Electronic Structure And Magnetic Characteristics For Spin/Orbital Ordering Systems: A First-principle Study

One of the important research fields in condensed matter physics is to study thephysical mechanism of spin/charge/orbital ordering, coupling, and its interactionstogether in the systems of complex materials. Especially in recent years, the rapiddevelopment of spintronics can also improve the development of the studies infoundational problems of spin/charge/orbital ordering systems. Among those, charge,spin, orbital and the lattice degree of freedom are coupled to each other, thecomplicated interactions give rise to some interesting phenomena and have thepotential technological applications in such as magnetic memory and magneticsensors, etc. In this dissertation, based on the first-principles calculations, weinvestigated the electronic structure and magnetic characteristics in some typicalspin/orbital ordering systems. There are six chapters in this thesis.In the first chapter, some ordering phenomena in condensed matter physics areintroduced. For example, the background of the orbital physics and phase transitionrelated to orbital degree of freedom （ODF） in oxides is reviewed, and the electroncorrelation effects are also introduced. The basic theory of metal clusters and the spinordering as well as spin-orbital interaction of transition-metal clusters are brieflymentioned. At the end of this chapter, the motivation and the main research contentsof this dissertation are given.In the second chapter, the theoretical methods used in this thesis are introduced.The three approximations of energy band theory and density-functional theory （DFT）are introduced first. In the following, we introduced the disposal method of potentialand wavefunctions, for example, plane-wave method and pseudopotential approximation, etc.In the third chapter, the electronic structures of the Fe-doped perovskiteruthenates BaRu_1-xFe_xO₃with x=0,0.25,0.50,0.625,0.75, and1are investigatedthrough density-functional calculations. Large exchange splitting and small crystalfield splitting are found in BaFeO3, and contrary scenario can take place for BaRuO₃as expected since Ru atom has highly extended4d orbital. The small exchangesplitting and extended4d states are the reasons that the obtained spin magneticmoment （0.628μB） is significantly lower than the spin only value （2μB） for thet_2g3↑t_2g1↓electronic configuration for Ru⁴⁺ion. The further investigations suggest thatFe substitution at the Ru sites can suppress the bandwidths of Ru4d orbital, leading tothe half-metallic behavior in BaRu_1-xFe_xO₃with x=0.625and x=0.75. The differentorbital feature of the Ru⁴⁺ions in BaRu_0.375Fe_0.625O₃is presented, which reflects theinfluence of Fe dopant on Ru4d orbitals.In the fourth chapter, the effect of spin-orbit coupling （SOC） on geometricalstructure and magnetism of M@Pb₁₂clusters （M=3d and4d atoms） were studied. Wefound that SOC may enhance the symmetry of geometrical configurations for someclusters, which can be ascribed to the increase of the numbers of delocalized electronsafter SOC is considered. The SOC marginally affects the local spin magneticmoments of3d atoms, whereas it can cause large influence on spin magnetism ofimpurities for most4d series, which can be explained based on the differenthybridization strength between M3d-Pb6p and M4d-Pb6p. The considerable orbitalmagnetic moments are produced by SOC, and the local orbital moments of Ti, V, Co,and Ru even exceed0.8μB. The variation trends of the local orbital magnetism of Matoms encapsulated in the Pb12cages for3d and4d series can well be comprehended from Hund’s rule.In the fifth chapter, we studied the properties of icosahedral bimetallic Ti_nMn_13-n（n=1-12） clusters. We found that for the most stable structures, the central position isoccupied by a manganese atom. The total magnetic moment （TMM） decreases withthe increase of Ti atoms and it reaches0μBat the cluster of Ti₁₁Mn₂, and thenincreases to3μBagain. The substitution of Mn atoms by Ti atoms and the chargetransfer from Ti to Mn atoms are mainly responsible for the variation of TMM. Forlocal magnetic moment （LMM）, all Ti atoms and most Mn atoms presentferromagnetic ordering, while the spins on Ti atoms are anti-parallel to those on Mnatoms except for Ti₁₂Mn₁, forming antiferromagnetic magnetic structure. The bindingenergy increases monotonically with the increase of Ti atoms, indicating the clusterwith more Ti atoms are more stable than those with more Mn atoms. At last, thevertical ionization potential （VIP） of the lowest energy structure is discussed and theresults show that the little exchange splitting will lead to a relatively larger VIP forTi₁₁Mn₂and Ti₁₂Mn₁.In the sixth chapter, we give a brief summary and outlook about the followingwork.