Computational studies on the structure and stabilities of magnetic inter-metallic compounds (Dissertation)

Abstract 

The purpose of this thesis is to further the understanding of the structural, electronic and magnetic properties of ternary inter-metallic compounds using density functional theory (DFT). Four main problems are addressed. 

First, a detailed analysis on the ternary Heusler compounds is made. It has long been known that many Heusler compounds (X2Y Z; X and Y transition elements, Z main group element) exhibit interesting half-metallic and ferromagnetic properties. In order to understand these, the dependence of magnetic and electronic properties on the structural parameters, the type of exchange-correlation functional and electron-electron correlation was examined. It was found that almost all Co2Y Z Heusler compounds exhibit half-metallic ferromagnetism. It is also observed that X and Y atoms mainly contribute to the total magnetic moment. The magnitude of the total magnetic moment is determined only indirectly by the nature of Z atoms, and shows a trend consistent with Slater-Pauling behaviour in several classes of these compounds. In contrast to experiments, calculations give a non-integer value of the magnetic moment in certain Co2-based Heusler compounds. To explain deviations of the calculated magnetic moment, the LDA+U scheme was applied and it was found that the inclusion of electron-electron correlation beyond the LSDA and GGA is necessary to obtain theoretical description of some Heusler compounds that are halfmetallic ferromagnets. The electronic structure and magnetic properties of substitutional series of the quaternary Heusler compound Co2Mn1−xFexSi were investigated under LDA+U. The calculated band structure suggest that the most stable compound in a half-metallic state will occur at an intermediate Fe concentration. These calculated findings are qualitatively confirmed by experimental studies. 

Second, the effect of antisite disordering in the Co2TiSn system was investigated theoretically as well as experimentally. Preservation of half-metallicity for Co2TiSn was observed with moderate antisite disordering and experimental findings suggest that the Co and Ti antisites disorder amounts to approximately 10 % in the compound. 

Third, a systematic examination was carried out for band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18-electron or half-metallic ferromagnet half-Heusler compounds. It was found that the most appropriate description of these compounds from the viewpoint of electronic structures is one of a Y Z zinc blende lattice stuffed by the X ion. Simple valence rules are obeyed for bonding in the 8- and 18-electron compounds. 

Fourth, hexagonal analogues of half-Heusler compounds have been searched. Three series of compounds were investigated: GdPdSb, GdAuX (X = Mn, Cd and In) and EuNiP. GdPdSb is suggested as a possible half-metallic weak ferromagnet at low temperature. GdAuX (X = Mn, Cd and In) and EuNiP were investigated because they exhibit interesting bonding, structural and magnetic properties. The results qualitatively confirm experimental studies on magnetic and structural behaviour in GdPdSb, GdAuX (X = Mn, Cd and In) and EuNiP compounds.

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