Authors: Ershadrad, Soheil
Source: KTH Publication Database DiVA (KTH Royal Institute of Technology) (article, 2025-01-01)
Field: Materials Science — Materials Chemistry

Ab Initio Study of Two-Dimensional Metallic Magnets : From Analysis of Synthesized Compounds to High-Throughput Screening for Novel Candidates Two-dimensional (2D) magnetic materials hold great promise for next-generation electronic and spintronic technologies due to their unique properties driven by quantum confinement and high surface-to-thickness ratios. This thesis presents a comprehensive theoretical and computational investigation of 2D metallic magnets, encompassing both experimentally realized compounds and predictions of novel materials through high-throughput (HTP) simulations. A major focus is placed on the FenGeTe2 family (n = 3–5), a recently synthesized class of van der Waals magnets. We uncover a √3 × √3 R30◦ superstructure that better aligns with experimental observations. In this structure, the outermost Fe sublattice undergoes a unique splitting behavior and is prone to spontaneous vacancy formation, significantly impacting the magnetic properties. Co doping is shown to preferentially substitute these Fe split sites, modifying magnetic anisotropy and enabling easy-plane magnetism. At higher doping levels, a coexistence of ferromagnetic and antiferromagnetic interlayer couplings emerges. By comparing Density Functional Theory (DFT), DFT with Hubbard U correction (DFT+U ), and DFT combined with Dynamical Mean Field Theory (DFT+DMFT), we demonstrate that static correlation treatments fail to reproduce experimental magnetic properties. Dynamical cor- relations included via DMFT yield more accurate exchange interactions and Curie temperatures. The exchange mechanism in Fe3GeTe2 is found to involve both direct and indirect pathways, with p-orbitals of Ge and Te mediating the latter. In the search for new 2D magnets, we propose Mn2Mg2X5 (X = S, Se) as a stable family of half-metallic ferromagnets, and studied exchange interactions and predicted magnetic transition temperatures. We also explore FeXZ2 (X = Nb, Ta; Z = S, Se, Te) and their Janus variants, identifying exotic features such as canted magnetic anisotropy and emergence of skyrmions in the absence of an applied magnetic field. Lastly, we develop a high-throughput framework and explored Fe3XY2 compounds, where X and Y are p-block elements. Out of 529 screened Fe3XY2 compounds, 31 were identified as stable. Several trends and correlations were observed among the stable candidates. The exchange interactions confirm a predominantly direct mechanism for first-neighbor coupling and an indirect, p-orbital-mediated mechanism for second-neighbor and longer-range interactions. Notably, Fe3SiTe2 exhibits enhanced perpendicular magnetic anisotropy, while halide-containing compounds surpass the magnetic transition temperatures of synthesized Fe3GeTe2 and Fe3GaTe2. Furthermore, compounds with non-centrosymmetric structural ground states, such as Fe3AsBr2, offer promising avenues for realizing skyrmion-hosting two-dimensional magnets. The large datasets generated here provide a foundation for future machine learning approaches aimed at accelerating the discovery and understanding of 2D magnetic materials. Density functional theory ; Ferromagnetism ; Antiferromagnetism ; Spintronics ; Ab initio ; van der Waals force ; Ab initio quantum chemistry methods ; Hybrid functional ; Curie temperature 2D Materials and Applications ; Heusler alloys: electronic and magnetic properties ; Chemical and Physical Properties of Materials