Comment by kragen

Ferrite is going to be pretty hard on your ball mill, since it's harder than steel, so at best you're going to get a lot of steel contamination in your ferrite. More to the point, though, if you buy 100-micron ferrite flour and you're trying to get a suspension of 1-micron particles, you need to break each of those flour grains into about a million pieces. My intuition is that, while in theory milling will eventually produce the desired result, it will probably take enormously longer than you can afford to wait. So generally the papers I've read about getting submicron particles† of one or another substance do it by synthesizing it in small particles in the first place, not by milling.

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† "nanoparticles", because calling them that allowed you to scam funds from the National Nanotechnology Initiative even if your research had nothing to do with Drexler's mechanosynthesis objectives!