We study reorientation of magnetization of a ferromagnetic film grown on a compensated antiferromagnetic substrate and propose a method to determine a lower bound of the interface exchange coupling strength. For uniaxial ferromagnetic films, we show that the frustration induced by the interface exchange leads to a reorientation of the magnetization only if the strength of the interface field is beyond a threshold value. For ferromagnetic films with fourfold crystalline anisotropy the reorientation of the magnetization occurs for any value of the interface exchange field strength. We use a self-consistent two spin mean field model and apply the theoretical model to Fe=FeF2 and Fe=MnF2 bilayers. For these systems the reorientation of the magnetization produces minor changes in the antiferromagnetic arrangement of the spins near the interface, allowing a simple analytical formula which relates the threshold value of the interface exchange coupling to the magnetic parameters of the antiferromagnetic substrate and to the anisotropy and thickness of the ferromagnetic film.

Group VI elemental 2D materials have been predicted to possess promising properties in electronics, optoelectronics, thermoelectricity, sensors, and detectors[1–5]. In this thesis, we present the electronic and optical properties of square selenene (s-Se), a 2D allotrope of selenium. We start by calculating the electronic properties using Density Functional Theory (DFT). These ab initio calculations show the formation of an anisotropic cone with semi-Dirac dispersion in a non-symmetric point, denoted P1, in the Brillouin zone, which forms a narrow gap (∼0.1 eV) due to spin-orbit coupling (SOC). Then, we uncover an intricate reciprocal space spin texture along the Brillouin zone, with spin-momentum locking around P1. To conclude the DFT calculations, we show the existence of edge states in two types of s-Se nanoribbons, with one of them presenting topologically non-trivial features, which are confirmed in other works[1,6]. We then replicate a k⋅p Hamiltonian, for the band structure around P1, from the literature[1] and add various SOC terms to obtain an effective model that describes the band structure data from fully relativistic DFT calculations and qualitatively reproduces the spin texture in reciprocal space. Using this effective model, we describe the exciton formation in the Dirac-like cone gap using the variational method while considering an anisotropic version of the Rytova-Keldysh potential[7]. This analysis provides a first characterisation of the exciton ground-state in square selenene, describing its binding energy (Eb=-24 meV) and wavefunction, which, to the best of our knowledge, have not been previously reported.