  Surface rotation rates of young solar-type stars display drastic changes at
the end of the pre-main sequence through the early main sequence. This may
trigger corresponding changes in the magnetic dynamos operating in these stars,
which ought to be observable in their surface magnetic fields. We present here
the first results of an observational effort aimed at characterizing the
evolution of stellar magnetic fields through this critical phase. We observed
stars from open clusters and associations, which range from 20 to 600 Myr, and
used Zeeman Doppler Imaging to characterize their complex magnetic fields. We
find a clear trend towards weaker magnetic fields for older ages, as well as a
tight correlation between magnetic field strength and Rossby number over this
age range. Comparing to results for younger T Tauri stars, we observe a very
significant change in magnetic strength and geometry, as the radiative core
develops during the late pre-main sequence.
