Recent experiments on polycrystalline Co1-x Fex S2 demonstrated composition control over the spin polarization by Fermi level manipulation. We report here the growth and characterization of Co S2 single crystals with fine control over the stoichiometry by chemical vapor transport. At the ideal Co:S atomic ratio we observe a minimum in the low temperature resistivity and the x-ray rocking curve width, coincident with a maximum in the residual resistivity ratio and the low temperature magnetoresistance. Point contact Andráev reflection on stoichiometric crystals indicates a spin polarization at the Fermi energy of 64%, a significant increase over the 56% observed in polycrystals.
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This work was supported primarily by the MRSEC program of the National Science Foundation.