Dan's Data letters #149
 

"I think it'd be physically possible for rare earth magnets close to your specs to hold the weight you need, but single solid magnets like that would be very hard to work with. Ceramic magnets of all kinds are, as you say, brittle; one 50 by 50 by 3mm (say) would be very easy to snap."

It's been awhile since I've read Dan's letters, he's already at #149.