Exploration of hole-doping-induced ferromagnetism in two-dimensional materials

Since the discovery of graphene in 2004, two-dimensional (2D) materials have triggered a lot of research interest, both scientifically and technologically. Their versatile electronic properties make them nteresting candidate materials to be integrated into future nanoelectronic devices. 2D magnetic materials have also received increasing research attention in the past few years, for their possible use in spintronic devices.

In this thesis, we investigate a new "family" of 2D magnetic materials, termed 2DHDFM, which are intrinsically non-magnetic semiconductors; upon hole doping, these materials are predicted to become erromagnetic half-metals. Inspired by recent theoretical work on 2D GaS, GaSe, and InSe, predicting such a magnetic transition upon hole doping, we first explore whether this transition also exists in heir oxide counterparts, i.e. 2D gallium oxides and indium oxides. By combining first-principles calculations with an advanced global crystal structure search method, stable atomic structures of 2D gallium oxides and indium oxides are identified, and a ferromagnetic transition upon hole doping is confirmed to occur in these materials. Their magnetic properties are systematically investigated, and their exchange coupling parameters are extracted, using a Heisenberg model Hamiltonian; the exchange couplings are subsequently used to compute their Curie temperatures, using Monte Carlo simulations, and they are found to range from 45 K to 125 K at hole densities of a few 10^{14} /cm^2.

We next perform a high-throughput theoretical investigation of 2DHDFM, based on DFT simulations. We consider a few thousands 2D non-magnetic semiconductors, using three different databases. We then identify 122 materials as promising 2DHDFM, and we find that almost half of them are 2D metal halides, next to chalcogenides, oxides, and nitrides. Their structural, electronic and magnetic properties are discussed in detail, based on the materials space groups. The exchange mechanisms responsible for the ferromagnetic coupling are also studied, and depending on the materials space group, we find that direct exchange coupling between anions p-orbitals, as well as indirect exchange coupling between the anions p-orbitals and metal p or d-orbitals can play an important role. Some of these 2DHDFM, especially few 2D metal halides, have predicted Curie temperatures exceeding 300 K, albeit at a high doping density of typically 6×10^{14} /cm^2.

Finally, the effects of intrinsic and extrinsic point defects on the electronic and magnetic properties of a few selected 2DHDFM candidates are also studied. The formation energies and charge transition levels of these defects are computed using DFT simulations. The results indicate that p-type doping of 2D nitrides (AlN, GaN, InN), sulfides (ZnS, SnS2) and oxides (Ga2O3) is very unlikely, most of the acceptor type defects producing deep states in their energy band-gap, rather behaving as trapping centers for charge carriers. On the other hand, more promising results are obtained for the considered 2D metal halides, namely PbBr2 and HgBr2. As a matter of fact, both metal vacancies and doping impurities, such as S, Se and Li, have relatively small formation energies, and produce shallow acceptor levels, typically less than 0.2 eV above the valance band edge.