Non-Fermi liquid behavior and the undersceened Kondo effect in Fe$_{1-y}$Co$_{y}$Si

Mn or Co substitutions into the narrow band-gap insulator FeSi introduce charge carriers, either holes or electrons, accompanied by an equal density of more localized magnetic moments resulting in an interesting insulator-to-metal transition (IMT). Mn doping of FeSi exhibits an IMT where the nascent metal displays intriguing field sensitive non-Fermi-Liquid (NFL) behavior due to the undercompensation of $S=$1 impurity moments by the spin-1/2 hole carriers. Here, we present the results of an investigation of Fe$_{1-y}$Co$_{y}$Si ( 0$\le y\le $0.1). Our magnetization and susceptibility measurements indicate that for $y$\textless 0.03 Co-impurities alsointroduce a $S=$1 magnetic moment that have a tendency to form singlets whereas for larger $y$a ferromagnetic interaction that grows with $y$. We have discovered a NFLbehavior for $y$\textless 0.03 that evolves into the standard disordered Fermi-liquid form either by applying a magnetic field or by increasing $y$. The results of specific heat measurements on Fe$_{1-y}$Co$_{y}$Si,performed to explore the underlying underscreened Kondo mechanism, to investigate its variation with field and composition,and to compare with our Fe$_{1-x}$Mn$_{x}$Si data will be presented.