentioned as a core factor of the project.Rachaen Mahfuz HuqNumayer Tahmid ShuvoPartha ChakrabortyMd. Anamul HoqueB. Electrical and Electronic Engineerin

Robust Spin Interconnect with Isotropic Spin Dynamics in Chemical Vapour Deposited Graphene Layers and Boundaries

The utilization of large-area graphene grown by chemical vapour deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of robust spin interconnects. Here, we report universal spin transport and dynamic properties in specially devised single layer, bi-layer, and tri-layer graphene channels and their layer boundaries and folds that are usually present in CVD graphene samples. We observe uniform spin lifetime with isotropic spin relaxation for spins with different orientations in graphene layers and their boundaries at room temperature. In all the inhomogeneous graphene channels, the spin lifetime anisotropy ratios for spins polarized out-of-plane and in-plane are measured to be close to unity. Our analysis shows the importance of both Elliott-Yafet and Dyakonov-Perel mechanisms, with an increasing role of the latter mechanism in multilayer channels. These results of universal and isotropic spin transport on large-area inhomogeneous CVD graphene with multilayer patches and their boundaries and folds at room temperature prove its outstanding spin interconnect functionality, beneficial for the development of scalable spintronic circuits

Observation of Spin Hall Effect in Weyl Semimetal WTe2 at Room Temperature

Discovery of topological Weyl semimetals has revealed the opportunities to realize several extraordinary physical phenomena in condensed matter physics. Specifically, these semimetals with strong spin-orbit coupling, broken inversion symmetry and novel spin texture are predicted to exhibit a large spin Hall effect that can efficiently convert the charge current to a spin current. Here we report the direct experimental observation of a large spin Hall and inverse spin Hall effects in Weyl semimetal WTe2 at room temperature obeying Onsager reciprocity relation. We demonstrate the detection of the pure spin current generated by spin Hall phenomenon in WTe2 by making van der Waals heterostructures with graphene, taking advantage of its long spin coherence length and spin transmission at the heterostructure interface. These experimental findings well supported by ab initio calculations show a large charge-spin conversion efficiency in WTe2; which can pave the way for utilization of spin-orbit induced phenomena in spintronic memory and logic circuit architectures