Hard Ferrite (ceramic) magnets, composed of barium or strontium, were developed in the early 1960's as an alternative to the more expensive metallic magnets. This class of magnet is very hard, brittle and possesses lower energy characteristics compared to other magnetic materials. However, it is very popular and widely used because of its excellent resistance to demagnetizaion, corrosion, and the advantageous pricing. http://www.sensmagnetics.com/