  The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have
been studied by means of ac-susceptibility measurements, polarized neutron
small angle scattering and spin echo spectroscopy. It is shown that these
compounds undergo the transition from the paramagnetic to helimagnetic phase
through continuous, yet well distinguishable crossovers: (i) from paramagnetic
to partially chiral, (ii) from partially chiral to highly chiral fluctuating
state. The crossover points are identified on the basis of combined analysis of
the temperature dependence of ac-susceptibility and polarized SANS data. The
whole transition is marked by two inflection point of the temperature
dependence of ac-susceptibility: the upper one corresponds to the crossover to
partially chiral state at $T^*$, where the inverse correlation length $\kappa
\approx 2 k$, the lower one corresponds to the transition to the spin helix
structure. The intermediate crossover to the highly chiral phase is observed at
the inflection point $T_k$ of the first derivative of ac-susceptibility, where
$\kappa \approx k$. The temperature crossovers to the highly chiral fluctuating
state is associated with the enhancing influence of the Dzyaloshinskii-Moria
interaction close to $T_c$.
