Doctor of Philosophy (PhD)
Physics and Astronomy
Diluted magnetic semiconductors (DMSs) and magnetoresistive materials are instrumental for the development of spintronic technology. Before we are able to engineer spintronic devices, it is necessary to understand the properties of materials from which spintronic devices are made, otherwise time and other resources will be wasted. In this thesis, we investigate some aspects of diluted magnetic semiconductors and a magnetoresistive material Ta2PdSe6 with first-principle and many-body methods. Particularly, we investigate the Mn valence in (Ga,Mn)N and derive low-energy models of (Ga,Mn)N by making use of a first-principles Wannier-function analysis. Additionally, we also study the disorder-driven localization in DMSs with the typical-medium theory (TMT). Finally, we present a discussion about the investigation of the mechanism behind Ta2PdSe6 magnetoresistance. From a first-principles Wannier-function analysis, we find unambiguously the Mn valence in (Ga,Mn)N to be close to 2+ (d5), but in a mixed spin configuration with average magnetic moments of 4μB. By integrating out high-energy degrees of freedom differently, we further derive for the first time from first-principles two low-energy pictures that reflect the intrinsic dual nature of the doped holes in the (Ga,Mn)N: 1) an effective d4 picture ideal for local physics, and 2) an effective d5 picture suitable for extended properties. Furthermore, our results further reveal a few novel physical effects and pave the way for future realistic studies of magnetism. Our study not only resolves one of the outstanding key controversies of the field, but also exemplifies the general need for multiple effective descriptions to account for the rich low-energy physics in many-body systems in general. Meanwhile, by implementing a geometrical average of the local density of states to define an order parameter, the typical density of states (TDOS), we find that the TDOS vanishes below a critical doping concentration, indicating an Anderson localization transition in the system. Our results qualitatively explain why at concentrations lower than a critical value DMSs are insulating and non-magnetic, whereas at larger concentrations they are ferromagnetic bad metals. On the investigation of Ta2PdSe6, our density functional theory (DFT) calculations show that the material is a semimetal that has two types of charge carriers with the same density. This result, together with the magnetoresistance behavior of the two-band model, leads to the conclusion that the mechanism responsible for Ta2PdSe6 magnetoresistance is charge compensation.
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Nelson, Ryky, "First-principles and Many-Body Methods: Implementations and Applications to Study Spintronic Materials" (2015). LSU Doctoral Dissertations. 1234.