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∗∼0.11 where the helimagnetic Bragg peaks disappear and between x∗ and the quantum critical concentration xC∼0.17, where TC 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∗.@en

We present a comprehensive and systematic magnetization and ac susceptibility study of Mn1-xFexSi over an extensive range of ten Fe concentrations between x=0 and x=0.32. With increasing Fe substitution, the critical temperature decreases but the magnetic phase diagrams remain qualitatively unaltered for x≤x∗≈0.11 with clear boundaries between the helical, conical, and skyrmion lattice phase as well as an enhanced precursor phase. A notably different behavior sets in for x=0.11,0.13, and 0.14, where certain characteristics of helimagnetic correlations persist, but without clear phase boundaries. Although a qualitative change already sets in at x∗, the transition temperature and spontaneous magnetization vanish only at xc=0.17 where also the average magnetic interactions change sign. Although the Curie-Weiss temperature reaches -12 K for x=0.32, no signature of long-range magnetic order is found down to the lowest temperature, indicating a possible significant role for quantum fluctuations in these systems. © 2018 American Physical Society.@en

We present a comprehensive small-angle neutron scattering study of the doping dependence of the helimagnetic correlations in Mn1−xFexSi. The long-range helimagnetic order in Mn1−xFexSi is suppressed with increasing Fe content and disappears for x>x∗≈0.11, i.e., well before xC≈0.17 where the transition temperature vanishes. For x>x∗, only finite isotropic helimagnetic correlations persist which bear similarities with the magnetic correlations found in the precursor phase of MnSi. Magnetic fields gradually suppress and partly align these short-ranged helimagnetic correlations along their direction through a complex magnetization process@en