Outrunner brushless motors have the coils inside and the magnets on the inner rim of an outside bell.
The bells are variously made of steel, aluminum, or plastic.
All else being the same, does the bell material make a (big enough) difference to the magnet field (to matter)?
Brushless or not, a permanent magnet structure can be designed to use the (inexpensive) steel as a flux path, or not. My lawn mower motor does have the steel in the magnetic field circuit, but my bicycle generator doesn't.
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