  We present here additional results of a spectropolarimetric survey of a small
sample of stars ranging from spectral type M0 to M8 aimed at investigating
observationally how dynamo processes operate in stars on both sides of the full
convection threshold (spectral type M4).
  The present paper focuses on early M stars (M0--M3), i.e. above the full
convection threshold. Applying tomographic imaging techniques to time series of
rotationally modulated circularly polarised profiles collected with the NARVAL
spectropolarimeter, we determine the rotation period and reconstruct the
large-scale magnetic topologies of 6 early M dwarfs. We find that early-M stars
preferentially host large-scale fields with dominantly toroidal and
non-axisymmetric poloidal configurations, along with significant differential
rotation (and long-term variability); only the lowest-mass star of our
subsample is found to host an almost fully poloidal, mainly axisymmetric
large-scale field ressembling those found in mid-M dwarfs.
  This abrupt change in the large-scale magnetic topologies of M dwarfs
(occuring at spectral type M3) has no related signature on X-ray luminosities
(measuring the total amount of magnetic flux); it thus suggests that underlying
dynamo processes become more efficient at producing large-scale fields (despite
producing the same flux) at spectral types later than M3. We suspect that this
change relates to the rapid decrease in the radiative cores of low-mass stars
and to the simultaneous sharp increase of the convective turnover times (with
decreasing stellar mass) that models predict to occur at M3; it may also be (at
least partly) responsible for the reduced magnetic braking reported for
fully-convective stars.
