First-principles, tight-binding and atomic spin model study of two-dimensional magnets

Abstract

The key ingredient for making better electronic devices is using materials with highly corresponding functional properties. The discovery of long-range magnetic order in two-dimensional (2D) van der Waals (vdW) materials paves a new avenue for spintronics research since these 2D magnets combines miniaturization (several angstrom), gate tunability, flexibility, high interface quality etc, and these advantages can be further inherited by electronics devices harnessing 2D magnets. However, there are some crucial problems still unsolved, which largely impedes practical applications of 2D magnets. For examples, most 2D magnets lack large perpendicular magnetic anisotropy (PMA) and high critical temperature. And inversion symmetry in 2D magnets could prohibits the emergence of topological magnetism. In this thesis, the first-principles calculations, tight-binding model and atomic spin model simulations are conducted to investigate the basic magnetic parameters, non-trivial transport phenomena of electrons and topological spin configurations in 2D magnets and their corresponding heterostructures (HSs), where spin-orbit coupling (SOC) plays crucial roles. The tight-binding model can give an accurate description of low-energy bands, thus helping us configure out crucial physical terms in materials; and the atomic spin model simulations can describe spin interaction in atomic level, thus helping us elucidate the spin configurations in 2D materials.

We first show that the PMA of NiI2 monolayer increases significantly when vdW interlayer distance of graphene/NiI2 (Gr/NiI2) HS decreases. This enhancement arises from the electronic states change of 5p orbitals of interfacial iodine. At the same time, the quantum anomalous Hall effect (QAHE) is realized in graphene layer. We second show that VSi2N4 is a ferromagnetic semiconductor harboring valley-contrasting physics. By tuning magnetization orientation from in-plane to out-of-plane, the valley polarization can be generated, resulting in the anomalous valley Hall effect (AVHE) in VSi2N4. We establish the mathematical relationship between valley splitting and magnetization orientation based on the model analysis. Moreover, via constructing Janus structures to break inversion symmetry, we achieve significant isotropic ii Dzyaloshinskii–Moriya interaction (DMI) and topological spin textures with ultra-small size in 2D magnets including CrXTe (X = S, Se), MnBi2Se2Te2 (MBST) and MBST/In2Se3 HS. Importantly, we demonstrate that strain and ferroelectric polarization are powerful tools for manipulating topological spin textures. The observed topological spin textures in 2D magnets have ultra-small size, high tunability and various morphology, which is hopefully considered as the ideal information carrier in electronic devices. We also find that critical temperature of strained CrXTe monolayer is even above the room temperature. This is a key factor required in realistic devices. Finally, we propose and demonstrate that a novel family of 2D magnets with 𝑃4𝑚2 crystal symmetry hold anisotropic DMI. This crystal symmetry-protected anisotropic DMI leads to various intriguing topological magnetism, including ferromagnetic chiral domain wall/antiskyrmion and antiferromagnetic chiral domain wall/antiskyrmion/vortex-antivortex pair in this family. These findings highlight potential applications of 2D magnets on the next-generation spintronic devices with high storage density, high speed and low energy consumption.