Computational Study on Spin Interactions and its Influence on Material Properties Pohang University of Science and Technology

Abstract

Spin is one of the fundamental elements consisting the quantum mechanical world. In condensed matter systems, under the Born-Oppenheimer approxima- tion, valence electrons are filled as water in the pool made of electrostatic poten- tial from fixed atomic ions. In systems where localized ionic potential and orbital exists, spin degree of freedom plays an important role in material properties. Systems with transition metal elements are such case, since they have 3d-orbital electrons as valence electrons which are strongly localized near the atomic center. This is why there are various magnets among the materials containing transition metal elements. In this dissertation, studies on the influence of spin interactions in different materials and properties will be described in a parallel manner for each case. The first case is two-dimensional (2D) magnetic material where magnetocrys- talline anisotropy (MAE) strongly influences on the Curie temperature TC and coercivity. In this study, the control of MAE via electric field perpendicular to plane which doesn’t cost energy loss has studied by Density Functional Theory (DFT). Resultingly, it has figured out that in FenGeTe2(n = 3, 4) the electric field can drastically change the MAE via intralayer charge redistribution, which is the mechanism allowed due to their peculiar 3D-like intralayer crystal structure. The second case is the thermoelectric material where weak ferromagnetism exists. In Mn15Si26 there is one hole per 4 f.u. leading to dilute carrier concen- tration close to semiconductor although it is half-metal in which only minority spin band crosses the Fermi level. In this system, slight doping of Fe or Co atom resulted in the increase of both conductivity and Seebeck coefficient, vio- lating the Pisarenko relation which originates from the simple fact that carrier concentration decreases as the increase of carrier excitation energy. In this study, thermoelectric enhancement resulting from antiferromagnetic coupling between host material and impurity has explained through a new mechanism. Specifically, two-current model has suggested and explained the mechanism of the simultane- ous increase of carrier energy and concentration resulting from the decrease of magnetic ordering via doping in half-metal. The third case is multi-orbital Hubbard model system where Hund’s rule coupling plays an important role. In degenerate multi-orbital Hubbard model, there are two sources of strong electron correlations, Hubbard and Hund inter- action. Hubbard interaction is a density-density type local Coulomb interaction which gives an energy barrier for electron transport. As the increase of Hubbard interaction, spectral weight moves from conduction band to Hubbard bands and at a certain point the system becomes insulator. Previous dynamical mean field theory (DMFT) studies on the metal-insulator transition (MIT) in single-band system has proven that this feature, namely Brinkman-Rice picture is clearly demonstrated. However, in case of multi-orbital systems where Hund interaction enters, things have changed. The influence of Hund interaction in strongly cor- related systems has studied vastly in last decade and made a great success by discovering Hund metal phase in which Hund correlation is strong while Hubbard correlation is weak. However the case where the two sources of correlation are both strong and cooperates has not been deliberately studied. In this study, such case where Brinkman-Rice picture is violated during MIT due to the Hund cor- relation will be described, and the origin of such behavior will be discussed under the concept of low-energy effectiveness of Hund correlation. Also, the discus- sion on low-energy effectiveness will be quantitatively systematized by defining the correlation factors in low- and high- energy scales. Finally, the origin of low-energy effectiveness of the Hund correlation will be explained through the discussion on spin and charge fluctuations. – III –