Low-temperature transport, optical, magnetic and thermodynamic properties of Fe1-xCoxSi

We report measurements of the electrical conductivity sigma(T), the optical reflectivity R(pi,T), the low-field ac magnetic susceptibility chi'(T,f), and the specific heat C-p(T) for polycrystalline samples of Fe1-xCoxSi with 0 less than or equal to x less than or equal to 0.03 at low temperatures. Between 130 and 300 K the temperature variation of sigma(pi,T) is consistent with that of a hybridization-gap semiconductor and is very similar for all samples. Below 50 K sigma(O,T) 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 less than or equal to x less than or equal to 0.02. For x less than or equal to 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 greater than or equal to 0.01 the chi'(T) curves show maxima at different temperatures T-f(x) manifesting a spin-grass-type freezing of magnetic moments below 1 K. The small values of T-f/x and the frequency dependences of T-f both suggest a weak magnetic interaction. The low-temperature specific heat C-p(T) 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 Fe0.97Co0.03Si. 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.