AbstractMagnetism in recently discovered van-der-Waals (vdW) materials such as CrI3 has opened new avenues in the study of fundamental spin interactions in two dimensions (2D). Such developments allow us to assess whether magnetism in 2D is a smooth continuation from low-dimensional magnetic materials, or if new physics and magnetic behaviour arise due to the reduced dimension.
In this thesis, we present an investigation of 2D magnetism through the use of a multiscale approach involving atomistic scale models with Hubbard-U corrected Density Functional Theory (DFT+U) and mesoscale models with Monte-Carlo (MC) simulations. Firstly, we performed a high throughput screening over 150 compounds, identifying 59 potential two-dimensional magnets. To computed their critical temperature Tc, we parametrised the generalised Heisenberg model (XXZ) using DFT+U, and then we ran MC simulations. Most of the materials have a critical temperature below 100 K, but two of them, i.e. VPS3 and VSe2, may potentially show magnetism at room temperature. Some of these materials also stabilise vortex-antivortex spin texture at low temperatures. These vortex structures are similar to those from the XY model. We find that the XXZ model can not reproduce the magnitude of the experimental Tc of CrI3 if only first-nearest neighbour interactions are included. We thus extended the XXZ model to include more neighbour interactions as well as higher-order exchange terms and Dzyaloshinskii-Moriya interactions. This extended model gives a better estimation of the critical temperature for several materials, including the chromium trihalides family (CrX3, X=F, Cl, Br, I). Modeling the magnon spectra using this model within the linear spin-wave theory gives good agreement with the experimental results. In particular, we show that for CrI3 the quantum effects are essential when it comes to describing the critical exponent in a Curie-Bloch law, and the shape of magnetisation versus temperature curve. We present a study of the magnetic domain in CrI3 monolayer and show that the domains are metastable. The boundary between the magnetic domains called the domain walls (DW) is in the majority of Néel type (97 %), with the rest being either Bloch type or hybrids. Finally, we show the nucleation of topological spin texture (TST) in CrCl3. We identify these TSTs as merons and anti-merons. The TSTs are stabilised through the dipole-dipole interaction. They are dynamic and can annihilate through non-trivial processes.
|Date of Award||Jul 2021|
|Supervisor||Jorge Kohanoff (Supervisor) & Myrta Grüning (Supervisor)|