  We carried out quartz crystal microbalance experiments of a 5 MHz AT-cut
crystal for superfluid 4He films on Grafoil (exfoliated graphite) with a small
amount of 3He up to 0.40 atoms/nm2. We found that the mass decoupling from
oscillating substrate is considerable sensitive even in a small amount of 3He
doping. In a 4He film of 29.3 atoms/nm2, we observed a small drop in resonance
frequency at T3 of ~0.4 K for a small amplitude, which is attributed to
sticking of 3He atoms on the 4He solid atomic layer. For a large amplitude, the
4He solid layer shows a reentrant mass decoupling at TR close to T3. This
decoupling can be explained by the suppression of the superfluid counterflow
due to the adsorption of 3He atoms on edge dislocations. As the 4He areal
density increases, TR shifts to the lower temperature, and vanishes around a
4He film of 39.0 atoms/nm2.
