Low-temperature transport, optical, magnetic and thermodynamic properties ofFe1−xCoxSi

We report measurements of the electrical conductivity $\sigma \mathrm{}\left(T\right)\mathrm{}$, the optical reflectivity $R(\omega ,T)$, the low-field ac magnetic susceptibility ${\chi }^{\prime }\mathrm{}(T,f)\mathrm{}$, and the specific heat $C_p\mathrm{}\left(T\right)\mathrm{}$ for polycrystalline samples of Fe${}_{1-x}$Co${}_x$Si with $0<~x<~0.03$ at low temperatures. Between 130 and 300 K the temperature variation of $\sigma (\omega ,T)$ is consistent with that of a hybridization-gap semiconductor and is very similar for all samples. Below 50 K $\sigma \mathrm{}(0,T)\mathrm{}$ strongly depends on the cobalt concentration $x$ and $d\sigma /dT$ decreases with decreasing $x$, changing sign from positive to negative in the range $0.01<~x<~0.02$. For $x<~0.0075$ the dc conductivity $\sigma$ tends to saturate below 0.1 K, indicating a semimetallic regime characterized by a very low density of itinerant carriers. For $x>~0.01$ the ${\chi }^{\prime }\mathrm{}\left(T\right)\mathrm{}$ curves show maxima at different temperatures $T_{\mathrm{f}}\mathrm{}\left(x\right)\mathrm{}$ manifesting a spin-glass-type freezing of magnetic moments below 1 K. The small values of $T_{\mathrm{f}}/x$ and the frequency dependences of $T_{\mathrm{f}}$ both suggest a weak magnetic interaction. The low-temperature specific heat $C_p\mathrm{}\left(T\right)\mathrm{}$ contains a term $\gamma T$ that increases with increasing $x$ from $2.1T$ mJ mol${}^{-1}$ K${}^{-1}$ for pure FeSi to $7.6T$ mJ mol${}^{-1}$ K${}^{-1}$ for Fe${}_{0.97}$Co${}_{0.03}$Si. Together with the results of the optical measurements this implies a respectable low-temperature mass enhancement of the itinerant carriers on the order of 30.