Abstract
Two-dimensional (2D) materials, especially transition metal based 2D materials, have been widely aroused tremendous attention because of their intriguing electronic, optical and magnetic properties. These 2D materials exhibit excellent electronic mobility, robust magnetism, flexible mechanical property, and easy tunability, thus being studied for potential applications in nanoscale electronics and spintronics. Although various 2D intrinsic magnetic materials have been discovered and developed, a systematical search for stable 2D intrinsic magnetic materials is still missing. Therefore, the aim of this thesis is to find more emerging 2D intrinsic magnetic materials and investigate their extraordinary physical properties. In this thesis, we theoretically design, search, and study a series of 2D intrinsic magnetic materials using a combination of evolutionary algorithm and first-principles calculations. We start from CoSe, which has been experimentally synthesized and has novel features. Here, we search for its stable or metastable structural phases in the 2D limit and identify three lowest energy 2D CoSe structures, termed as the -, -, and -CoSe. We show that - and -CoSe are two rare examples of 2D antiferromagnetic metals, which are related to their Fermi surfaces nesting features, and meanwhile, -CoSe is a ferromagnetic metal. They possess a range of interesting physical properties, including anisotropic valleys connected by crystalline symmetries, strain-tunable valley polarization, strain-induced metal-semiconductor and/or magnetic phase transitions, as well as topological band features such as the magnetic Weyl point and the magnetic Weyl loop. Remarkably, all the topological features here are robust against spin-orbit coupling. Then, we extend our attention to cobalt pnictides (Pn) and chalcogenides (Ch), which have similar structure as that of the most stable -CoSe. Compare the CoCh, CoPn exhibit an exceptional electronic behavior, being in proximity to the Fermi-surface topology of FeSe. In five CoPn (Pn = N, P, As, Sb, Bi), CoP is proven as the most stable structure in thermodynamic, mechanical and compositional checking, which indicates it is possible to be synthesized. We further reveal its unique physical properties, such as magnetic ordering and Fermi surfaces. The strain engineering can be exploited to effectively tune the Fermi surface and realize the the magnetic states transitions. Given that tetragonal structures show numerous incredible properties, we hypothesize that a bunch of transition metal compounds with the similar structural motif can be involved to expand the family of 2D intrinsic magnetic materials. After a comprehensive searching, we prefer TiAs, CrAs, and TiSb as archetypes to discuss in iii detail for their exotic magnetic and electronic properties. TiAs is an ideal spin-polarized Weyl half-semimetal, and is able to transform to a quantum anomalous Hall phase under SOC. CrAs is an ferromagnetic semiconductor with high Curie temperature, while TiSb is an antiferromagnetic semiconductor with anisotropic valleys. Notably, strain engineering is still applicable to tune these physical properties for both CrAs and TiSb, such valley polarization and metal-semiconductor transition. The work in this thesis has shown a map of 2D intrinsic magnetic materials, and the property-tuning can viably achieved by applying tensile strain. It not only paves the way for guiding and identifying novel materials with fascinating properties in experiment, but offers a plethora of opportunities for promising applications in electronics and spintronics.