Physical properties of (Mn1−xCox)Si at x≃0.060–0.100 : Quantum criticality

We have grown and characterized three samples of Co-doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity, and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results it follows that impurities (Co and Fe) eliminate the first-order phase transition peaks and spread the fluctuation maxima in such a way that the low-temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low-temperature parts of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The $d\ensuremath{\rho}/dT$ data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio ${C}_p/T$ and hence to the diverging effective electron mass. We found that at a large concentration of the dopant there are no distinct phase transition points. What we observe is probably a region of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0 K.