  We present a comprehensive and systematic magnetization and ac susceptibility
study of Mn$_{1-x}$Fe$_{x}$Si over an extensive range of ten Fe concentrations
between $x$ = 0 - 0.32. With increasing Fe substitution, the critical
temperature decreases but the magnetic phase diagrams remain qualitatively
unaltered for $x$ $\leq$ $x^*$ $\approx$ 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 $x_C$ =
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.
