Exploring Fe$_{1-y}$Co$_{y}$Si near the insulator-to-metal transition

FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. Charge carriers, either holes or electrons, accompanied by a more localized magnetic moments, can be introduced by doping FeSi with Mn or Co. It has been reported that Mn doping of FeSi near the insulator-to-metal transition (IMT) exhibits an intriguing field sensitive non-Fermi-Liquid behavior due to an undercompensated Kondo effect where the spin-1/2 carriers underscreen the $S=$1 impurity moments. To compare with the case of Mn substitution (hole doping), we investigate the effect of Co substitution (Fe$_{1-y}$Co$_{y}$Si, 0$\le y \le $ 0.1)(electron doping)results. Our magnetic property measurements indicate an interesting evolution of the impurity magnetic moments with $y$. Our transport studies indicate a temperature and field dependence that does not conform to the standard disordered Fermi-liquid form for small $y$. Standard semiconducting behavior is restored either by applying a magnetic field or increasing $y$. A more detailed analysis is underway to compare with disordered Fermi liquid theory as well as to the previously reported behavior of Fe$_{1-x}$Mn$_{x}$Si.