Quantum criticality features in the Co,Fe doped MnSi.

An universal line revealing an independence of spin fluctuation contributions to the heat capacity on impurity content and its nature is discovered in the helical phase of Mn(Co,Fe)Si. This situation declares an invariance of the heat capacity of spin subsystem under doping, which probably arises as a result of relative stiffness of the helical spin structure in respect to the impurity spins. On the other hand the situation drastically changes at the helical fluctuation region when no long range spin order exists. At low temperatures the spin fluctuation contributions to the heat capacity for a whole set of compositions of Mn(Co,Fe)Si are described by a single power expression with an exponent less then unity, which implies divergence of the ratio C p / T at T → 0. The current data are revealing that a singular quantum critical point does not exist in the system under study. In its place, one can see some sort of a quantum critical cloud covering a significant range of dopant concentrations. • The helical magnet MnSi has played a special role in efforts of investigations of quantum criticality phenomena. • We used so-called "chemical" pressure technique • Mysterious universal line of spin fluctuation heat capacity of Mn(Fe,Co)Si was found. • The latter shows that a singular quantum critical point does not exist in the system under study. • Instead, one can see a sort of quantum critical cloud covering a range of dopant concentrations. [ABSTRACT FROM AUTHOR]