Of course, the best way is FIB, but if you are not equipped by FIB as me, then you have to rely on electropolishing or chemical thinning. I never tried to prepare Mg foils but if I would like by these ways I will affraid of strong oxidation potential of Mg and its alloys. I have got some recipes for chemical and electrochemical thinning dedicated to Mg. Here you are:
Mg and its alloys:
Chemical thinning:
- 2 to 15% HCl in H2O or CH3OH at room temperature
- 2 to 15% HNO3 in H2O or CH3OH at room temperature