Magnetic Properties of Large-Scale Nanostructured Graphene Systems

The on-going progress in two-dimensional (2D) materials and nanostructure fabrication motivates the study of altered and combined materials. Graphene—the most studied material of the 2D family—displays unique electronic and spintronic properties. Exceptionally high electron mobilities, that surpass those in conventional materials such as silicon, make graphene a very interesting material for high-speed electronics.

Simultaneously, long spin-diffusion lengths and spin-life times makes graphene an eligible spin-transport channel. In this thesis, we explore fundamental features of nanostructured graphene systems using large-scale modeling techniques. Graphene perforations, or antidots, have received substantial interest in the prospect of opening large band gaps in the otherwise gapless graphene.

Motivated by recent improvements of fabrication processes, such as forming graphene antidots and layer-by-layer stacking, we consider a hybrid bilayer graphene system: Graphene on graphene antidot lattice (GOAL). These systems can be engineered to select attractive features from either bilayer and monolayer graphene.

For a certain set of optimized geometries, we obtain linearly dispersing bands with a high corresponding mobility, resembling that of monolayer graphene. Nevertheless, these linearly dispersive GOALs can be made gapped by breaking layer symmetry, using e.g. perpendicular electric fields. In the area of graphene spintronics, the formation of magnetic moments is predicted as the result of breaking the graphene sublattice symmetry.

We take advantage of this, and explore the fundamental features of zigzag-edged triangular graphene antidots (zz-TGAs). Their specific edge configurations give rise to highly desirable spin-filtering and spin-splitting transport features. The mechanisms behind these functionalities are studied in detail in lattices, devices, and finally in disordered systems of experimentally feasible scale.

We demonstrate that superlattices of triangular antidots exhibit large bands gaps, induced by sublattice symmetry breaking. Spin-polarized TGAs are shown to become half-metallic near the Fermi level, giving rise to perfectly spin-polarized densities of states. By studying the transport properties of devices with embedded zz-TGAs, we highlight an interesting spatial spin-splitting feature analogous to the spin Hall effect.

Unlike the conventional spin Hall effect, this feature is obtained without spin-orbit interactions or topologically protected transport channels. Motivated by spin Hall effect measurements, we calculate transverse resistance signals in zz-TGA devices and show that these can provide a general diagnostic tool to detect the presence of zigzag edge magnetism.

The extraordinary features of zz-TGAs at small scales motivate our study of their underlying mechanisms in larger, more realistically sized TGAs. Half-metallic, semiconducting and highly anisotropic transport behaviors can be induced in these systems. Furthermore, these properties are extremely robust in the face of substantial disorder, in stark contrast to what is seen for many other antidot-based devices.

Ultimately, these properties may prove useful in spintronic devices, graphene-based transistors and integrated electronic circuits where a particular transport direction is preferred.