| This thesis demonstrates that alloy composition can be used to fine-tune the position of the Fermi level in Co1-xFexS2 alloys leading to composition controlled spin polarization and the ability to engineer high conduction electron spin polarizations of up to 85 %. We present here a comprehensive experimental investigation of the structure, stoichiometry, magnetic, magnetotransport, and thermodynamic properties of bulk polycrystalline solid solutions of Co1-xFexS 2. These data are supplemented with direct measurements of the spin polarization at the Fermi level by point contact Andreev reflection (PCAR) and detailed first principles electronic structure calculations. The compositions studied are in the range 0.0 < x < 0.30, the most relevant part of the phase space in terms of composition control over high spin polarization. By measuring the Fe doping dependence of the saturation magnetization, high field magnetoresistance, and anisotropic magnetoresistance, and combining them with PCAR, we are able to show that spin polarization can be tuned by alloy composition between -57 % (x = 0) and +85 % (x = 0.15). The density of states at the Fermi level determined by the band structure calculation, heat capacity measurement and nuclear magnetic resonance agree with each other very well.; Following these successes with polycrystalline Co1-xFe xS2, we show that the sulfur stoichiometry of CoS2+/-delta single crystals can be tuned by the sulfur vapor pressure in a chemical transport growth process. We present an investigation of the structural, stoichiometric, magnetic, magnetotransport, and thermodynamic properties of single crystal CoS2+/-delta. The sulfur content has a dramatic effect on the transport characteristics. At the optimal sulfur vapor pressure we observe a minimum in the width of the x-ray diffraction rocking curve and a minimum in the low temperature resistivity, as well as sharp maxima in the residual resistivity ratio and low temperature positive magnetoresistance. Under optimal conditions the residual resistivity ratio is well over an order of magnitude better than in typical polycrystals.{09}Co1-xFexS 2. single crystals have also been successfully synthesized using the chemical transport methods. Preliminary results are presented. |
