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

Doctoral thesis, 2021

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.