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 M _ { \odot } , 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 .