Tunnel barriers for spin injection into graphene

Abstract

This thesis describes the fabrication and characterisation of tunnel barriers on graphene for spintronic devices. Tunnel barriers are ultrathin, smooth, homogeneous and electrically insulating layers and a prerequisite for successful spintronic experiments, since they prevent the conductivity mismatch between a spin transport material and its ferromagnetic contacts. During the course of this thesis, which constitutes the first systematic graphene spintronics study in our group, we experimented with the oxides Al2O3 (grown by atomic layer deposition/ALD) and MgO (grown by physical vapour deposition/PVD) as barrier materials. A detailed description of the fabrication process with all the recipes we used is given in the thesis. Before and after the barrier fabrication, the samples were characterised with Raman spectroscopy, allowing the determination of the quality of graphene below its top layer, and with AFM measurements from which we extracted the barrier’s smoothness and homogeneity. Finally, we fabricated graphene spin valves in a non-local geometry by depositing permalloy (Ni80Fe20) contacts onto the covered graphene flakes. In order to further investigate the performance of these devices, we conducted charge and spin transport (i. e., non-local resistance and Hanle spin precession) measurements. We successfully achieved spin injection with both oxides. Compared with the literature, most of our results hint at pinhole-dominated or transparent barriers in the devices. In a novel approach, we deposited thin exfoliated layers of the semiconductor MoS2 onto graphene using an in-house-built transfer microscope, where MoS2 acted as tunnel barrier between the ferromagnetic contacts and graphene. Using three-terminal measurements on the MoS2-graphene samples, we observed large magnetic-field-dependent jumps in the MoS2 barrier resistance. Finally, we also present the first spin signals achieved with a multilayer MoS2 barrier.