First-principles Study And Design Of Novel Magnetic Materials

Magnetic materials, as a kind of functional materials, are widely used in many apparatus, ranging from the ancient compass to the modern telephone. With the development of information technology, magnetic materials are also utilized for information storage. As we know, for traditional information devices, electron charge is utilized in the information processing, while electron spin is used in the information storage. To fabricate smaller devices, combining electron charge with spin has attracted many interests.This new area in magnetic materials has been marked as spintronics. However, many novel properties of spintronics have not been fully understood. Thus, theoretical studies are required to provide the physical insights. Due to the progress in computationalmethods and enhancement of computational ability, density functional theory (DFT) has become one of the most important methods in such magnetic materials.The dissertation contains five chapters, which are divided into three parts. The first part containing three chapters focuses on the magnetic and electronic properties of graphene. The second part is to study the magnetic and electronic properties of diluted magnetic semiconductors (DMS), and the third part are devoted to discuss the ferromagnetism of Cs₂AgF₄.In chapter 1, we introduce the basic idea of DFT and review its recent progress. DFT, which describes that any properties of a many body system can be determined by the charge density at ground state. With the help of Kohn-Sham equation, many body interaction is included in the exchange-correlation energy and the many body problem becomes an effective single particle problem. Finding good approximation of exchange-correlation functional is one of the main targets in DFT. At the end of this chapter, we briefly introduce some DFT based simulation packages used in the dissertation.In chapter 2 and chapter 3, we begin to focus on the graphene. First of all, we give a simple introduction of graphene. Due to its novel properties, such as anomalous quantum hall effect and massless dirac fermions, graphene has been widely stud- ied. In chapter 3, by theoretical treatments, we give several materials designs based on graphene nanoribbons (GNRs). Firstly, we predict that the strong hybridization between the titanium chain and the GNRs gives rise to ferromagnetism and metallicity for one-dimensional titanium chains adsorbed on semiconducting armchair GNRs. Meanwhile,the adsorption system may offer half-metallicity when the width of GNRs is less than 2.1 nm. Next, for zigzag GNRs, they are semiconductors with two localized electronic edge states, which are ferromagnetically ordered, and antiferromagnetically coupling each other. The calculated results show that such zigzag GNRs can be converted to half metal under enough transverse electric field. However, the high electric field makes the applications difficult. As an alternative way, we theoretical predict that half metal can also be gained by chemical approaches. The first method is edge-modifications. We show that by modified the zigzag GNRs with different chemical groups, the ribbons can become half metal. With the reduction of chemical groups, the ribbons become more stable. We also predict that the ribbons can be changed to half metal by implanting a BN row into zigzag GNRs. Our molecular-dynamics simulations confirm such hybrid structures are stable under room temperature.In chapter 4, we study two kinds of DMS. For Co-doped ZnO, we find that Co atoms tend to distribute on the nearest neighbor sites, and antiferromagnetic (AFM) ordering between the Co ions is favored over ferromagnetic (FM) ordering. But FM ordering can be induced by electron doping. To investigate the surface effect of ZnO on the magnetic properties, we study the Cu-doped ZnO surfaces. Theoretical calculations suggest that surfaces have great influence on the distribution of magnetic atoms and the magnetic coupling.In chapter 5, we investigate one kind of new materials: Cs₂AgF₄, which is an ideal isostructural analog of layered cuprates La₂CuO₄. Magnetic measurements reveal that Cs₂AgF₄ is FM. Based on the structures resolved by neutron diffraction, we find that the ground state has a staggered order of z²-y² and z²-x² orbitals. The superexchange interaction through the Ag(z²-y²) - F - Ag(z²-x²) bridge stabilizes the ferromagnetism.