Here you have some of the solutions given in the literature:
5%HNO3 in ethanol, 15V, 243K
50cm3 perchloric acid, 25cm3 2-butoxyethanol, 450cm3 ethanol, 230-240K, 8-10 V
I will also look for the solution I've used, however, I'll be able to do so on monday-tuesday.
It has to be also mentioned that magnesium alloys are easily oxygenating, and, I have to say that polishing in my solution (also based on the perchloric acid) may lead to some significant oxygenation of the specimen surface.
I think that the best way to do the TEM specimens is ion thinning.