First-principles-based study of magnetism in van der Waals systems

Abstract

Magnetism in a two-dimensional material is an interesting topic due to the different quantum mechanical behaviors from conventional three-dimensional materials and their atomic-scale applicability. Studying two-dimensional magnetism through van der Waals materials is expected to give a fundamental understanding and technological development of quantum materials. However, it is not straightforward to understand the magnetism of van der Waals materials due to various variables and difficulties of a many-body system. In this dissertation, first-principles-based calculation methods are implemented and used to deal with magnetism and related phenomena in van der Waals materials. First, the calculation methods for magnetic interactions and spin excitations are described. The magnetic force theorem was implemented within a first-principles framework to calculate relativistic exchange interactions. The reliability of the calculation is confirmed by applying it to various transition metal materials. The relativistic exchange interactions can be controlled by a change in crystal symmetry. The spin wave excitations can be also calculated using the first-principle-based spin Hamiltonian. The relativistic spin Hamiltonian of $CrI_3$ is obtained by the magnetic force calculation. Based on the spin Hamiltonian, the magnon and their topology in monolayer and stacked $CrI_3$ are investigated. Second, the interplay of spin, charge, and lattice degrees of freedom in the bulk and monolayer of $VTe_2$ are studied. Single-crystalline $VTe_2$ shows two different charge density waves with magnetism. In particular, the newly discovered charge density wave can be stabilized only in the presence of magnetism. On the other hand, the single-layer $VTe_2$ has various charge density waves and the role of magnetism has never been investigated. Our comprehensive density functional theory calculation reveals that magnetism plays an important role in stabilizing the structure. In addition, by noticing this intriguing interplay between magnetism and other degrees of freedom, we suggest a possible strain engineering. The last part describes magnetic interactions and anisotropies in $MPS_3$. We performed a complete mapping of the magnetic anisotropy of prototype Ising van der Waals magnet $FePS_3$ combining the torque magnetometer measurement, magnetostatic model, and relativistic density functional theory analysis. For $NiPS_3$, a generalized spin Hamiltonian is constructed to understand the magnetic properties. Based on this spin Hamiltonian, we found that the characteristic torque pattern originates with a spin-flop, and the magnetic anisotropy is principally determined by magnetic dipole-dipole interactions.