More and more permanent magnet motors are starting to use rare earth magnets, which have a high energy product, in order to achieve higher output density.
Magn Res Med 2012; 68: 882—889;.
NeuroImage 60 1 : 562-570; doi:.
Also, does the number of coils you wrap around affect the efficiency i.
I'm assuming that's straightforward, but if I run into trouble when I try doing it myself I'll add that info here.
Hum Brain Mapp 7: 254-266.