| Generally, the magnetism of condensed matters originates from the spin angular momentum and orbital angular momentum of electrons. First-principles calculation based on density functional theory is an important method to investigating magnetic materials. The electronic structures of condensed matters can be obtained through directly solving quantum mechanism equations. Therefore, the method could reveal the microscopic origin of the magnetism, and provide guidelines for designing magnetic functional materials. Based on density functional theory, this dissertation investigates the selected two-dimensional and quasi-two-dimensional magnetic systems. The investigations are summarized as followings.Firstly, the structure, electronic structure and magnetic properties of the Gd adsorbed graphene monolayer are calculated. The adsorption is of chemical properties, and the system has considerable magnetic anisotropy. In the situation of neutral system, the easy axis is perpendicular to the graphene plane, and the magnetic anisotropy energy is3.6meV. When the supercell misses an electron, the magnetic anisotropy energy is greatly enhanced to15.65meV. When the system obtains two electrons, the perpendicular easy axis is turned to the easy plane parallel to the the graphene sheet. Furthermore, the origin of the charge mediated magneto-electrical effect is explained by discussing the electronic structures of distinguished charge configurations.Secondly, the structure, electronic structure and magnetic properties of the hexagonal boron nitride monolayer doped by3d transition metal at boron-site are calculated. The doping systems could have considerable magnetic anisotropy energies. For example, in Fe-doped boron nitride monolayer, the calculated magnetic anisotropy is1.19meV with an easy axis perpendicular to the boron nitride plane; in the Mn-doped system, the easy plane is parallel to the boron nitride monolayer, and the magnetic anisotropy energy is0.63meV. Analytical analyses indicate:in the Mn-doped system, the magnetic anisotropy energy mainly originates from the unquenched orbital angular momentum; in the Fe-doped system, the magnetic anisotropy energy is mainly derived from the second-order spin-orbit coupling.Thirdly, following the theoretical predictions for the d-wave Dirac cone in two-dimensional triangular lattice, the3d transition metals doped artificial spinel structures are designed. Taking the advantage of first-principles calculations, the d-wave Dirac cones are found in these quasi-two-dimensional systems, which indicates the property of massless fermions transport in the artificial spinels.In summary, taking the advantage of first-principles calculations based on density functional theory, three selected systems with two-dimensional or quasi-two-dimensional structures are investigated. The works mainly focus on the structures, electronic structures and magnetic properties of these selected systems. The investigations illustrate the two-dimensional magnetic materials possess some novel properties, and provides theoretical guidelines for rationally designing two-dimensional magnetic function materials. |
