First-principles Studies Of Half-metallic XO(X=Li,K,Rb,Cs) And XKMg(X=C,Si)、CKCa

In this paper we predicted some half-metallic compounds. Half-metallic materials have very special property that one spin channel is conductive while the other channel shows semiconductor or insulator properties. The materials with special features have great application potential in spintronics. Seen from the beginning in their development, half-metallic materials have been the favorite in industry application and scientific research. Finding new half-metallic materials has become the focus of scientific research since the half-metallic materials was first found by Rob de Groot in 1983. Both experiment and theory have all been the best way of human beings to explore the unknow world. That, in one hand, means predicting properties of materials by theoritical computations is helpful for experiments, including the study of halfmetallics. Predicting half-metallic materials based on first-principle density function theory plays an active role in guiding the experimental study. In the paper, we predicted the electronicmagnetism property, structure property of rocksalt XO(X=Li, K, Rb, Cs) and half-Heusler compounds XKMg(X=C, Si)、CKCa and studied the influence of external pressure on the lattice constant and half-metallic property.In the second chapter, we introduce the density function theory(DFT), which is one of the most useful theory in the computational physics. In 1927, H. Thoms and E. Fermi firstly propose that the physics property of a system can be described only by the density of particles.And based on the assume of uniform electronic gases, they propose the Thomas-Fermi model,which is the rough theory about DFT. Based on the Thomas-Fermi model, P. Hohenberg and W.Kohn propose the DFT we used now by taking into consideration of the exchange-correlation function between the electrons.In the third chapter, by using first-principle calculation based on density function theory,we investigated the electronic structure and magnetic properties of the XO(X=Li, K, Rb, Cs).The results show that all the four compounds have half-metallic properties at equilibrium lattice constant. we calculated the curie temperature of the four compounds of which results show that the curie temperature of each compound is higher than room-temperature. It shows that their ferromagnetism can be maintained at room-temperature. By calculating the formation energy and the cohesive energy, we get that the four compounds have stable structure and can be synthesised. We discuss the origin of the magnetic moment and the orbital hybridization of the four compounds. We also discuss the influence of external pressure on the half-metallic property of the compounds, the results showing that the XO(X=Li, K, Rb, Cs) compounds can keep half-metallic property in a wide range of external pressure. Then we can get the conclusion that the four compounds have great potential in practical application.In the fourth chapter, the structural, electronic, and magnetic properties of half-Heusler alloys CKMg, Si KMg and CKCa are studied by using first-principle density functional theory.The calculations reveal the Si KMg and CKCa alloys are half-metallic ferromagnets with the magnetic moment of 1 μBper formula unit at equilibrium lattice constant. Si KMg can preserve half-metallic character when its lattice constant is compressed within 17.6 %. CKMg alloy is nearly half-metallic with a spin polarization of 99.99 % at equilibrium lattice constant, but it is a good half-metallic alloy when a low pressure(0.23 GPa) is applied. This shows CKMg is a very promising spintronic functional material. By using the GIBBS pseudo-code, we calculated the thermodynamic property of the three half-Heusler alloys. We study the influences of pressure and temperature on thermal capacity Cv, coefficient of thermal expansion αand bulk modulus B for the three half-Heusler alloys.In the final chapter, we make a conclusion for this thesis, and made a plan for the following work.