The rare-earth magnets are very expensive. However, they are the most
cost effective in very small motors.
From Wikipedia: “Developed in the 1970s and 80s, rare-earth magnets are
the strongest type of permanent magnets made, producing significantly
stronger magnetic fields than other types such as ferrite or alnico magnets.
The magnetic field typically produced by rare-earth magnets can be in
excess of 1.4 T, whereas ferrite or ceramic magnets typically exhibit fields of
0.5 to 1 T. There are two types: neodymium magnets and samarium-cobalt
magnets. Rare earth magnets are extremely brittle and also vulnerable to
corrosion, so they are usually plated or coated to protect them from
breaking and chipping.”
In general, alnico magnets are used in very large motors up to 200 hp. It is
also possible to use special combinations of magnets and ferromagnetic
materials to achieve high performance (i.e., high torque, high efficiency, and
low volume) at a low cost