  Using muon spin rotation it is shown that the field of first flux penetration
H_entry in Nb is enhanced by about 30% if coated with an overlayer of Nb_3Sn or
MgB_2. This is consistent with an increase from the lower critical magnetic
field H_c1 up to the superheating field H_sh of the Nb substrate. In the
experiments presented here coatings of Nb_3Sn and MgB_2 with a thickness
between 50 and 2000nm have been tested. H_entry does not depend on material or
thickness. This suggests that the energy barrier at the boundary between the
two materials prevents flux entry up to H_sh of the substrate. A mechanism
consistent with these findings is that the proximity effect recovers the
stability of the energy barrier for flux penetration, which is suppressed by
defects for uncoated samples. Additionally, a low temperature baked Nb sample
has been tested. Here a 6% increase of H_entry was found, also pushing H_entry
beyond H_c1.
