  There is a large change in surface rotation rates of sun-like stars on the
pre-main sequence and early main sequence. Since these stars have dynamo driven
magnetic fields, this implies a strong evolution of their magnetic properties
over this time period. The spin-down of these stars is controlled by
interactions between stellar winds and magnetic fields, thus magnetic evolution
in turn plays an important role in rotational evolution. We present here the
second part of a study investigating the evolution of large-scale surface
magnetic fields in this critical time period. We observed stars in open
clusters and stellar associations with known ages between 120 and 650 Myr, and
used spectropolarimetry and Zeeman Doppler Imaging to characterize their
large-scale magnetic field strength and geometry. We report 15 stars with
magnetic detections here. These stars have masses from 0.8 to 0.95 Msun,
rotation periods from 0.326 to 10.6 days, and we find large-scale magnetic
field strengths from 8.5 to 195 G with a wide range of geometries. We find a
clear trend towards decreasing magnetic field strength with age, and a
power-law decrease in magnetic field strength with Rossby number. There is some
tentative evidence for saturation of the large-scale magnetic field strength at
Rossby numbers below 0.1, although the saturation point is not yet well
defined. Comparing to younger classical T Tauri stars, we support the
hypothesis that differences in internal structure produce large differences in
observed magnetic fields, however for weak lined T Tauri stars this is less
clear.
