What's the size difference between the AFM needle and the area of stored magnetic flux on a hard drive platter? If you used an AFM as a sort of record player, scanning along lines of little pits, what sort of theoretical information density could be achieved over the whole surface of the disk?
This is an advertisement, but it's one of the few I actually enjoy watching, and it suggests a track is "2500 times smaller than a human hair" which puts an upper bound on the size of a bit: https://www.youtube.com/watch?v=ZXs_9OXRnQo
This doesn't answer your question but your question made me think of this and I thought I'd share for anyone else.