I've farmed 120 atmas and all I can say is that all of mine were obtained from golded, successful FATEs. Most of the FATEs I did fell in that category, so my experience isn't exactly indicative of anything, but over all the time farming there were still quite a few that didn't fall in that category. I just never happened to get an atma from any of them.

What I am interested in knowing is if you have different odds (presumably lower chances) of getting an atma if you don't win or if you get a lower grade.