  The reference chiral helimagnet MnSi is the first system where skyrmion
lattice correlations have been reported. At zero magnetic field the transition
at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region
dominated by precursor phenomena, neutron scattering shows the build up of
strong chiral fluctuating correlations over the surface of a sphere with radius
$2\pi/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that
these fluctuating correlations drive the helical transition to first order
following a scenario proposed by Brazovskii for liquid crystals. We present a
comprehensive neutron scattering study under magnetic fields, which provides
evidence that this is not the case. The sharp first order transition persists
for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and
start to concentrate along the field direction already above 0.2 T. Our results
thus disconnect the first order nature of the transition from the precursor
fluctuating correlations. They also show no indication for a tricritical point,
where the first order transition crosses over to second order with increasing
magnetic field. In this light, the nature of the first order helical transition
and the precursor phenomena above $T_C$, both of general relevance to chiral
magnetism, remain an open question.

Materials Science