Spin and magneto transport in van der Waals heterostructures of graphene with ferromagnets
Doktorsavhandling, 2021

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.

Opponent: Professor Masashi Shiraishi, Kyoto University, Japan

Författare

Bogdan Karpiak

Chalmers, Mikroteknologi och nanovetenskap (MC2), Kvantkomponentfysik

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Unconventional Charge–Spin Conversion in Weyl-Semimetal WTe2

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Today the electronic age is in full swing, and the advancements in information technology stand behind the technological revolution that has touched upon all aspects of our lives. This progress has mainly been possible due to scaling down memory and logic device dimensions and engineering new ways to mitigate the challenges accompanied by this trend by e.g. utilizing new materials and employing novel device geometries. However, we are reaching fundamental limits of this approach when the device features are only few atoms thick. This leads to significant heat dissipation problems and, more importantly, quantum uncertainties start to contribute to device behavior at this scale, making device operation unreliable. To keep up with the fast pace of technological developments, alternative approaches are being considered.

In contrast to charge-based logic and memory devices mainly utilized today, spintronics introduces a new paradigm in device operation by employing electron’s magnetic property, i.e. angular momentum or spin. Manipulating the direction of spins as a state variable in contrast to rearranging charge carriers is promising for developing novel spin-based non-volatile electronics with lower energy consumption and faster operations.

The appropriate choice of functional materials is the key to the successful development of nanotechnology. With the advent of graphene in 2004 a whole new field of materials science, i.e. two-dimensional (2D) materials and their heterostructures has emerged. A vast plethora of 2D materials with complementary electronic properties have been discovered, such as insulating hexagonal boron nitride, magnets and topological semimetals. Based on the heterostructures of such atomically thin layered materials, a new generation of electronics has been envisioned with the long-term goal of creating electronic devices with novel spin functionalities. By assembling such layers of distinct 2D materials, we can tune the required functionality and combine different properties of the assembled structures in one ultimate unit. This offers the possibility of creation of new phases of matter and novel opportunities in device design. For example, in this thesis graphene is shown to be magnetic because of proximity-induced interactions with a magnetic insulator in a 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 insulating hexagonal boron nitride with one-dimensional ferromagnetic edge contacts and a path for optimizing such device geometry is outlined. These experimental findings can contribute to developing 2D materials-based devices for future spintronics, as well asquantum device architectures and technologies.

Styrkeområden

Nanovetenskap och nanoteknik (SO 2010-2017, EI 2018-)

Ämneskategorier

Materialteknik

Fysik

Nanoteknik

Fundament

Grundläggande vetenskaper

Infrastruktur

Nanotekniklaboratoriet

ISBN

978-91-7905-473-1

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4940

Utgivare

Chalmers tekniska högskola

Online

Opponent: Professor Masashi Shiraishi, Kyoto University, Japan

Mer information

Senast uppdaterat

2021-04-08