Spin and magneto transport in van der Waals heterostructures of graphene with ferromagnets

The increasing demand for information and communication technologies has augmented the requirements of electronic devices with improved speed, sensitivity, and reduced power consumption. The utilization of novel electronic materials and the use of the spin degree of freedom as a state variable for information processing and storage are expected to fulfill these demands. In this direction, two-dimensional (2D) materials have attracted a significant research effort with the long-term goal of creating electronic devices with novel functionalities. Graphene has shown excellent potential for future device applications due to its outstanding electronic carrier mobility and spin coherence time at room temperature. Followed by the successful advent of graphene, a vast plethora of 2D materials with complementary electronic properties have been discovered, such as insulating hexagonal boron nitride (hBN), magnets and topological semimetals. We observed that engineering 2D material heterostructures by combining the best of different materials in one ultimate unit offers the possibility of the creation of new phases of matter and novel opportunities in device design. For example, graphene is shown to acquire magnetic properties because of proximity-induced interactions with a magnetic insulator in van der Waals heterostructure. On the other hand, topological semimetal candidates such as WTe2 and ZeTe5 allowed us to observe unconventional charge-to-spin conversion and anomalous Hall effects due to their enormous spin-orbit coupling, lower crystal symmetry, and larger fictitious magnetic field in the crystals. Furthermore, the performance of heterostructures comprised of graphene and hBN with one-dimensional ferromagnetic edge contacts and a path for optimizing such device geometry is outlined. These experimental findings on 2D materials and heterostructure device architectures can contribute to developing a new platform for spintronic as well as quantum science and technology

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

Ghosts in a Wilderness: Problems and Priorities of Faculty at Mid-Career and Mid-Life

This article explores the experiences of mid-career, older faculty members in higher education through a qualitative study of 20 associate professors (15 men and 5 women) between the ages of 41 and 59 at a Canadian university. The study was undertaken to explore how university faculty experience the mid-career and mid-life period in their academic and community work, their personal and family relationships, and their sense of self. The findings focus on a variety of issues revealed in this sample group: the undervalue of teaching as a vocation, noncaring university administrators, rejection in career progression, external personal relationships, impediments to research, failing health, adapting to change, and the question of self-knowledge. While acknowledging several limitations of the study, the perspective of adult development theory points to the potential for greater integration of personality on the part of faculty members, and for its recognition on the part of administrators in fostering a more humanizing, responsive, and caring environment for the conduct of teaching and research. The recognition of significant differences between younger and older faculty, and how the genuine problems facing the latter group may be addressed, will contribute to enhancing the quality of university life generally.Cet article présente les résultats d'une étude qualitative menée dans une université canadienne auprès de vingt professeurs agrégés (15 hommes et 5 femmes) dont l'âge se situe entre 41 et 59 ans. Cette recherche portait sur ce que vivent les professeurs d'université parvenus au milieu de leur vie et de leur carrière, en ce qui concerne leur travail universitaire et leur engagement communautaire, leurs relations personnelles et familiales et la perception qu'ils on d'eux-mêmes. Les réponses des participants ont permis de dégager diverses préoccupations, dont le peu de valeur accordée à la vocation de l'enseignement, le manque de sensibilité des administrateurs à leurs difficultés, les rejets qui freinent l'avancement professionnel, les relations personnelles à l'extérieur du travail, les obstacles à la recherche, les problèmes de santé, l'adaptation au changement et la question de la connaissance de soi. Bien que cette recherche comporte de nombreuses limites, la perspective de la théorie du développement à l'âge adulte laisse entrevoir la possibilité qu'une meilleure intégration de la personnalité chez les professeurs d'université et la reconnaissance de cette intégration part les administrateurs pourraient contribuer à créer un milieu de vie plus humain et axé sur la personne et ses besoins et, par conséquent, plus propice à l'enseignement et à la recherche. Le fait de reconnaître qu'il existe des différences significatives entre les jeunes professeurs d'université et leurs collègues plus âgés et de savoir comment tenir compte des préoccupations réelles de ces derniers permettront d'améliorer la vie de l'ensemble de la communauté universitaire

Gate-tunable Hall sensors on large area CVD graphene protected by h-BN with 1D edge contacts

Graphene is an excellent material for Hall sensors due to its atomically thin structure, high carrier mobility and low carrier density. However, graphene devices need to be protected from the environment for reliable and durable performance in different environmental conditions. Here we present magnetic Hall sensors fabricated on large area commercially available CVD graphene protected by exfoliated hexagonal boron nitride (h-BN). To connect the graphene active regions of Hall samples to the outputs the 1D edge contacts were utilized which show reliable and stable electrical properties. The operation of the Hall sensors shows the current-related sensitivity up to 345 V/(AT). By changing the carrier concentration and type in graphene by the application of gate voltage we are able to tune the Hall sensitivity

Origin and evolution of surface spin current in topological insulators

The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing out the bulk conductivity in semiconducting Bi1.5Sb0.5Te1.7Se1.3 below 100K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2 and 100K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology

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

Charge-spin conversion in layered semimetal TaTe2 and spin injection in van der Waals heterostructures

A spin-polarized current source using nonmagnetic layered materials is promising for next-generation all-electrical spintronic science and technology. Here we electrically created spin polarization in a layered semimetal TaTe2 via the charge-spin conversion process. Using a hybrid device of TaTe2 in a van der Waals heterostructure with graphene, the spin polarization in TaTe2 is efficiently injected and detected by nonlocal spin-switch, Hanle spin precession, and inverse spin Hall effect measurements. Systematic experiments at different bias currents and gate voltages in a vertical geometry prove the TaTe2 as a nonmagnetic spin source at room temperature. These findings demonstrate the possibility of making an all-electrical spintronic device in a two-dimensional van der Waals heterostructure, which can be essential building blocks in energy-efficient spin-orbit technology

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