Evolution of helimagnetic correlations when approaching the quantum critical point of Mn1-xFexSi

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn1-xFexSi with increasing doping. By combining polarized neutron scattering and high resolution neutron spin echo spectroscopy we investigate three samples with x = 0.09, 0.11, and 0.14, i.e., with compositions on both sides of the concentration x* similar to 0.11 where the helimagnetic Bragg peaks disappear and between x* and the quantum critical concentration x(C) similar to 0.17, where T-C vanishes. We find that the abrupt disappearance of the long range helical periodicity at x* does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing x and the abrupt disappearance of long range helimagnetic periodicity at x*.