Spintronic devices based on van der Waals heterostructures

With myriad potential applications, van der Waals (vdW) ferromagnetic materials have attracted considerable attention over the past few years. Two-dimensional (2D) ferromagnetism has been confirmed in several monolayer vdW materials, such as CrI3, Cr2Ge2Te6 (CGT) and Fe3GeTe2 (FGT). Although the vdW ferromagnetic metal FGT has been studied for some time, the suitability of vdW FGT as a ferromagnetic metal for spintronic devices has only recently become clear. The weak coupling between vdW layers means that vdW heterostructures can be assembled from lattice mismatched materials. This is in contrast to conventional thin-film heterostructures and with this significant design restriction removed, there are almost limitless possibilities for novel spintronic device architectures. Giant magnetoresistance (GMR) is one of the first spintronic effects exploited in devices. These consist of two ferromagnetic metals separated by a nonferromagnetic metal. Most GMR devices have been fabricated from metallic thin films grown in a high-vacuum chamber. Recently, interest has been drawn to replacing the nonferromagnetic metal in GMR with a conductive metal exhibiting special properties, such as graphene. Within vdW heterostructures, non-magnetic conductors such as graphene can be sandwiched between two vdW ferromagnets with clean interfaces. In Chapter 4, FGT/graphite/FGT devices were fabricated using a pick-up transfer technique. The magnetoresistance of these devices was investigated. When typical GMR devices are subjected to measurements they show a symmetric MR effect with high- and low-resistance states. The FGT/graphite/FGT devices revealed an intermediate-resistance state. This was revealed to occur when the magnetic moments of the ferromagnets were parallel. The three resistance states exhibited by the FGT/graphite/FGT heterostructure were attributed to the unique properties of the graphite/FGTs interfaces and spin-orbit coupling (SOC) in the FGT. Chapter 5 in this thesis concerned exchange bias in vdW magnet FGT nanoflakes. By employing exchange bias, strong magnetic coupling was induced by proton intercalation in vdW FGT nanoflakes. This finding suggests that proton intercalation is a promising tool for enabling strong interface coupling and better device performance in a wide range of vdW heterostructure devices including 2D ferromagnetic (FM) insulators-topological insulator heterostructures for high-temperature quantum anomalous Hall effect and FM-ferroelectric heterostructures for 2D multiferroics. In chapter 6, a ferromagnetic-antiferromagnetic heterostructure was fabricated and tested. FePS3 (FPS) was selected as an antiferromagnetic vdW nanoflake. After fabrication, exchange bias was observed in F5GT/FPS heterostructures. By introducing a protonic solid conductor, it became possible to switch the exchange bias within the F5GT/FPS heterostructures on and off. This switchable exchange bias opens up new spintronic applications for vdW materials/devices. Applying gating induced proton will enhance interlayer coupling in vdW materials. This would significantly increase potential use in high-temperature devices utilising quantum anomalous Hall effect, and in 2D multiferroics.