Ferrites are iron oxide-based ceramic materials with magnetic properties, useful in many types of electronic devices and building materials. Ferrites are hard, brittle, generally gray or black, and typically have a crystal structure which has more than one type of site for the cations. Usually the magnetic moments of the metal ions on sites of one type are parallel to each other and anti-parallel to the moments on at least one site of another type. Ferrites are produced by sintering finely powdered precursor compounds in molds, milling the resulting material into a fine powder, and then re-sintering, with the second sintering round usually conducted in a magnetic field to ensure that the particles align properly.
Commercial ferrites can be classified as hard or soft.
Soft ferrites are used in transformer or electromagnetic cores. They have low coercivity and high resistivity, and contain iron, oxygen, zinc, and either manganese or zinc.
Hard ferrites have high coercivity, and therefore are used as permanent magnets. Since ferrite permanent magnets are cheaper, and also weaker, than other available permanent magnets, they are frequently used in household products such as refrigerator magnets, and as mediums for magnetic recording. Strontium ferrite, barium ferrite, and Cobalt Ferritecobalt ferrite are all considered hard ferrites.